Detektering af sandlinser i moræneler ved brug af cross-borehole ground penetrating radar

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2 Detektering af sandlinser i moræneler ved brug af cross-borehole ground penetrating radar Majken Caroline Looms Zibar (projektansvarlig), Københavns Universitet Lars Nielsen, Københavns Universitet Kort metodebeskrivelse Ground penetrating radar (GPR) er en elektromagnetisk metode hvor en elektromagnetisk bølge udsendes fra en transmitter antenne. En receiver antenne bruges til at registrere hvorledes bølgen udbredes i det undersøgte medie. GPR kan bruges i to konfigurationer, refleksions GPR og transmissions GPR. Ved refleksions GPR placeres de to antenner ved siden ad hinanden og den elektromagnetiske energi der optages ved receiveren skyldes hovedsagligt refleksioner der opstår på grænseflader mellem medier af forskellig dielektriske egenskaber. Refleksions GPR bruges derfor især til kortlægning af geologiske aflejringsstrukturer. Ved transmissions GPR, eller cross-borehole GPR, placeres antennerne i to nærtliggende borehuller således at den energi der optages ved receiveren hovedsagligt stammer fra den direkte bølge der har passeret i gennem mediet mellem borehullerne. Fordelen ved cross-borehole GPR er at mediets dielektriske egenskaber og elektriske egenskaber kan estimeres ud fra en viden om antennernes eksakte indbyrdes placering og antagelser omkring bølgetype og udbredelsesmønster. Cross-borehole GPR er i de sidste årtier blevet brugt til at bestemme vandindhold i umættede medier eller porøsitet i de mættede medier grundet den store forskel i vands og lufts/sediments dielektriske egenskaber. Langt de fleste studier har beskæftiget sig med ucementeret sandede aflejringer, men der er også foretaget enkelte undersøgelser i kalkaflejringer samt sandsten. Materialer, med høj elektrisk ledningsevne, såsom ler, undgås som regel, eftersom det elektromagnetiske signal dæmpes med en reduceret indtrængningsevne som følge. GPR borehul geometri og geologi For at undersøge om cross-borehole GPR kan bruges til at kortlægge sandlinser i moræneler blev der i efteråret 2015 lavet fire GPR borehuller ved Kallerup grusgrav (RT 1, RT2, RT3 og RT4). De indbyrdes afstande varierer fra m og borehuller er boret m ned under tærren, se Tabel 1 og Figur 1. Tabel 1. Information vedrørende de fire GPR borehuller indbyrdes afstand og højde forskel. Rør Vand i Højde over Højde over Indbyrdes afstand dybde rør tærren havniveau RT1 RT2 RT3 RT4 RT m Tør 0.50 m m m 3.24 m 4.79 m RT m 5.77 m 0.52 m m m 2.64 m RT m 5.81 m 0.50 m m m RT m Tør 0.62 m m - 1

3 Figur 1. Indbyrdes placering af de fire GPR borehuller ved Kallerup grusgrav. Ved de fire GPR borehuller er geologien blevet tolket, se figur 2. Højderne i Figur 2 er korrigeret i forhold til det estimerede tærren-niveau ved RT1. Det ses at der eksisterer et m tyk sandlag i de fire borehuller mellem m dybde, og at Hedelands formationen nås ved m dybde. RT1 RT2 RT3 RT Clay Sand Air Depth in m Tube Unknown Figur 2. Tolkede boreprofiler ved de fire GPR borehuller. 2

4 GPR-måle rutiner Der blev foretaget tre forskellige GPR dataindsamlingsrutiner (Rubin and Hubbard, 2005). 1) Calibration (Figur 3A og 4A-C); Her måler man igennem luft til forskellige afstande mellem sender og modtager antennerne. Dermed opnås en bestemmelse af Absolut Time Zero (ATZ), dvs. det eksakte tidpunkt udstyret udsender det elektromagnetiske signal. 2) ZOP (Figur 3B og 4D); Zero-offset profile, hvor en transmitter og receiver antenne nedsænkes i to borehuller med samme centre-antenne-højde i faste intervaller. Dermed opnås en endimensionel bestemmelse af de dielektriske egenskaber af materialet mellem borehullerne. 3) MOG (Figur 3C); Multiple-offset gather. Her holdes sender antennen fast ved en given dybde og modtager-antennen nedsænkes i faste intervaller. Derefter gentages proceduren med senderantennen ved en ny dybde. Gentages dette et tilstrækkeligt antal gange kan man bruge målingerne i en geofysisk inversion og man opnår et todimensionelt billede af hvorledes materialet mellem borehullerne varierer. Figur 3. De tre forskellige dataindsamlingsrutiner. A: Calibration, B: Zero-offset profile, og C: Multiple-offset gather. 3

5 Figur 4. Billeder af to dataindsamlingsrutiner. A-C: Calibration ved forskellige antenne afstande og D: Zero-offset profile. GPR-målinger I alt blev der udført 44 målinger, se Appendiks A for detaljer. Overordnet set resulterede det i 11 kalibreringsfiler, seks zero-offset profileringer (de seks mulige mellem de fire borehuller, jf. Figur 1) og en multiple-offset gather mellem RT1 og RT3. De tolkede kalibreringsmålinger fremgår af Appendiks B, hvor ATZ observeres ændret med på op til 1.9 ns fra start til slutningen af dataindsamlingsforløbet. En enkel ZOP måling tog mellem minutter at indsamle og blev indsamlet med et meget fint indsamlingsinterval på 6.25 cm. Allerede under første måling kunne sandlaget ved m dybde identificeres ved en lavere ankomsttider og stærkere signal af den elektromagnetiske bølge. Et eksempel på dette er angivet med * for ZOP13 i Figur 5. 4

6 Figur 5: De målte ZOP data. Øverste panel illustrerer rådata, mens signaler i det nederste panel er normaliseret i forhold til det enkelte signals maksimum amplitude. * i ZOP13 indikerer placeringen af sandlinsen. Ved at identificere ankomsttiderne af de enkelte signaler samt amplituden af første peak blev sand forekomsterne endnu tydeligere, se Figur 6 for et eksempel af ZOP resultatet mellem RT1 og RT3. Tilsvarende figurer for de resterende fem ZOP er vist i Appendiks C. Figur 6. Det analyserede ZOP data mellem borehul RT1 og RT3, samt information om geologien ved de to borehuller. 5

7 Af Figur 6 fremgår det at det elektromagnetiske signal har en højere hastighed gennem sandlaget ( m/ns) i forhold til moræneleret ( m/ns). Dette skyldes at der i sandlaget er mindre porevand end i moræneleret og at hastigheden af et elektromagnetisk signal gennem vand er stærkt reduceret (0.033 m/ns) i forhold til gennem luft ( m/ns). Der observeres også et kraftigt forøgelse af amplituden af det første peak fra omkring mv til > mv sammenfaldende med de observerede sandlinser/lag. Som nævnt tidligere, vil den højere elektriske ledningsevne af leret dæmpe det elektromagnetiske signal dersom energien bliver omdannet til varme. Endelig kan man af Figur 6 observere at sandlinsens/-lagets eksakte tykkelse og placering fremgår bedre bestemt af amplituden af det elektromagnetiske signal og at tilstedeværelsen af sandet påvirker hastigheden af det elektromagnetiske signal op til ca. 1 m væk fra den bestemte laggrænse ved borehullerne. Det elektromagnetiske signal udsendes som en kugleform fra senderantennen og derfor er det ikke altid det direkte signal (her gennem leret) der registreres først ved modtagerantennen. I stedet for vil det signal der når op og rammer grænsen mellem sandlinsen/laget og leret overhale det direkte signal grundet den højere hastighed i det tørre sand. Dette kaldes den kritisk refrakterede bølge. Data fra multi-offset gather indsamlingsrutinen mellem RT1 og RT3 er analyseret ved brug af en simpel inversionsrutine som antager rette stråle-baner (Cordua et al., 2008). I denne inversion indgår kun information vedrørende placeringerne af sender og modtager antenne samt tilhørende løbetider. Resultatet er vist i Figur 7 nedenfor. Figur 7. Det inverterede MOG data mellem borehul RT1 og RT3, samt information om geologien ved de to borehuller. Som ved ZOP målingen ses det i Figur 7 at sandlinsen ved m dybde bliver identificeret som et højhastighedslag, men at tykkelsen af laget er overestimeret. Det interessante ved Figur 7 er at den svage hældning af sandlaget mod RT3 bliver fanget korrekt, samt at den modsatte stigning af Hedelands formationen ved omkring 6 m dybde også fremgår (dog meget svagt). Figur 8 illustrerer amplituden af det elektromagnetiske signal under antagelse af rette strålebaner. Signaler med amplituder over mv antages kun at have passeret igennem sand. Som det også fremgik af Figur 6 er størrelsen af amplituden i langt højere grad påvirket af sedimenttype i forhold til EM hastigheden. I forhold til Figur 7, ses nu meget tydeligere afgrænsning samt hældning af sandlinsen i Figur 8. 6

8 Figur 8. Elektromagnetisk signal dæmpningsplot mellem borehul RT1 og RT3, samt information om geologien ved de to borehuller. Syntetiske modelresultater For nærmere at undersøge potentialet af cross-borehole ground penetrating radar til at detektere sandlinser i moræneler af forskellige tykkelser, har vi lavet nogle indledende syntetiske test i MATLAB (Irving and Knight, 2005). Konkret har vi undersøgt to syntetiske modeller, se Figur 9. De fysiske egenskaber for sand og ler brugt i modellen er vist i Tabel Baseret på observationerne ved Kallerup grusgrav. 2. Baseret på observationerne ved Kallerup grusgrav, men sandlinsetykkelsen er reduceret til 10 cm. 0 Synthetic Model 1 0 Synthetic Model 2 Clay 1 1 Sand Depth in m Distance in m Distance in m Figur 9. De to syntetiske modeller brugt i MATLAB. Tabel 2. De fysiske egenskaber brugt i de syntetiske modelkørsler i MATLAB. Sand Ler Dielektrisk permittivitet 5 15 EM hastighed m/ns m/ns Konduktivitet S/m S/m Resistivitet 500 Ωm 40 Ωm 7

9 Formålet med Syntetisk Model 1 er at sikre at MATLAB koden producerer resultater der er i overensstemmelse med målinger i Kallerup Grusgrav. Resultatet af Syntetisk Model 1 ses i Figur 10. Ligesom ved feltdata plottes tracenormaliseret rådata, det estimerede hastighedsprofil samt amplituden af første peak. Modelresultaterne i Figur 10 stemmer overraskende godt overens med de målte data vist i Figur 5 og 6. Igen observeres at sandlinsens eksakte tykkelse og placering er bedst bestemt af amplitudeværdien af det elektromagnetiske signal og at tilstedeværelsen af sandet påvirker hastigheden af det elektromagnetiske signal op til ca. 1 m væk fra den korrekte laggrænse. Desuden observeres det at amplituden af den første peak er lavere for den kritisk refrakterede bølge end det direkte signal gennem moræneleret og at der opstår en lille forøgelse af amplituden (markeret med * på Figur 10) på grænsen mellem det refrakterede og det direkte signal. Dette skyldes konstruktiv interferens mellem de to bølgetyper og ses også i feltdata (se Figur 6). Figur 10. Syntetisk modelresultat 1. Samme sandlinse tykkelse som ved Kallerup Grusgrav. Figur 11. Syntetisk modelresultat 2. Sandlinsetykkelse reduceret til 10 cm. I Figur 11 ses resultatet af Syntetisk Model 2. Denne modelkørsel viser at selv om sandlinsen kun er 10 cm tyk burde den kunne observeres som et udslag, både i hastigheden og i amplituden af det elektromagnetiske signal. 8

10 Delkonklusion I denne rapport er det blevet vist at det er muligt at bruge cross-borehole ground penetrating radar (GPR) til at detektere sandlinser og -lag ved Kallerup grusgrav. Metoden var hurtigt og simpel at bruge. Allerede efter den først måling (som tog 20 minutter at indsamle) kunne dybden til den sammenhængende sandlinse ved m dybde identificeres. Det er derfor potentielt muligt at bruge cross-borehole GPR i forbindelse med anlæggelse af borehuller for at afgrænse eventuelle gennemgående sandlag/linser. Sandlinsens og Hedelands formationens placering fremgik til dels af den målte elektromagnetiske hastighed, idet sandets dårlige retentionsegenskaber medførte lave vandindhold. Men også amplituden af det elektromagnetiske signal gav udslag, hvilket muligvis medfører at sandlinser også kan detekteres under mættede forhold. En geofysisk inversion af data indsamlet mellem RT1 og RT3 bidrog derudover med information vedrørende hældning af sandlinsen mod RT3, hvilket er i overensstemmelse med borehulsloggene. Det var muligt med en syntetisk modelkørsel i MATLAB at genskabe feltmålingerne, og dette numeriske værktøj blev derfor brugt til at påvise at en sandlinse på 10 cm tykkelse ville fremgå af cross-borehole GPR data. Perspektiver De indsamlede cross-borehole data og de konklusioner anført ovenfor repræsenterer de lokal specifikke forhold ved Kallerup Grusgrav. Det er derfor vigtigt at undersøge hvorvidt disse kan overføre til lokaliteter med andre moræneaflejringer. En mere avanceret inversionsrutine (f.eks. full-waveform inversion), hvor amplitude information også indgår, vil højst sandsynligt bidrage til bedre at afdække grænsefladen mellem sand og moræneler og identificere de tilfælde hvor sandlinsen er brudt mellem de to borehuller. Full-waveform inversion vil muligvis også bidrage med information vedrørende den geologiske variabilitet indenfor morænelerlaget, og eventuelt lokalisere områder med mere sand og/eller store stenblokke. Endelig vil cross-borehole GPR metoden kunne benyttes under et kunstigt infiltrationsforsøg for at illustrere vand/stof transport i moræneafleringer. Referencer Cordua, K.S., M.C. Looms, and L. Nielsen Accounting for correlated data errors during inversion of cross-borehole ground penetrating radar data. Vadose Zone Journal 7, doi: /vzj Irving, J., and R. Knight Numerical modeling of ground-penetrating radar in 2-D using MATLAB. Comput. Geosci. 32: Rubin, Y. and S.S. Hubbard (eds.) Hydrogeophysics. Springer, Dordrecht, the Netherlands 9

11 APPENDIKS A. Indsamlede målinger GPR measurements (Before lunch, collection time 1 h 29 min): Line Time Type Tx (LN) Rx (PJ) Traces Notes 01 12:20 Test MT 3 m MT 2 m 5 1 m 02 12:23 CAL MT 3 6 m MT 2 m 31 1 : 0.1 : 4 m 03 12:37 ZOP43 RT4 RT : : 7.75 m 04 12:43 CAL MT 3 6 m MT 2 m 31 1 : 0.1 : 4 m 05 12:52 ZOP13 RT1 RT : : 7.75 m 06 12:57 CAL MT 3 6 m MT 2 m 31 1 : 0.1 : 4 m 07 13:05 ZOP23 RT2 RT : : 7.75 m 08 13:08 CAL MT 3 6 m MT 2 m 31 1 : 0.1 : 4 m 09 13:17 ZOP21 RT2 RT : :? m (Error at ~ 6 m depth) 10 13:24 ZOP21 RT2 RT : : 7.75 m 11 13:27 CAL MT 3 6 m MT 2 m 31 1 : 0.1 : 4 m 12 13:36 ZOP41 RT4 RT : : 7.75 m 13 13:38 CAL MT 3 6 m MT 2 m 31 1 : 0.1 : 4 m 14 13:46 ZOP24 RT2 RT : : 7.75 m 15 13:49 CAL MT 3 6 m MT 2 m 31 1 : 0.1 : 4 m MT= Measuring Tape, LN= Lars Nielsen, PJ=Peer Jørgensen, MZ= Majken Zibar GPR measurements (After lunch, collection time 1 h 20 min): Line Time Type Tx (fixed) Rx (MZ) Traces Notes 16 14:31 CAL MT m MT 2 m 25 2 : 0.1 : 4.4 m 17 14:34 CAL MT m MT 2 m 25 2 : 0.1 : 4.4 m 18 14:39 MOG RT1: 1.00 m RT3 25 Rx: 1 : 0.25 : 7 m 19 14:43 MOG RT1: 1.25 m RT3 25 Rx: 1 : 0.25 : 7 m 20 14:45 MOG RT1: 1.50 m RT3 25 Rx: 1 : 0.25 : 7 m 21 14:48 MOG RT1: 1.75 m RT3 25 Rx: 1 : 0.25 : 7 m 22 14:51 MOG RT1: 2.00 m RT3 25 Rx: 1 : 0.25 : 7 m 23 14:53 MOG RT1: 2.25 m RT3 25 Rx: 1 : 0.25 : 7 m 24 14:56 MOG RT1: 2.50 m RT3 25 Rx: 1 : 0.25 : 7 m 25 14:58 MOG RT1: 2.75 m RT3 25 Rx: 1 : 0.25 : 7 m 26 15:01 MOG RT1: 3.00 m RT3 25 Rx: 1 : 0.25 : 7 m 27 15:04 MOG RT1: 3.25 m RT3 25 Rx: 1 : 0.25 : 7 m DVL BATTERY CHANGED 28 15:09 CAL MT m MT 2 m 25 2 : 0.1 : 4.4 m 29 15:12 MOG RT1: 3.50 m RT3 25 Rx: 1 : 0.25 : 7 m 30 15:15 MOG RT1: 3.75 m RT3 25 Rx: 1 : 0.25 : 7 m 31 15:17 MOG RT1: 4.00 m RT3 25 Rx: 1 : 0.25 : 7 m 32 15:19 MOG RT1: 4.25 m RT3 25 Rx: 1 : 0.25 : 7 m 33 15:22 MOG RT1: 4.50 m RT3 25 Rx: 1 : 0.25 : 7 m 34 15:25 MOG RT1: 4.75 m RT3 25 Rx: 1 : 0.25 : 7 m 35 15:27 MOG RT1: 5.00 m RT3 25 Rx: 1 : 0.25 : 7 m 36 15:30 MOG RT1: 5.25 m RT3 25 Rx: 1 : 0.25 : 7 m 37 15:33 MOG RT1: 5.50 m RT3 25 Rx: 1 : 0.25 : 7 m 38 15:35 MOG RT1: 5.75 m RT3 25 Rx: 1 : 0.25 : 7 m 39 15:38 MOG RT1: 6.00 m RT3 25 Rx: 1 : 0.25 : 7 m 40 15:40 MOG RT1: 6.25 m RT3 25 Rx: 1 : 0.25 : 7 m 41 15:43 MOG RT1: 6.50 m RT3 25 Rx: 1 : 0.25 : 7 m 42 15:45 MOG RT1: 6.75 m RT3 25 Rx: 1 : 0.25 : 7 m 43 15:48 MOG RT1: 7.00 m RT3 25 Rx: 1 : 0.25 : 7 m 44 15:51 CAL MT m MT 2 m 25 2 : 0.1 : 4.4 m 10

12 APPENDIKS B. Estimering af Absolute Time Zero (ATZ) Travel time in ns Distance in m Figur B.1. Eksempel på kalibreringsresultat Line02 fra den tilhørende ZOP43 målingen. Skæringen med y-aksen, her ns, angiver Absolute Time Zero (ATZ). Absolute time zero in ns R RMSE in ns Adquisition time in minuts Figur B.2. Resultatet af alle udførte kalibreringer. 11

13 1 2 ZOP13 MOG Depth in m 4 5 MOG y = 0.993x R 2 = rmse= EM velocity in m/ns ZOP13 Figur B.3. Sammenligning af ZOP13 med de tilsvarende horisontale målinger taget i løbet af MOG13, under antagelsen at ATZ= ns kan bruges for alle MOG målinger. 12

14 APPENDIKS C. ZOP resultaterne 1 ZOP43 1 ZOP43 1 RT3 1 RT4 Clay Sand Depth in m EM velocity in m/ns Amplitude in mv 4 x ZOP13 1 ZOP13 1 RT1 1 RT3 Clay Sand Depth in m EM velocity in m/ns Amplitude in mv 4 x ZOP23 1 ZOP23 1 RT2 1 RT3 Clay Sand Depth in m EM velocity in m/ns Amplitude in mv 4 x

15 1 ZOP21 1 ZOP21 1 RT1 1 RT2 Clay Sand Depth in m 4 4 Depth in m EM velocity in m/ns Amplitude in mv 4 x ZOP41 1 ZOP41 1 RT1 1 RT4 Clay Sand Depth in m 4 4 Depth in m EM velocity in m/ns Amplitude in mv 4 x ZOP24 1 ZOP24 1 RT2 1 RT4 Clay Sand Depth in m EM velocity in m/ns Amplitude in mv 4 x

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17 Cross borehole DCIP Kallerup Report number , November 2016

18 Table of Contents Project information...3 Introduction...4 Location General geology...5 Methodology DC Method Time Domain Induced Polarization (TDIP) Surface measurements Borehole Measurements Array types, single borehole Array types, cross-borehole Array types, cross-borehole to surface...10 Synthetic Model...12 Data Acquisition Field Campaign Acquisition settings Contact Resistance...15 Data processing and inversion DC processing IP processing Inversion...18 Depth of investigation...20 Results...21 Conclusion...28 References...29

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20 PROJECT INFORMATION Cross-borehole DCIP Kallerup Gravelpit Contact person Thue Bording, Aarhus University Locality Kallerup Gravelpit Fieldwork period November, 2015 Report Prepared by: Thue Bording, Aarhus University Anders V. Christiansen, Aarhus University Table 1: Project information

21 INTRODUCTION The present study investigates the use of cross-borehole DCIP measurements to determine thin sand-layers in a moraine setting. In the Capitol Region of Denmark, a large amount of smaller sites is expected to be contaminated due to various human activities. A large fraction of these sites are in clayey moraines, where the flow of contaminants is predominantly in sand lenses, or sandy layers. Multiple boreholes are normally drilled in order to describe the geology, but boreholes alone do not always provide the necessary resolution to map out such sand lenses. The test site was situated in an uncontaminated gravel pit near Hedehusene, Zeeland, Denmark (Kallerup grusgrav). After measurements the entire test site was dug out, and the geology was described. The cross-borehole measurements were carried out by custom developed and constructed electrode tubes. Three boreholes along a surface DCIP profile were used. 3 dataset were obtained using electrodes in one borehole, 3 dataset were obtained using electrodes in two boreholes and 3 dataset were obtained using electrodes in two boreholes and on the surface Before the measurements were performed, a synthetic model test was carried out. The results showed that the using the chosen methodology and measurement configurations, thin sand lenses could indeed be identified by the inversion procedure.

22 LOCATION An overview map of the survey area is shown in Figure 1. The site is an active gravel pit (Kallerup Grusgrav) near Hedehusene, Zeeland, Denmark. Four profiles (A-D) were excavated and described geologically. Due to the nature of the excavation method, the positions of the geological profiles are only approximate. Figure 1: Location map, with positions of surface profile electrodes, boreholes and geological profiles A-D. 3.1 General Geology The main resource for gravel pit at the test site is the Hedeland Fm., which is overlain by 6-10 m of moraine. A few thin sand layers are observed in the moraine, and these are the target of the investigation. The water table was more than 20 below terrain.

23 METHODOLOGY 4.1 DC Method Resistivity measurements are carried out by injecting electrical current into the ground and subsequently measuring the resulting potential differences at the surface. The measured resistivity depends on the material, porosity and the pore fluid resistivity. For instance clayey material is characterized by a low resistivity, because water can diffuse between the mineral grains and thereby increase the specific surface area, which supports the surface conductivity. In the field, measurements are carried out by sending current into the ground through two electrodes (A and B) and measuring the potential difference at a different set of electrodes (M and N), and thereby getting an estimate of the resistivity of the ground called the apparent resistivity. The current pattern and equipotential surfaces for a homogenous and isotropic ground is shown on Figure 2. Figure 2: (a) Electrical current and potential fields in a homogenous ground. A and B are current electrodes and M and N potential electrodes. (b) Sketch showing that by increasing the current electrode spacing, information on the deeper part of the ground is obtained. Figure from Christensen (2008). The potential at any observation point at the surface of a homogenous isotropic half space can be expressed V(r) = ρi 2πr where I is the transmitted current, ρ is the resistivity of the ground and r is the distance from the observation point at the surface to the point source. With an electrode configuration like the one shown in Figure 2 the potential difference in a homogenous halfspace can be calculated from: ΔV = ρi 2πr ( 1 AM 1 BM 1 AN + 1 BN ) Where AM, BM, AN and BN denote the distance between current and potential electrodes.

24 In reality the measured potential rarely comes from a homogeneous halfspace and the above equation is rewritten to give the apparent resistivity: ρ a = ΔV I 2π ( 1 AM 1 BM 1 AN + 1 BN ) 1 = ΔV I K Where K is the geometrical factor which depends on the geometry of the chosen electrode configuration. The apparent resistivity is the resistivity a homogenous halfspace should have to give the actual measurement. Different electrode configurations have different penetration depths, but generally when increasing the distance between current electrodes, information on deeper parts of the earth is obtained (Figure 4). For borehole measurements the current lines between electrodes require a full space to be resolved, and the geometrical factor is changed to the following: K = 4π ( 1 AM 1 BM 1 AN + 1 BN ) 1 Combinations of surface and borehole electrodes complicates the analytical expression, and requires the use of mirror sources to be resolved analytically (Nielsen, 2007) 4.2 Time Domain Induced Polarization (TDIP) In the present survey IP data were collected together with the resistivity data, using a 100% duty cycle (Olsson et al., 2015). Time Domain IP (TDIP or IP) consists of measuring the voltage charge up resulting from an exciting current pulse. Figure 3 summarizes the basic principles of TDIP signal acquisition, and all the following denotations refer to this figure. Immediately after the current is turned on, a potential rise across the potential electrodes. After a charge-up effect the primary voltage (VDC) is measured for the computation of the direct current resistivity just before the polarity of the current is changed. The charge up effect is characteristic of the medium (in terms of initial magnitude, slope and relaxation time), and represents the target of TDIP. Because of inductive signals, similar to a time-domain EM system, occurring just after the current shut-off, a time gap or delay is applied before performing the measurements. The length of this gap is typically in the order of 1-5 ms. The signal decay is usually integrated over n time windows or gates for the computation of the chargeability M. The integral chargeability [mv/v] is defined as following (Schön, 2015, Slater and Lesmes, 2002) Ma i V DC 1. t i 1 t i t i 1 t i V ip dt where Vip is the intrinsic or secondary potential [mv] that can be seen as the transient response resulting from the ground polarization when the current is turned on.

25 ti and ti+1 are the opening and closing times [s] for the gate over which the signal is integrated. Figure 3: Principle of Time domain IP measurement. a) injected current waveform and voltage response. b) a single IP decay curve. 4.3 Surface measurements In order to measure the resistivity variation in both vertical as well as horizontal direction (2D), multi electrode surveys are routinely applied on the surface. The method is known as Electrical Resistivity Tomography (ERT), Continuous Vertical Electrical Sounding (CVES), or Multi Electrode Profiling (MEP). The latter is mainly a term used in Denmark. Regardless of the name, a number of electrodes, typically around 60 are installed in a line and all connected to the instrument using multicore cables. The instrument measures a number of potential differences for each current injection, which speeds up the acquisition time dramatically over older system capable of measuring just one channel at a time. With the ABEM Terrameter LS up to 12 channels can be measured simultaneously, but due to other restrictions typically only 7 channels are feasible in typical data collection setups

26 The surface profile was of the gradient type. Gradient type of measurements are carried out by injecting current between two electrodes (current electrodes) and simultaneously measuring the potential differences between several sets of electrodes placed between the current electrodes. Figure 4 shows one set of current injection and 6 sets of potential measurements for each current injection. The dipole distance for the potential measurements is increased when the current injection distance is increased. In the example 6 channels are used whereas we often use 7 channels, but other instruments use only 4 or 3. Figure 4: measurements with a 6-channel acquisition setup. For each current injection (red), 6 potential measurements (blue) are taken between the two injection points. The number to the right of each configuration states the vertical focus point and the lateral focus point is marked by the cross. For instruments with a different number of channels available, the number of potential measurements are adjusted accordingly 4.4 Borehole Measurements In order to get a good vertical resolution, borehole measurements can be performed by either the Ellog method or by fixed positions borehole electrodes. Fixed positions borehole measurements, as performed in this study, are carried out in manner similar to the surface measurements. The number of channels used is dependent on the array type.

27 Array types, single borehole In the single borehole measurements, collinear dipole-dipole arrays are used (Figure 5, a). Similar to the gradient array, multichannel measurements can also be done here by stepping the potential measurements away from the current injection. Figure 5: a) collinear array type used in single boreholes. b) parallel dipole-dipole configuration used in cross-borehole measurements. c) equatorial dipole-dipole configuration used in cross-borehole measurement Array types, cross-borehole The cross-borehole measurements consist of two different array types: parallel dipole-dipole and equatorial dipole-dipole (Figure 5, b-c). In parallel dipole-dipole configurations the current injections are in one borehole and the potential measurements are in the other borehole. The equatorial dipole-dipole configuration use a current and potential from each borehole. Current injections and potential measurements are performed between electrodes at the same depth level Array types, cross-borehole to surface In the cross-borehole to surface measurements, perpendicular dipole-dipole arrays were used (Figure 6). In a perpendicular dipole-dipole configuration the current electrodes are in two different boreholes at the same depth level, while the potential electrodes are at the surface. Similar to the gradient array, multichannel measurements can also be done here by stepping the potential measurements away from the current injection. These measurements were all too noisy, and were not used in the inversions.

28 Figure 6: perpendicular dipole-dipole configuration used in surface to borehole measurements:

29 SYNTHETIC MODEL Prior to the field campaign, a synthetic model test was performed in order to see if the collected data and the inversion would be able to resolve thin sand layers. A synthetic resistivity model was constructed, based on the expected geology at Kallerup, with thin resistive sand layers in a conductive clayey setting. This model is shown in Figure 7 below. Figure 7: Synthetic model of expected geology. Positions of boreholes A synthetic dataset was constructed by forward models using the electrode configurations specified in chapter 4. Two 10 m deep boreholes were assumed to be present at positions 28 m and 33 m along the profile coordinate axis. The data was then inverted assuming a noise model with 2 % error on the DC data. The resulting model is seen in Figure 8. Between the boreholes, the synthetic model is really well resolved, with almost identical models. Going away from the boreholes in either direction, the resolution of the fine layers decrease, but up to a few meters outside the cross borehole area, the geology is still well resolved. From this simple synthetic model, it is seen that by DC data alone, even really thin sand layers (20 cm) can be resolved, and that the method is indeed promising in resolving thin layers. Figure 8: Inversion result from synthetic model. Boreholes present at 28 m and 33 m along the coordinate axis

30 DATA ACQUISITION 6.1 Field Campaign The field measurements was done during November The datasets collected are summarized in Table 2. The surface profile was acquired with a uniform electrode spacing of 1 m. The original plan was to drill the boreholes the using an updated Ellog auger drilling method (Sorensen, 1994), but the field conditions forced us to use a drilling rig where this was not possible. Three boreholes were drilled with a 6 diameter drill bit to a depth of ten meters. Custom made electrode tubes (Figure 9) with an outer diameter 60 mm and a vertical electrode spacing of 20 cm were inserted in the boreholes and the boreholes were backfilled with sand. The electrode tubes got down to 9.0 m in B0635, 10.0 m in B1215 and 9.4 m in B1720, and as such the number of electrodes is different in each borehole. The measurements were carried out using array types as stated above. Figure 9: Electrode tubes being inserted into borehole. The electrodes can be seen as the copper rings every 20 cm along the tube. The cabling for the electrodes is on the inside of the tube.

31 Dataset Boreholes Protocols No. of quadrupoles Surface - Gradient 1519 B_0635 B0635 Collinear dipole-dipole 463 B_1215 B1215 Collinear dipole-dipole 564 B_1720 B1720 Collinear dipole-dipole 508 CH_0635_1215 B0635,B 1215 Equatorial dipole-dipole, parallel dipoledipole 462, 517 CH_0635_1720 B0635, B1720 Equatorial dipole-dipole, parallel dipoledipole 462, 369 CH_1215_1720 B1215, B1720 Equatorial dipole-dipole, parallel dipoledipole 487, 540 SH_0635_1215 B0635, B1215 Perpendicular dipole-dipole 780, not used SH_0635_1720 B0635, B1720 Perpendicular dipole-dipole 780, not used SH_1215_1720 B1215, B1720 Perpendicular dipole-dipole 858, not used Table 2: Acquired datasets, the differing number of quadrupoles stem from the differing number of electrodes involved 6.2 Acquisition settings The ABEM Terrameter LS was used for measuring, and DC and IP measurements were performed simultaneously. A total of 7 channels were used for measuring, and 25 IP gates were defined, logarithmically spaced from 1 ms to 4000 ms. The full waveform dataset was recorded, and the IP data was extracted after signal processing. The instrument settings are summarized in Table 3 and the widths of the IP gates are compiled in Table 4.

32 Instrument Maximum Voltage used Maximum Current used Maximum Power used Current injection time DC Delay Time DC Acquisition Time ABEM Terrameter LS 600 V 500 ma 250W 4 seconds 3.9 sec 0.1 sec Number of channels 7 Number of IP gates 25 Full waveform recording Sample Rate Yes 3750 Hz Table 3. Instrument settings. Gate number Gate width (ms) Gate center (ms) Gate number Gate width (ms) Gate center (ms) Table 4. Gate width and center time 6.3 Contact Resistance The contact resistances measured in the boreholes just after the borehole installation were in the order of a few kω, but after a few hours they increased to tens of kω, due to the drainage of the water into the Hedeland Fm. at the bottom of the boreholes (the water table being well below the borehole depth). A test acquisition

33 showed poor data quality due to the high contact resistance. Consequently, around 100 liters of salt water were poured in each borehole, resulting in a permanent decrease of contact resistance in the kω range. The surface electrodes did not have any noteworthy problems with contact resistance.

34 DATA PROCESSING AND INVERSION The data processing was done in the Aarhus Workbench program package, which has been developed at Aarhus University by the HydroGeophysics Group. In the processing procedure all bad data points are removed. Bad data points can be caused by several factors, but the most common is poor contact between the electrodes and the soil. Other typical causes are couplings, noise of unknown origin or instrument errors. 7.1 DC processing When processing the resistivity data, the data profiles are inspected and all the data points with a jiggered appearance are removed. An example including data points that are typically removed are shown on Figure 10 Figure 10: An example of DC processing. Bad data points caused by noise or poor electrode contact stands out as data points with a jiggered appearance and are removed. The removed data points are grey. The example shown is not from the present survey. 7.2 IP processing In the processing of the induced polarization data, all the induced polarization decay curves are evaluated one by one. All the bad IP decays are removed. IP decays showing one of the following appearances (Figure 11) are considered as bad decays and are culled: 1. Has a strange shape (e.g. an abrupt change in slope) 2. Not consistent in magnitude with surrounding decays (i.e. having abnormally high magnitude) 3. An increase in magnitude in later gates 4. A very flat curve as compared to neighboring decays

35 Figure 11: Examples of IP data processing. Blue curves in the figure are showing the good quality IP signals and gray curves in the figure are showing examples of bad IP decays. The decays shown are not from the present survey. 7.3 Inversion For this survey the time-domain DC and IP data has been inverted with a new 2D- DCIP Inversion code developed at Aarhus University by the HydroGeophysics Group (Auken et al., 2014). With the code, the polarization occurring at the mineral grain-fluid interface immediately after the current is shut off, is described with the empirical Cole-Cole model, which describes the complex resistivity, ζ, as a function of the four Cole-Cole parameters ρ, m0, τ and C: 1 ζ = ρ (1 m 0 (1 1 + (iωτ) C)) where ρ is the resistivity, M0 the magnitude of the chargeability and describes the polarization magnitude, τ is a constant which characterizes the decay and C a constant that controls the frequency dependence. In addition to the Cole-Cole parameters, a normalized chargeability is also calculated as m0/ρ or σ m0. The advantage of the new code is that it uses the entire IP decay curve, resulting in a better understanding of the polarization phenomena and in the end a more detailed description of the soil. Previously IP inversion involved only the integral chargeability where the decay is integrated into one single data point.

36 As the Cole-Cole model is an empirical model that describes the decay, the model parameters are not easily translated into geological terms. The parameter τ relates to the diffusion length of the ions and grows with the size of pore spaces. The normalized chargeability is related to the surface conductivity of the grains, and reduces the effect from a changing pore fluid resistivity when compared to the chargeability itself. As clay minerals have a higher surface conductivity as sand grains, this can be used as a lithological discriminator, at least in uncontaminated field sites.

37 DEPTH OF INVESTIGATION Depth of investigation (DOI) is a useful tool for evaluation of inversion results and holds useful information when a geological interpretation is made. A calculation of the DOI is crucial for interpreting the geophysical models, as the validity of the model varies considerably with data noise and parameter distribution. Without the DOI estimate, it is difficult to judge when the information in the model is datadriven or is strongly dependent on the constraints and/or on the starting value. The method is based on an approximated covariance analysis applied to the model output from the inversion while considering the data standard deviations. Furthermore, the cross-correlations between intrinsic parameters are taken into account in the computations, which is crucial when strong cross-correlations are expected. The details of the method can be found in (Christiansen and Auken, 2012, Fiandaca et al., 2015). The DOI approach presented here is based only on part of the Jacobian referring to the observed data. Hence, plotting the DOI on top of a section will allow discrimination of the data-driven parts, and the constraints and a priori driven parts of the model. This information can assist to quickly evaluate the results and estimate the validity of the results.

38 RESULTS The inversions result for each Cole-Cole parameter are shown (Figure 12-13) for the combined datasets. Figure 12 show the inversion results using data from all boreholes, while Figure 13 show the inversion results using only the outermost boreholes. DOI is shown with a white line below which model sections are faded with colors. The table below shows the inversion settings used for inversion. The profile is subdivided into two different model groups with different constraints settings for layers : models containing boreholes, 2: models not containing boreholes. Layers share the same constraints for all models, and is shown below as setting 3. Value Software AarhusInv 7.12 Starting model Constraint settings 1 Constraint settings 2 Constraint settings 3 Number of layers Resistivities (all layers) [Ωm] Chargeabilities (all layers) [mv/v] Tau (all layers) [s] C (all layers) [-] Thickness of layers 1-50 [m] Depth to layer 51 [m] Depth to last layer [m] Thickness distribution of layers Vertical constraints (Cole-Cole parameters) [factor] Horizontal constraints (Cole-Cole parameters) [factor] Vertical constraints (Cole-Cole parameters) [factor] Horizontal constraints (Cole-Cole parameters) [factor] Vertical constraints (Cole-Cole parameters) [factor] Horizontal constraints (Cole-Cole parameters) [factor] 60 From surface data inversion From surface data inversion From surface data inversion From surface data inversion Log. increasing with depth Table 5. Inversion settings Because of the conductive anomaly near the boreholes induced by the saltwater injection, the model cells at the boreholes have been horizontally decoupled from the rest of the models. Still, these cells have very low resistivities and to get a more readable image the models containing boreholes have been blanked out. For resistivity, 1D DC inversions of borehole data before salt water injection have been plotted instead. The test site was excavated and geological profiles were obtained. From these, a geological profile along the DCIP profile was constructed. This model is seen in Figure 14. Figures 15 and 16 show the inversion results with the geological model on top.

39 The inversion results including all boreholes show that the sand layer present around 2 meters is precisely determined. It is clearly visible in the resistivity, m0, tau and normalised chargeability sections, and also to some extent visible in the C section. The bottom of the moraine clay is also clearly defined in the resistivity, normalised chargeability and tau sections while lesser determined in the C and m0 sections. Some resistivity variation is seen in the moraine clay which is corresponding to a part with large stones and boulders present in the moraine between 4-6 meters depth. The inversion results using only boreholes at 6.35 m and m also show the sand layer around 2 meters depth, most notably in the resistivity section, but also in the m0 and normalised chargeability sections. The resolution is decreased compared to the inversions using all three boreholes and the bottom sand layer is present in the resistivity section.

40 Figure 12: Inversion result. Models containing boreholes greyed out. 1D DC inversion of borehole data before saltwater injection is plotted in resistivity section.

41 Figure 13: Inversion result using only data from boreholes 0635 and Models containing boreholes greyed out. 1D DC inversion of borehole data before saltwater injection plotted in resistivity section.

42 Figure 14: Geologic profile showing the truth from the excavation. Yellow: sand, orange: gravel, brown: clayey moraine. Coordinates are identical to the DCIP profile. Dashed green lines indicated where the excavations lines are.

43 Figure 15: Inversion result with geologic truth. Models containing boreholes greyed out. 1D DC inversion of borehole data before saltwater injection plotted in resistivity section.

44 Figure 16: Inversion result using data from boreholes 0635 and 1720, and geologic truth. Models containing boreholes greyed out. 1D DC inversion of borehole data before saltwater injection plotted in resistivity section.

45 CONCLUSION We have shown that cross-hole DCIP measurement may be used to accurately determine the presence and structure of sand layers in a clayey moraine. The main challenge was to get a good contact between the borehole electrodes and borehole wall. This is a general problem in the vadose zone and needs to be addressed. The saltwater used in this project induced a conductive anomaly near the boreholes, which proved a bit problematic. Preliminary research into a gel-based solution looks promising but more research is necessary. For a more sustainable system, a reusable borehole electrode setup is required. This may be carried out by placing the electrodes on a towing rope of sorts, which can be pulled out of the ground. Alternatively the electrodes can be left in the boreholes if a monitoring setup is wanted. The maximum distance between boreholes to accurately determine the subsurface structures depends on the expected variations in the geology and the field conditions, but more than 10 m is probably not advisable. We believe this methodology has a great potential and would be a good choice for precisely mapping sand lenses at contaminated sites. Though, we have yet to investigate the lower limit for detection of sand lenses with this method, the synthetic model test show that methodology has the possibility of mapping really thin sand lenses between boreholes. The methodology also has the possibility to determine sand lenses outside of the area defined by the boreholes.

46 REFERENCES Auken, E., Christiansen, A. V., Kirkegaard, C., Fiandaca, G., Schamper, C., Behroozmand, A. A., Binley, A., Nielsen, E., Effersø, F. and Christensen, N. B. (2014) 'An overview of a highly versatile forward and stable inverse algorithm for airborne, groundbased and borehole electromagnetic and electric data', Exploration Geophysics, 46(3), pp Christensen, N. B. (2008) 'Environmental applications of geoelectrical methods. Lecture notes'. Christiansen, A. V. and Auken, E. (2012) 'A global measure for depth of investigation', Geophysics, 77(4), pp. WB171-WB177. Fiandaca, G., Christiansen, A. and Auken, E. 'Depth of Investigation for Multi-parameters Inversions'. Near Surface Geoscience st European Meeting of Environmental and Engineering Geophysics. Nielsen, E. (2007) Cylindersymmetrisk DC-Respons. Master Degree, Aarhus University. Olsson, P.-I., Dahlin, T., Fiandaca, G. and Auken, E. (2015) 'Measuring time-domain spectral induced polarization in the on-time: decreasing acquisition time and increasing signal-to-noise ratio', Journal of Applied Geophysics, 123, pp Schön, J. H. (2015) Physical properties of rocks: fundamentals and principles of petrophysics. Elsevier. Slater, L. D. and Lesmes, D. (2002) 'IP interpretation in environmental investigations', Geophysics, 67(1), pp Sorensen, K. 'The Ellog auger drilling method'. Proceedings of the symposium on the application of geophysics to engineering and environmental problems, Boston, Massachusetts.-Environmental and Engineering Geophysical Society,

47 * 1 ) /!

48 0 Crosshole S wave seismic experiment Testing the method in shallow boreholes Egon Nørmark, Department of Geoscience, Aarhus University Giulio Vignoli, GEUS

49 1 Introduction The present project is motivated by numerous of cases in Region Hovedstaden where pollution has been detected in places with limited extent. The geological setting is here often a clayey till with sand lenses being embedded. When a site is contaminated, the sand lenses tend to act as an aquifer for the pollution. Even sand lenses as thin as 10 cm may be a problem in such cases. Normally, a significant number of wells are made to map the geological setting and the pollution. The present project has been initiated to reduce the number of boreholes needed for mapping the polluted site. The main purpose of the project is to test if thin sand lenses can be detected in such an environment by using geophysical methods. The geophysical methods are applied in shallow boreholes combined with surface measurements. It was decided to carry out the test over an area with the extent of 10 m x 10 m. The maximum depth to be tested was set to 10 m. The test was carried out as a controlled experiment where the site was excavated afterwards. Under this process a detailed geological description was made. Three geophysical methods have been applied in the present project: Georadar, DCIP (DC resistivity measurements combined with induced polarization) and Crosshole S wave seismics. Only the results of crosshole S wave seismic experiment will be described here. The test site It was decided to select a test site next to an open sand/gravel pit near Kallerup not far away from Hedehusene on Seeland, Denmark. Using a pit for the test made it possible to find a site with the desired geological properties. A place where the sand lenses had a thickness between 10 cm and 50 cm was chosen. In Fig 1 and 2 the location of test site is shown.

50 2 Fig. 1 Position of test site. Google Earth Fig. 2 The test site is marked by a red symbol.

51 3 Geological Setting After making the measurements the excavation of the pit continued and the geological structures were exposed. During this process a geological description of the site was made. See examples on the geological outline in Fig In Fig. 7 a cross section from the southern end of the site is shown, which represents the overall geological setting quite well. Fig. 3 Geological setting of the volume being excavated. Fig. 4 Photo from the excavation. Profiles shown in Fig. 5 6 are indicated.

52 4 Profile C Fig. 5 Profile C Profile D Fig. 6 Profile D

53 5 Fig. 7. Cross section from the test site representing the overall geological setting quite well. Defining the crosshole seismic experiment When carrying out the S wave seismic data acquisition, it was planned to make as many sets of borehole measurements as possible. One set of borehole measurements is defined as all measurements made between each source and receiver well. A set of measurements involve all combinations of source depths, source orientations and receiver depths. Since only two borehole geophones were available, a set of measurements requires many individual records. Therefore, it was clear from the start that all combinations boreholes sets could not be acquired. Initially, we aimed at 3 or 4 sets of measurements. The crosshole measurements were made by transmitting S waves from a source borehole to a receiver borehole. In Fig. 8 a sketch of the seismic measurements is shown. The plane spanned by the boreholes will in this context named as the crosshole profile.

54 m Fig. 8. Sketch of the borehole experiments. (from Geotomographie manual). The optimum distance between boreholes was not clear when planning the experiment. The borehole equipment had been acquired for the present project, so our experience with the equipment was limited. Moreover, there was no option for testing the equipment on a similar site. This would require new boreholes with a similar geological setting and with a suitable distance between the boreholes. Still, the borehole source was tested in a shallow well near Horn outside of Aarhus. Data were recorded by 3 component surface geophones. The test was carried mainly to verify that the borehole source was working correctly. Based on scattered information about the borehole equipment, it was estimated that 5 10 m would be the optimum distance to use between the boreholes. Thus, four boreholes were placed so that they formed the corners of a rectangle with side length of 5 and 10 m. The position of the boreholes will be described in the following section. The boreholes for the S wave experiment were cased with PVC. They were originally 10.0 m deep, but when the measurements were carried out the maximum depth was actually m. In order to improve the acoustic coupling to the formation, the well has been backfilled with bentonite, which should give good acoustic contact to the formation. Boreholes for S wave seismic measurements The relative position of boreholes for the seismic experiment and the crosshole profiles are shown in Fig. 9. Positions of boreholes are given as UTM coordinates (Euref89 Zone 33). Coordinates have been read from google kml map provided by Orbicon. This reading might not be completely accurate. Unfortunately, two names for the boreholes have been used. Names starting with ST were used when making the boreholes

55 7 and the names starting with B were used the field. The original ST names are used in the present report. Both set of names are listed in Fig. 9. Fig. 9 The relative position of boreholes and borehole profiles covered with seismic crosshole data. Positions of boreholes for seismic tomography are shown to the right in (UTM Zone 33 Euref89). The crosshole profiles are named STsr, where s and r are respectively the source and receiver borehole numbers. Thus, the two profiles covered with seismic data are ST32 and ST34. The profile ST34 has a length of 4.40 m and ST32 of 12.15m. The distances between the boreholes have been measured in the field. Numerous of other shallow boreholes were present when making the seismic experiment. These boreholes were used for the other geophysical experiments. It might be a concern that other boreholes could disturb the present measurements. However, this is not expected to be the case, since the other wells were scattered all over the test site and the boreholes had a limited borehole diameter (typically mm) with a casing consisting of PVC or a similar material. When making the S wave measurements, the pit was approximately m away from the test site. However, there were no steep walls leading into the pit which might have generated undesired side reflections. A gentle slope was present into the pit.

56 8 Borehole Equipment The equipment for the S wave seismic experiment consisted of a borehole source and a power supply for it. A borehole receiver unit with two 3 component geophones and eight 3 component surface geophones was used for the data acquisition. Borehole Source The borehole source was a BIS SH delivered by Geotomographie (Fig. 10). Fig. 10 Borehole Source BIS SH The borehole source must be clamped to the casing in borehole. This is done by inflecting an air bladder. Inflection of it is done by using a simple manual pump. The orientation of the source was relatively easy to control since the hose covering the high voltage power cables was very stiff. Even at maximum depth (8 m depth in the present case) the orientation of device could be handled quite easily. Thus, proper source orientation could be ensured before clamping the borehole source. A power supply from Applied Acoustics (CSP D2400), normally used for marine sparker data acquisition, has been applied for activating the borehole source (Fig. 11). Maximum source level on the power supply was 2400 Joule, but in the present survey only 1000 Joule was used. This might be too high energy level in the present application. Applying higher energy than needed is not expected to have caused negative effects, but it has not given any additional seismic source energy either. Before using the power supply a hardware modification was made, so that the power supply did turn into idle mode after each shot. If it happens it needs to be restarted again. Normally, this will take place after 5 sec for safety reasons.

57 9 Fig. 11. Power supply used for the borehole source. Borehole Receiver The borehole receiver (BKG3) is also from Geotomographie. It consists of two 3 component geophones. The distance between the receiver units is 2.0 m apart (Fig. 12). The orientation of the borehole receiver was not as easy to control as for the borehole source. A relatively soft cable holds the equipment which can easily rotate in the borehole. However, the orientation of the device is monitored by a fluxgate compass. It was attempted to keep the same orientation for all measurements in the receiver well. This was accomplished by rotating the device gently in the hole and clamping it to the borehole when the compass returned the desired orientation. In practice it was not possible to have a precision better than 15 degree.

58 10 Fig. 12 a) Recording equipment consists of two 3 component geophones. There is a distance of 2.0 m in between them. An air bladder is mounted on the side of each geophone so that they can be clamped to side of the borehole. b) The geophones after the removal of the bladder and the extension part cover. Surface geophones Eight 3 component (3C) 10 Hz surface geophones (Fig. 13) were also used for the last crosshole profile ST32 (B14). When acquiring data on the first crosshole profile ST34 (B12), no surface geophones were used. Initially, it was not expected that the surface geophones would give good data due to the very wet surface conditions. Moreover, moving the geophones to another position would be a challenge. Especially, connecting the geophones again after they have been submerged into mud would be a problem. Therefore, surface geophones were only used on the last crosshole profile. Fig. 13. A 3 component surface geophone.

59 11 Acquisition units The data acquisition was made with 2 Geodes from Geometrics (Fig. 14). Each Geode handles 24 channels. Borehole data were acquired by using one Geode, while the other was utilized for the surface recordings. Fig. 14 Geode from Geometrics The Geodes are connected to a server, a laptop: Dell Laptop XFR P21G. As software for the data acquisition, Geometrics Land Seismics was used. The equipment for the data acquisition was placed in the tent, mainly to protect the high voltage power supply from the rain. In Fig. 15 the server is shown. A single record from the data acquisition can be seen on the laptop screen. No editing and/or comparison of the measurements were practically possible in the field. Fig. 15 A Dell XFR laptop used as server for the two acquisition units. Data from a single shot in seen on screen.

60 12 Recording geometry Acquisition was made with two source directions. Both directions were perpendicular to the crosshole profile. In principle, these directions maximize the S wave energy and make minimum the P wave contribution. The two opposite source directions were expected to make the identification of S waves easier because of their different polarity. Moreover, changing polarity gives the possibility of suppressing noise via stacking of additional data after changing the polarity of one of the dataset. Source well Displacement Wave propagation Receiver well Fig. 16 Sketch of source directions being used relative to the source and the receiver well. View from above. The direction of for the S wave displacements of and direction of wave propagation is shown. Since only two receiver units were available, many shots were necessary to complete one set of borehole measurements with a sufficient of density of observations. The goal was to acquire as many different wave directions as possible, which, in theory, would give the best chances to resolve horizontal velocity variations. Another issue is spatial sampling interval. A suitable compromise between acquisition time and spatial data coverage was decided to be a source spacing of 0.5 m and receiver spacing of 1.0 m. Thus, three/four receiver positions would be sufficient for one set of borehole measurements. At each receiver position acquisition at all source positions and directions were made. In the Fig. 17 a sketch of the resulting raypaths for the present source and receiver configuration is shown (assuming a homogeneous velocity distribution).

61 m Source well Receiver well Fig. 17. Sketch of the resulting raypath geometry for a constant velocity field. Field Work The acquisition of the crosshole S wave data toke place Nov 30 and Dec 1, The field work was carried out by Per Trinhammer (Department of Geoscience, Aarhus University), Giulio Vignoli (GEUS) and Egon Nørmark (Department of Geoscience, Aarhus University). Over the two days of field work, two crosshole profiles were covered. The same source well was used: ST3. Measurement in two receiver wells, ST2 and ST4, were collected. 10 records were made for each source and receiver configuration. The shots were triggered shortly one after another. Thus, it did not require much extra time to get 10 records compared to just making a single measurement. Most of the acquisition time was actually spent on rearranging the source and receiver devices. In the beginning of the survey, heavy machinery (a caterpillar) was working in the pit right next to the survey site, which was a serious concern as it could be a significant source of noise. When making 10 records at each position it would give an opportunity to select data with the lowest noise level. However, based on our previous experiences working with reflection seismic data in places with such noisy machinery it was clear that it might bury the seismic signals in noise. Still, it was not clear how far away the machinery should be in order to state that noise from it would not be a problem. Thus to make sure that the noise source would not jeopardize the survey it was arranged so that they stopped working with the equipment. However, some of the data from the first crosshole profile (ST34) were already acquired with the machinery in action. For the consequences see the section about seismic processing.

62 14 Due to heavy rain in the weeks before the survey, the condition at the site was extremely wet and muddy, which made it very difficult to handle the equipment (Fig. 18). However, during the S wave experiment, the weather was relatively good with no rain of importance and only moderate wind. Fig. 18 Photos from the field work. Unfortunately, the survey had to be stopped before completing all measurements due to technical problems. The reason was that the borehole source started to backfire, meaning all the power was not only released in the source device but also in the power supply itself. Eventually, it led to a hardware failure which could not be repaired in the field and the survey had to be terminated. Since excavation of the site for the geological description would take place shortly after the completion of the S wave experiment, there was no time for repairing the equipment and continuing the measurements.

63 15 Acquisition procedure The procedure used for the data acquisition consisted in placing the receiver cable at a starting position with a well defined source direction. At each acquisition step, the source was lowered into the hole by 0.5 m. For each source depth, 10 shots were fired. Then, at the maximum source depth, the source direction was inverted and the procedure was repeated, now moving the source upward. After the downhole and an uphole series were completed, the receiver cable was moved to the subsequent position and a new downhole and uphole series could start. This was done until all the desired source receiver configurations (including changes in the source direction) were systematically covered. In total 2141 shots were acquired for the two sets of borehole observations (ST34 and ST32). Acquisition parameters Acquisition format Seg D Sample interval 62.5 microsec Number samples 8000 Record length 500 ms Number of active channels 6+24 channels Acquisition filter Antialias Problems during the data acquisition A potential problem when acquiring data was that the depth indicator started to slip along the cable. This was mainly a problem at the end of the survey. This added uncertainty to the receiver position measurements. In the field, it is estimated that for a few measurements, the depth indicator could have moved up to m. Later, during the processing, data were inspected for such errors. This was done by comparing traveltimes for reversed source directions. The errors being detected were so few and so small that they are considered to be insignificant for the overall results. No further action has been taken to correct the errors. As mentioned in the section describing the equipment there was also a major problem with power supply at the end of survey. This error could not be corrected in the field and the survey had to be terminated.

64 16 Processing borehole data Geometry One of the first steps in the processing was to define the geometry for the data being acquired. That is receiver depth, source depth and source directions were defined as header variables for all traces. Timing of the shots It turned out that timing of the shots were inconsistent. That is the time for triggering the recording instruments was not completely accurate and variable time delays were introduced on the data. An example showing the variable time delays is given in Fig. 19. The data does not represent seismic signals but crosstalk from the high voltage signal and the source device onto the receiver cable. Thus the present signals mark the shot time and is detected approximately 1 ms after triggering the system. This may be ms before the seismic signal arrivals. The variable shot delays were picked in each shot gather and a static correction has been applied to all traces in the gather. Fig. 19 Variable time delays were present on the shots. Crosstalk signals from the source device mark the actual shot time. The time for it is picked (marked by a red symbol) and applied as a static correction on the data.

65 17 Trace Editing Trace editing data has been performed on all data. If data were considered to be of degraded quality they were deleted. With 10 records for each source and receiver configuration, it is acceptable to delete quite a lot of data without leaving holes in stacked data. Especially, on the first dataset (ST34), where heavy machinery was working next to the pit, a significant number of records were deleted. Noisy data from this set of borehole measurements are shown in Fig. 20. Thus, only the records with a relatively low noise level were kept for further processing. Fig. 20 Raw data. The records to the right are affected by noise from the heavy machinery. Bandpass filter Preferably no bandpass filter should be applied to the seismic data when picking arrival time. However, especially low frequent noise may affect the appearance of the data and may affect a consistent picking of the traveltimes. Thus, a gentle low cut filter has been applied on all data. Early in the processing a Hz Ormsby low cut filter was applied. Hence, all data below 50 Hz are removed, while all data above 70 Hz are kept. Between 50 Hz and 70 Hz, a taper zone was defined. Later in the processing, a similar Hz low cut filter was applied. On most records, the filter did not change the appearance of the data significantly, but on some data it had the desired effect in suppressing low frequent noise.

66 18 Reorganizing borehole data On the raw shot records, there was difficult to see repeatability in the data and there was no option for inspecting how well data where correlating. See the example in Fig. 21. Fig. 21 Raw borehole data. Each record consists of data from the two 3 component borehole geophones. Data show repeated records at three different source depths. (For Source depth see the annotations above the traces). When selecting the horizontal component perpendicular to the crosshole profile data appear as in Fig. 22. This component is supposed to give maximum amplitude of the transmitted S wave. Data from borehole profile ST34 and from receiver depth 4.0 m is shown. It is observed that the repeatability of the signal is very high, which confirms that valid data have been acquired.

67 19 Fig. 22 Raw data ST34. Data of the horizontal component, perpendicular to the profile, acquired at 4.0 m receiver depth and with the same source direction. Each group of traces has the same source depth. Source direction Data has been acquired in two opposite source direction perpendicular to the crosshole profile. Opposite source directions should give reversed polarity of the S wave signals. In Fig. 23, data from opposite source direction are shown for three different source depths. Data from crosshole profile ST32 are shown. The reversed polarity is very clear also when inspecting the details in the data.

68 20 Fig. 23 Data showing the effect of source direction. Blue: + 90 deg. Red 90 deg. Receiver depth 4.0 m. Source depth m. Each group of traces is acquired at the same depth and direction. Stacking After the initial processing has been performed, traces with same receiver component, source depth, source direction and receiver depth was stacked. This made a significant reduction in the amount of data. Further stacking has also been attempted by reversing the polarity for one source directions and adding it to the stack of data from the other source direction. This will give an additional improvement in the signal to noise ratio. However, in practice the effect was very limited. Thus most of the data shown here originates from one of the source directions. See appendix for borehole data. Data are shown without automatic gain. However, scaling factors which vary from trace to trace have been applied. Spectrum of stacked data Spectrum of stacked data from the two borehole profiles and the surface recordings are shown in Fig. 24. Spectrum for the borehole data is made for recordings at receiver depth of 3.0 and 4.0 m. For the surface data, the horizontal distance from the source well is 4.0 m. A Hz low cut filter was applied before calculating the spectrum. Peak frequency for borehole data is approximately 400 Hz. Assuming a velocity of 400 m/s the dominating wavelength is 1.0 m. When comparing the dominating wavelength with the goal of achieving a resolution of

69 m this is indeed a challenge. In order to achieve this resolution one would prefer significantly higher frequencies. However, in the present environment this was not possible. Fig. 24a Amplitude spectrum of borehole ST34 (B12) data acquired at a receiver depth of 4.0 m Fig. 24b Amplitude Spectrum for ST32 (B14). Depth borehole record: 3.0 m Fig. 24c Amplitude spectrum of surface observation from profile ST32 (B14). Horizontal distance from source well: 4.0 m A lower peak frequency at about 200 Hz is observed on the surface observations presumably related to higher absorption close to the surface.

70 22 Polarity of data In order to make a joint inversion of borehole data and surface observations a consistent receiver direction must be used and data must be shown in the same polarity. Here the following considerations are made. The direction H1 for the borehole geophones (channel 1 and 4) was perpendicular to the borehole profiles. The similar direction on surface geophones is named H2 on the surface geophones (channel 27,30,33 ). Unfortunately, H1 on the borehole geophones and H2 on the surface geophones are oriented in opposite directions. However, a test of the polarity of the borehole geophone and surface geophone revealed that the surface geophones also have reversed polarity compared the borehole geophones. Thus, no further action was needed in order to pick the corresponding phases on borehole data and on the surface recordings. The three component signal Unexpectedly, the most clear three component data was acquired on the surface geophones. Thus, surface data will be used for demonstrating the relative amplitude of the seismic source. Data are acquired in a horizontal distance of from the borehole. See in Fig. 25. Constant scaling has been applied on all data. On the vertical component, both P wave and the S wave are visible. The P wave arrives at ms on the present data, whereas the S wave arrives at ms. The S waves have as expected higher amplitude than P waves. Moreover, the S waves registered on the horizontal component perpendicular to borehole profile have also higher amplitude than on the inline horizontal component.

71 23 Fig 25a Vertical component (Geo_comp=1) Fig 25b Horizontal component parallel to borehole profile. (Geo_comp=2) Fig 25c Horizontal component perpendicular to borehole profile. (Geo_comp=3)

72 24 A hodogram of the horizontal components is shown in Fig. 26 at source depths: 5.0 m, 5.5 m and 6.0 m. Time window for the analysis is ms. Clearly, the direction H2 component perpendicular to borehole profile has highest amplitude and displacements are well aligned in this direction. Thus, when reading the S wave traveltimes on surface data it can be justified that the H2 component must be used. V H1 H2 V H1 H2 V H1 H2 H2 H1 H1 H1 Fig 26 a) Three components surface observations. Source depths: 5.0 m, 5.5 m and 6.0 m. H1 is parallel to borehole profile, and H2 perpendicular to borehole profile. b) Hodogram of the horizontal components. Since the components are well aligned with the borehole profile there is no point in rotating the data from the horizontal component. On the surface recorded data it is obvious that the component perpendicular to the borehole profile is most well suited for reading the traveltime observations. On the borehole data the same clear polarization has not been observed. As mentioned earlier the orientation of the borehole data was not as well defined as for the surface data. However, this is not the only explanation for the rather poor polarization of horizontal data component. Reorientation of the horizontal component did not show significant improvement. The reason is most likely that a higher noise level. Another reason might be the effect from the borehole itself.

73 25 On the borehole data, the traveltime observations have also been read on the horizontal component perpendicular to the borehole profile (Chan 1 and 4). Generally, this component carry the most clear S wave signal (Fig. 25c) but the difference in amplitude between the horizontal components is not as pronounced as on the surface based data. Interpretation of the data When picking traveltimes it is generally best to pick the onset of the arriving signal. However, on the present data this is not possible to do it in a consistent way due to a relatively high noise level on some of the data. In order to achieve a consisting traveltime picking the phase with maximum positive amplitude has been used instead. Traveltime inversion method Traveltime inversion is carried out in order to estimate slowness field (reciprocal of velocities). The inversion is carried out as iterative linearized inversion reducing the difference between traveltime model responses and the traveltime observations. When estimating the raypath, partial derivatives of traveltimes with respect to slowness, is estimated as well (Sun, 1991). Conjugate gradient is used as optimization method with some modification in step length in order to reduce the maximum change in the slowness field. A multigrid procedure is applied. That is first optimization is carried out in a coarse model and after some iterations (5 10) the model is transferred to a finer model with more and the optimization procedure is repeated. In the present case the starting model it is a constant velocity field and optimization is initiated with just a few layers. Subsequently, more layers are introduced and during the inversion horizontal variations are allowed as well. (Nemeth et al., 1997). Synthetic inversion examples In order to test the performance of the traveltime inversion procedure a synthetic example has been constructed. The dataset is based on the setup used for the real borehole profile ST32. The test is supposed to demonstrate to what extent velocity anomalies can be detected under optimal (noise free) conditions. The model (Fig. 27) was constructed in a very simple way. It consists of background velocity field with a constant velocity gradient. The background velocity field ranges from a velocity of 250 m/s at the surface to a velocity of 400 m/s at the bottom (at 8.0 m depth). Two velocity anomalies have been introduced,

74 26 forming two dykes with velocities deviating 40 m/s from the background velocity field. The dykes are introduced as diagonal and horizontal anomalies. Tests with both positive and negative anomaly have been performed (for example, Fig. 27a, b). In addition, two versions with different dyke thicknesses has been created: one with thicknesses approximately equal to 1.0 m (Fig. 27 b, d) and another where they are around 1.75 m thick (Fig. 27a, c). These reconstructions are significantly less challenging than the goal of the present experiment aiming at a detection level of ~0.1 m. The reason why not attempting to make the structures smaller in the present traveltime inversion is that it might easily violate the assumption made in the raypath theory. Raypath modelling is a high frequency approximation, which implies that the structures must have a significant size compared to wavelength being used. Exactly, where the limit is hard to say. However, when making the structures smaller the actual effect from an anomaly will be reduced and the anomaly may become invisible, which may not be revealed in the raypath modelling. The spectral analysis suggested that the dominating wavelength for the borehole observations was approximately 1.0 m. Under these circumstances it will be too optimistic to resolve structures as thin as 10 cm. Therefore, only relatively large structures have been tested in this context. The model with the velocity field is made up by cells with a cell size of 0.25 m vertically and m horizontally. Each cell has a constant velocity.

75 m/s 400 Fig. 27 Synthetic velocity fields used to test the performance of the traveltime inversion procedure.

76 28 Performance of the inversion In Fig. 29, all traveltime observations and model responses are shown for the model with a thickness of the anomaly of 1.75 m. The observations originate from the model with a positive velocity anomaly compared to background velocity field. In Fig. 29, observations from a single receiver depth (1.0 m) are shown. It can be seen that a relatively good fit between observations and model responses is obtained. The velocity anomalies are resolved reasonably well by traveltime inversion (Fig. 28). Both the thin and the thick layer anomaly for both positive and negative velocity contrasts are detected. It is observed that positive velocity anomaly is resolved slightly better than the negative anomaly. This is quite natural since positive velocity anomalies so to say attract the seismic waves opposed to negative velocity anomalies, where the seismic waves tends to deviate from them. It can be also observed that, for thin layer anomaly, only a few observations (rays) define the anomaly with the present geometry. This would make the interpretation, if this were a real dataset, very sensitive to errors in the observations. Thus, it can be expected that, in case of noise contaminated data, it would be difficult to resolve such features with the experimental geometry, especially if layers were thinner than in the present case. Additionally, there might be problems getting sufficient energy through such layers as discussed in the previous paragraph. Generally, inversion for many parameters (cells) is performed compared to the number of observations. Thus, it estimated that it is hard to achieve a better performance of the traveltime optimization unless the solution is constrained to a desired type of solution.

77 Fig. 28 Result of the traveltime inversion. Should be compared with true model shown in Fig. 27. m/s 400

78 30 Fig. 29 All traveltime observations (circles) and model responses (crosses) for the model with layer thickness of 1.75 m and positive velocity anomaly. Fig. 30 Traveltime observations (circles) and model responses (crosses) as shown in Fig. 29, but only for single receiver depth: 1.0 m.

79 31 Traveltime inversion of crosshole data from profile ST34 Data used for picking traveltimes on crosshole profile ST34 is shown in Fig Data represents the horizontal component perpendicular to the crosshole profile. The seismic data are shown in two scaling modes: A scaling mode which varies from trace to trace (Fig. 31) and a scaling mode which is constant for all traces (Fig. 32). The latter makes it possible to inspect the relative amplitude variations. Significant amplitude variations are observed. Especially, data acquired near the surface shows relatively low amplitude. This is most likely caused by unconsolidated sediments near the surface. Also at a receiver depth of 7.0 m relatively low amplitudes are present. In Fig. 33 the traveltimes used for the velocity estimation are plotted on the seismic data. Traces with very low amplitude have not been used for traveltime inversion. This is generally the case for the data acquired at a receiver depth of 1.0 m. Fig. 31 Data acquired on the horizontal component perpendicular to crosshole profile ST34. Data is organized by Receiver depth (DEPREC) Source depth (DEPSOU).

80 32 Fig. 32. Data as in Fig. 31 but shown with constant scaling for all traces. Fig. 33. Data as in Fig. 31. Observations used for traveltime inversion are marked as red points and lines.

81 33 Traveltime inversion has been carried out using the inversion procedure described previously. Model responses are given in Fig. 34, imposed on the seismic data. It is observed that the model responses compare reasonably well to the traveltime observations. Both observation and model responses for all receiver depths are given in Fig. 35 and the resulting velocity model is shown in Fig. 36. The velocity model shows a general increase in velocity by depth. This can most likely be referred to consolidation and compaction of sediments by depth. For comparison the geological setting at the source and the receiver position is given in Fig. 37. The geological description is made by Knud Klint, GEUS. The geological setting at the two places is quite similar except for the fact the depth to the top of the sand layer is 6.0 m at ST3 and 7.0 m at ST4. The sand layer continues downward to much greater depth but is also confined by a thin till layer at approximately 8.0 m depth. At approximately 2.0 m depth, a 1.0 m thick layer described as gravel is reported. One would expect lower velocity here compared to the surrounding till. However, in the resulting velocity model this layer of gravel has not been detected by seismic tomography. Still, an anomaly in the traveltimes (a delay less than 1.0 ms) have been observed, most clearly seen at a receiver depths between 2.0 m and 5.0 m (Fig. 34). Such an anomaly might have been generated by the presence of sand or gravel, but is here observed slightly too deep to match the gravel layer mentioned above. Actually, the anomaly has been modelled as lateral variations in the resulting velocity model. At the very bottom of the model the sand layer seems to be recognized with lower velocities compared to the velocities in the surroundings till. These differences in the velocities for sand and till have also been observed for shallow VSP measurements (Jørgensen et al, 2003) although these observations are based on P wave velocities. The top of the sand is present at greater depth in the receiver well compared to the source well (6.0 m in ST3 and 7.0 m in ST4). This depth variation is also recognized in the velocity model. The presence of sand has most likely also introduced the amplitude reduction, which is observed at a receiver depth of 7.0 m (Fig. 32). Low velocity together with high absorption is common feature for unconsolidated sediments. The lowermost observations at a receiver depth of 8.0 m and at source depth of m show again significantly higher amplitudes, which might by related to the thin layer of till.

82 34 Fig. 34. Model responses shown on the observed seismic crosshole traces. Fig. 35 Comparison between traveltime observations and model responses

83 35 ST3 ST4 250 m/s m 8.0 m Fig. 36. Velocity model. Depth to sandlayer: 6.0 m 7.0 m Fig. 37. Geological setting at source and receiver wells.

84 36 Traveltime inversion of crosshole data from profile ST32 Seismic observations for traveltime inversion along profile ST32 is shown in Fig Fig. 38 and 39 show the borehole observations and Fig. 40 and 41 the surface observations. The horizontal component perpendicular to profile is given. Data is also here viewed in two scaling modes: Individual scaling on traces and constant scaling on all traces. The latter allow inspection of amplitude variations from trace to trace. Data shown with constant scaling for all traces (Fig. 39) reveals significant variations in the amplitudes. Reduced amplitudes at shallow receiver depth are, like for the other profile, probably related to unconsolidated for the near surface sediments. Fig. 38 Borehole observations along ST32 (B14). A trace scaling which vary from trace to trace has been applied. Depth of the receiver (DEPREC) and the depth of the source (DEPSOU) are annotated above the seismic traces. Fig. 39 Borehole observations along ST32 (B14). Constant scaling on all traces has been applied. DEPREC: receiver depth. DEPSOU: source depth.

85 37 Data for receiver depth of 6.0 m also shows a significant reduction in amplitude level for most source depth. Data acquired right below at 7.0 m has, on the other hand, significantly higher amplitudes. These variations might reflect waves trapped in a layer. The surface based observations used for traveltime inversion is given in Fig. 40 and Fig. 41. Again, the data are shown in two different scaling modes. Distance from source borehole: (m) Fig. 40 Surface observations (individual trace scaling). Depth of the source (DEPSOU) is annotated. Distance from source borehole: (m) Fig. 41 Surface Observations. (Constant trace scaling). DEPSOU: source depth

86 38 The picked traveltime arrivals used for the inversion are shown in Fig. 42 and Fig. 43 imposed on the seismic data. Observations made for shallow sources and acquired near the source show some deviations from data observed at greater source depth, in particular, close to the source borehole. Such observations are expected to be influenced by surface waves and have not been included in the traveltime inversion. Fig. 42. Borehole Observations. DEPREC: receiver depth. DEPSOU: source depth. Distance from source borehole: (m) Fig. 43 Surface observations. DEPSOU: source depth.

87 39 Fig. 44 Borehole observations with model responses from the traveltime inversion are imposed. Distance from source borehole: (m) Fig. 45 Model responses on surface observations.

88 40 ST3 ST2 250 m/s m Fig. 46 Velocity model estimated by traveltime inversion. 7.0 m The model responses are imposed on the well observations and the surface observations, in Fig. 44 and Fig. 45, respectively. The estimated velocity model is given in Fig. 46. Again, compaction by depth is reflected in the velocities. There seems to be some inconsistencies between observations made on the surface and in the borehole, maybe because the same phase has not been picked on surface observations and on well observations. A joint inversion seems to be difficult and it is probably the reason why relatively curved raypaths are introduced. The fact that rays concentrate in in a narrow interval between the two boreholes is probably not correct, but is just an example one of the many possible models equally compatible with the observations (i.e.: one of the infinite models fitting the data within the noise level). There is not much control on the lateral variations in velocity model. If better lateral control on the velocities should be achieved, observations with swapped source and receiver configuration, combined with surface observations along the entire profile, would have been beneficial. It was not possible to include data acquired at the receiver depth of 6.0 and 7.0 m in a joint traveltime inversion. In Fig. 47, the phases which would be natural to include are shown. Significant differences in the traveltimes compared to the other traveltime observations (Fig. 44) are evident. The reason for this is probably that data recorded at 6.0 and 7.0 m represent another event, possibly a guided wave in a medium with considerably lower velocity. The traveltimes for those observations suggest velocities of about 400 m/s, which would not be in contradiction with the velocities estimated in the sand layer along the profile ST34. Also reduced amplitude is observed for those observations, but here even more pronounced than on ST34. The reason is most likely, again, the higher attenuation in the sand compared to the till. The sand layer is reported in geological setting shown in Fig. 48. When amplitude reduction is even more

89 41 pronounced along the present profile, it is caused by the fact the distance between the source and the receiver well is 3 times bigger here than on ST34. On the present data, a high amplitude response is observed at maximum source depth, and most pronounced for the maximum a receiver depth (Fig. 38). A similar high amplitude response is also observed on profile ST34. It would be quite natural if it had the same source. On profile ST34, it was suggested that it represented a thin layer of till. It would fit well into the interpretation, if it also represented the till layer along the present profile. However, on the borehole seismic data this high amplitude event is observed at a depth, which is a bit too shallow compared to the geological description of the till. However, the interpretation of this feature might be a bit suspicious, since it is only observed on single traces at the lowermost source point position. Therefore, one might wonder if it is the position of the source in the well that is responsible for the higher amplitude response rather than the geological conditions. The presence of gravel at about 2.0 m depth, which is indicated in geological setting, has not been detected in the velocity model. Fig. 47 Observations at maximum receiver depth.

90 42 Depth to sandlayer: 6.5 m 6.0 m Fig. 48 Geological setting along profile ST32

91 43 Conclusion The present project has demonstrated that the S wave seismic data can be generated in boreholes and that S waves can be transmitted between boreholes. It has also been demonstrated that the traveltime observations may be used for estimating S wave velocity field between the boreholes. Unexpectedly, surface observations of the source in the borehole signals gave surprisingly good data and have on one of the cross profiles been included in the traveltime inversion. However, it was not trivial to ensure that the exact same phases were picked on the two types of observations. In the resulting velocity model relatively poor lateral control on the velocities were recognized. This mainly applies to the profile with a relatively big distance between the source and receiver borehole (hence, characterized by lower angular coverage). The thin layer of sand/gravel reported in the geological description at about 2.0 m depth and announced as the main target for the survey, has not been recognized by S wave tomography. This should not be surprising when one compares the wavelength of the seismic signal to the layer thickness, especially not for a layer that is expected to have a lower velocity compared to the velocity of the surrounding till. (A low velocity layers tends to deviate the seismic waves into the medium with higher velocity). Moreover, a significant velocity contrast may not be present either. Still, it has been demonstrated that sand can be detected and recognized as a lower velocity structure compared to the velocity in the surrounding till. It just requires that sand has a significant layer thickness compared to the wavelength. Absorption might also be way to detect such layers. Especially for observations made over a significant distances (here more than 10 m), there was a substantial amplitude reduction related to the sand compared to the amplitude attenuation for waves travelling in the till.

92 44 Acknowledgement The present project was initiated and supported by Region Hovedstaden. Orbicon has been consultant and coordinator of the project. Referfences Jørgensen F., Lykke Andersen H., Sandersen P.B.E., Auken E., Nørmark E., 2003, Geophysical investigations of buried Quaternary valleys in Denmark: an integrated application of transient electromagnetic soundings, reflection seismic surveys and exploratory drillings, Journal of Applied Geophysics. Nemeth, T., Normark, E., and Qin, F., 1997, Dynamic smoothing in crosswell traveltime tomography, Geophysics, Vol. 62, No. 1, p Sun, Y., 1991, Ray tracing in general 3 D media by parameterized shooting: Presented at the 61st Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, Appendix In the appendix all observations are shown. That is all 3 component data are presented. Most data consist of a stack of 10 records. Data from each component is shown for both source directions.

93 45 Borehole profile ST34 Distance between wells: 4.4 m Horizontal component perpendicular to borehole profile.

94 Horizontal component parallel to borehole profile. 46

95 Vertical component 47

96 48 Borehole Profile ST32 Distance between wells: m Horizontal component perpendicular to borehole profile.

97 Horizontal component parallel to borehole profile. 49

98 Vertical component. 50

99 51 ST32: Observations from borehole source made on 3 component surface geophones Horizontal component perpendicular to borehole profile.

100 Horizontal component parallel to borehole profile. 52

101 Vertical component 53

102 * 1 ) / "

103 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,62 m MORÆNELER, ret fed, lysebrunt, enk. kalkfragmenter MORÆNELER, ret fed, plastisk, forvitret, lysebrunt, enk. kalkfragmenter MORÆNELER, ret fed, plastisk, sv. forvitret, lyst brungråt, st. khl MORÆNELER, fed, sandet, plastisk, sv. forvitret, kalkgruskorn, lyst brungråt, st. khl MORÆNELER, siltet, st. sandet, sv. forvitret, lyst brungråt, khl SAND, fint - mellem, leret, lyst brungult SAND SAND, fint - mellem, leret, siltet, lyst brungråt +27 X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :58:13 B1 1 S. 1/1 Miljøprofil

104 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,98 m 0 1 MORÆNELER, ret fed, st. forvitret, rødbrunt MORÆNELER, ret fed, forvitret, lyst olivenbrunt, khl. MORÆNELER, sandet, kalkgruskorn, lyst olivenbrunt MORÆNELER, sandet, tynde sandslirer, kalkgruskorn, lyst olivenbrunt SAND, fint - mellem, sv. gruset, lyst brungråt SAND, fint - mellem, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :17:01 B2 1 S. 1/1 Miljøprofil

105 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +35,12 m SAND, fint - mellem, brunt MORÆNELER, sv. forvitret, lyst olivenbrunt, khl MORÆNELER, sv. forvitret, rustslire, lyst olivenbrunt, khl MORÆNELER, sv. forvitret, lyst olivenbrunt, khl SAND, fint - mellem, lyst brungråt SAND, fint. mellem, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :30:00 B3 1 S. 1/1 Miljøprofil

106 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,80 m 0 MORÆNELER, st. forvitret, fast, rødbrunt SAND, fint - mellem, lysebrunt MORÆNELER, sv. forvitret, sandet, lyst olivenbrunt, khl MORÆNELER, sv. forvitret, lyst olivenbrunt, khl MORÆNELER, sv. forvitret, rustslire, lyst olivenbrunt, khl SAND, fint - mellem, lyst brungråt SAND, fint - mellem, sv. gruset, lyst brungråt +28 X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :02:07 B4 1 S. 1/1 Miljøprofil

107 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +35,07 m MORÆNELER, st. forvitret, ret fed, rustslire, rødbrunt MORÆNELER, sv. forvitret, olivenbrunt, khl MORÆNELER MORÆNELER MORÆNELER, st. sandet, lyst olivenbrunt, khl SAND, fint - mellem, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :28:27 B5 1 S. 1/1 Miljøprofil

108 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +35,22 m FYLD: MULD, brunt SAND, fint - mellem, lysebrunt MORÆNELER, sv. forvitret, olivenbrunt, khl. O Re MORÆNELER, sv. forvitret, lyst olivenbrunt, khl MORÆNELER, sv. forvitret, store kalkgruskorn, lyst olivenbrunt, khl MORÆNELER, sv. forvitret, lyst olivenbrunt, khl SAND, fint - mellem, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :42:00 B6 1 S. 1/1 Miljøprofil

109 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,35 m MORÆNELER, forvitret, ret fed, brunt, khl. 1 SAND, fint - mellem, lyst brungråt SAND MORÆNELER, forvitret, ret fed, olivembrunt +31 MORÆNELER, sv. forvitret, lyst olivenbrunt, khl MORÆNELER stop pga sten +29 X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :33:56 B7 1 S. 1/1 Miljøprofil

110 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,49 m MORÆNELER, st. forvitret, brunt, sv. khl. 1 LER, fed, lyst olivenbrunt MORÆNELER, st. forvitret, brunt, sv. khl. +33 SAND, fint - mellem, lyst brungråt SAND, mellem, sv. gruset, rustslire, lyst brungråt 3 MORÆNELER, sv. forvitret, lyst olivenbruntkhl MORÆNELER, sv. forvitret, ret fed, lyst olivenbrunt, st. khl X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :50:48 B8 1 S. 1/1 Miljøprofil

111 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,46 m MORÆNELER, st. forvitret, olivenbrunt, khl MORÆNELER, sv. forvitret, ret fed, olivenbrunt 2 SAND, mellem, lyst brungråt MORÆNELER, sv. forvitret, lyst olivenbrunt MORÆNELER, sv. forvitret, lyst olivenbrunt, khl MORÆNELER, sv. forvitret, rustslire, lyst olivenbrunt, khl. +29 MORÆNELER, st. siltet, st. sandet, lyst brungråt MORÆNELER MORÆNELER SAND, fint - mellem, sv. leret, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :47:04 B9 1 S. 1/1 Miljøprofil

112 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,58 m MORÆNELER, forvitret, ret fed, olivenbrunt MORÆNELER SAND, fint - mellem, gulbrunt MORÆNEGRUS, st. flintholdigt, olivengråt MORÆNELER, forvitret, ret fed, lyst olivenbrunt, khl MORÆNELER, sv. forvitret, lyst olivenbrunt SAND, fint - mellem, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :01:12 B10 1 S. 1/1 Miljøprofil

113 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,87 m 0 MORÆNELER, st. forvitret, rustslire, brunt MORÆNELER, forvitret, ret fed, forvitrede sten, olivenbrunt SAND, fint - mellem, gulbrunt MORÆNELER, forvitret, olivenbrunt MORÆNELER, forvitret, st. flintholdigt, lyst olivenbrunt MORÆNELER, forvitret, lyst olivenbrunt SAND, fint - mellem, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :13:06 B11 1 S. 1/1 Miljøprofil

114 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,86 m 0 MORÆNELER, st. forvitret, rustslire, brunt MORÆNELER, forvitret, kalkgruskorn, olivenbrunt SAND, fint - mellem, usorteret MORÆNELER, forvitret, lyst olivenbrunt, khl. MORÆNELER, sv. forvitret, kalkgruskorn, lyst olivenbrunt MORÆNELER MORÆNELER, sv. forvitret, kalkgruskorn, mere plastisk, lyst olivenbrunt MORÆNELER, plastisk, kalkgruskorn, lyst olivengråt SAND, fint - mellem, usorteret, lyst gulbrunt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :55:25 B12 1 S. 1/1 Miljøprofil

115 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +35,04 m MORÆNELER, st. forvitret, brunt, khl. MORÆNELER, st. forvitret, kalkgruskorn, olivenbrunt, khl MORÆNELER, forvitret, plastisk, kalkgruskorn, lyst olivenbrunt MORÆNELER, sv. forvitret, lyst olivenbrunt MORÆNELER, sv. forvitret, siltet, sandet, lyst olivenbrunt SAND MORÆNELER, sv. forvitret, siltet, sandet, lyst olivenbrunt SAND, fint - mellem, lyst gulbrunt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :06:42 B13 1 S. 1/1 Miljøprofil

116 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,61 m 0 FYLD: MORÆNELER, st. forvitret, ret fed, muldholdigt O Re MORÆNELER, st. forvitret, rustslire MORÆNELER, fast, st. forvitret, olivenbrunt, khl. +32 MORÆNELER, fast, forvitret, kalkgruskorn, lyst olivenbrunt, khl MORÆNELER, fast, sv. forvitret, olivenbrunt, khl MORÆNELER - - MORÆNEGRUS MORÆNELER, sv. forvitret, st. sandet, lyst olivenbrunt MORÆNELER SAND, fint - mellem, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :03:51 B14 1 S. 1/1 Miljøprofil

117 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,45 m MORÆNELER, fast, forvitret, brunt MORÆNELER, plastisk, forvitret, olivenbrunt 2 SAND, gulbrunt +32 MORÆNELER, sv. forvitret, lyst olivenbrunt, khl MORÆNELER MORÆNELER MORÆNELER, plastisk, sv. forvitret, lyst olivengråt MORÆNELER SAND, fint - mellem, lyst gulbrunt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :24:33 B15 1 S. 1/1 Miljøprofil

118 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,18 m MORÆNELER, st. forvitret, st. siltet, brunt MORÆNELER, st. forvitret, siltet, brunt 2 MORÆNELER, forvitret, olivenbrunt, khl. +32 SAND MORÆNELER, sv. forvitret, lyst olivenbrunt, khl MORÆNELER MORÆNELER, sv. forvitret, lyst olivenbrunt, st. khl MORÆNELER, sv. forvitret, lyst olivenbrunt, khl MORÆNELER, uforvitret, lyst olivengråt SAND, sammenkittet med okker, rustrødt SAND, fint - mellem, lyst gulbrunt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :45:09 B16 1 S. 1/1 Miljøprofil

119 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,06 m SAND, fint. mellem, gulbrunt MORÆNELER, fast, st. forvitret, brunt MORÆNELER, sv. forvitret, olivenbrunt SAND MORÆNELER, sv. forvitret, olivengråt MORÆNELER MORÆNELER, sv. forvitret, lyst olivengråt MORÆNELER, sv. forvitret, st. siltet, plastisk, lyst olivenbruntt MORÆNELER, sv. forvitret, rustslire, lyst olivenbrunt SAND, mellem, lyst gilgråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :01:56 B17 1 S. 1/1 Miljøprofil

120 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,04 m MORÆNELER, st. forvitret, ret fed, olivenbrunt MORÆNELER MORÆNELER, forvitret, plastisk, olivenbrunt MORÆNELER, fast, sv. forvitret, lyst olivenbrunt MORÆNELER MORÆNELER, sv. forvitret, sandet, lyst olivenbrunt MORÆNELER, uforvitret, olivengråt, st. khl MORÆNELER, uforvitret, olivengråt, khl SAND, sammenkittet med okker SAND, usorteret, lyst gulbrunt Fortsættes X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :31:10 B18 1 S. 1/2 Miljøprofil

121 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. Fortsat X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :31:10 B18 1 S. 2/2 Miljøprofil

122 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,65 m 0 MORÆNELER, st. forvitret, olivenbrunt +34 SAND, mellem, brungråt MORÆNELER, sv. forviotret, lyst olivenbrunt, khl MORÆNELER MORÆNELER, sv. forviotret, st. sandet, lyst olivenbrunt, khl. 4 MORÆNELER SAND, fint - mellem, lyst gulbrunt +29 SAND, fint - mellem, lys brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :44:50 B19 1 S. 1/1 Miljøprofil

123 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,79 m 0 MORÆNELER, st. forvitret, olivenbrunt MORÆNELER, forvitret, lyst olivenbrunt, khl SAND, mellem, lyst, brungråt +32 MORÆNELER, fast, sv. forvitret, lyst olivenbrunt 3 MORÆNELER MORÆNELER SAND, mellem, lyst brungråt X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :55:31 B20 1 S. 1/1 Miljøprofil

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125 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,56 m MORÆNELER, gruset 1 2 MORÆNELER SAND, fint - mellem 2 4 MORÆNELER, gruset MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER SAND 7 14 SAND SAND 8 16 SAND +26 Pejlerør: 1: X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :23:39 RT1 1 S. 1/1 Miljøprofil

126 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,26 m MORÆNELER, sandet 1 2 MORÆNELER MORÆNELER SAND, fint - mellem MORÆNELER, gruset 3 6 MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER SAND, fint SAND SAND Pejlerør: 1: X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :14:29 RT2 1 S. 1/1 Miljøprofil

127 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,60 m MORÆNELER, sandet 1 2 MORÆNELER MORÆNELER SAND, fint - mellem MORÆNELER, sandet, gruset 3 6 MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER, enk. sandslirer MORÆNELER, sandet SAND, fint 7 14 SAND SAND SAND Pejlerør: 1: X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :21:38 RT3 1 S. 1/1 Miljøprofil

128 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,38 m MORÆNELER 1 2 MORÆNELER MORÆNELER 2 4 SAND MORÆNELER 3 6 MORÆNELER MORÆNELER 4 8 MORÆNELER MORÆNELER 5 10 MORÆNELER MORÆNELER 6 12 MORÆNELER SAND, fint - mellem 7 14 SAND SAND SAND Pejlerør: 1: X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :33:38 RT4 1 S. 1/1 Miljøprofil

129 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,64 m MORÆNELER, sandet, gruset 1 2 MORÆNELER MORÆNELER SAND MORÆNELER, sandet, gruset 3 6 MORÆNELER MORÆNELER MORÆNELER, sandet, gruset, stenet MORÆNELER MORÆNELER MORÆNELER MORÆNELER SAND, fint 7 14 SAND SAND SAND Pejlerør: 1: X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :40:08 ST1 1 S. 1/1 Miljøprofil

130 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,66 m MORÆNELER 1 2 MORÆNELER MORÆNELER 2 4 SAND SAND 3 6 MORÆNELER MORÆNELER 4 8 SAND MORÆNELER MORÆNELER 5 10 MORÆNELER MORÆNELER 6 12 MORÆNELER MORÆNELER 7 17 SAND, fint SAND Pejlerør: 1: X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :54:40 ST2 1 S. 1/1 Miljøprofil

131 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,35 m MORÆNELER 1 2 MORÆNELER MORÆNELER 2 4 SAND MORÆNELER 3 6 MORÆNELER MORÆNELER 4 8 MORÆNELER MORÆNELER 5 10 MORÆNELER MORÆNELER 6 12 MORÆNELER SAND, fint - mellem 7 14 SAND SAND Pejlerør: 1: X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :03:29 ST3 1 S. 1/1 Miljøprofil

132 Dybde (m) Forsøgsresultater Kote (m) Geologi Prøve Nr. Jordart Karakterisering Aflejring Alder Lugt Misfarv. DVR90 +34,03 m MORÆNELER, gruset MORÆNELER MORÆNELER SAND, fint 5 SAND MORÆNELER, gruset 7 MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER MORÆNELER SAND, fint SAND - - Pejlerør: 1: X = Prøve udtaget til analyse 0 = Ingen lugt 1 = Svag lugt 2 = Lugt 3 = Stærk lugt + = Misfarvet - = Ikke misfarvet Sag : DP & Geofysik Boremetode : Tørboring 6 uden foring X: (m) Y: (m) Plan : Strækning : Boret af : Boregruppen Dato : DGU-nr.: Boring : Udarb. af : sjoe Kontrol : Godkendt : Dato : Bilag : GeoGIS ddh PSTMDK :10:44 ST4 1 S. 1/1 Miljøprofil

133 * 1 ) / $

134 Figur 1 Profil A Uden polygoner. Farveforskelle skyldes belysning og ikke redoxstatus. Figur 2 Profil A med indtegnede sand og gruslag. Farveforskelle skyldes belysning og ikke redoxstatus.

135 Figur 3 Profil B Uden polygoner. Farveforskelle skyldes belysning og ikke redoxstatus. Figur 4 5 Profil B med indtegnede sand og gruslag. Farveforskelle skyldes belysning og ikke redoxstatus.

136 Figur 6 Profil C Uden polygoner. Farveforskelle skyldes belysning og ikke redoxstatus. Figur 7 Profil C med indtegnede sand og gruslag. Farveforskelle skyldes belysning og ikke redoxstatus.

137 Figur 8 Profil D Uden polygoner. Farveforskelle skyldes belysning og ikke redoxstatus. Figur 9 Profil D med indtegnede sand og gruslag. Farveforskelle skyldes belysning og ikke redoxstatus.

138 * 1 ) / %

139 Bilag 7 Sammenfatning af SWOT analyse på geofysiske metoder SWOT analyse af DCIP metoden Styrker IP-signal er korreleret med litologi Kendt teknologi Stor datasampling tæthed Iterativ dataopsamling Fluteliner -> genbrug af elektroder Fleksibilitet elektroder Anvendes på ejendomme med firmaer i drift 2D- tolkning kan testes umiddelbart Muligheder 3D tolkningskode Ny teknologi muligheder ikke endelig kortlagt Test i 2D -> evaluering -> 3D Tolkning af 3D geologisk viden Svagheder Tolkningskode skal udvikles Lange regnetider Opløsning afhænger af borehulsafstand Følsom overfor elektromagnetisk støj Modstands- og IP kontraster ikke fuldstændig kendte Trusler Opløselighed ikke kendt Tidsforbrug ved fremstilling af elektroder til borehuller og ved udvikling af tolkningskode SWOT analyse af GPR metoden Styrker Hurtig kan bruges i dynamiske undersøgelser Kan anvendes i eksisterende boringer eller kernehuller Udstyret findes ved KU og kommercielt Stor kontrast mellem ler og sand Muligt at beregne begrænsning ved metoden Kan anvendes på industrielle lokaliteter Billig Muligheder Quick and dirty test Kombination med refleksions-georadar Optimering af dynamisk undersøgelse Svagheder Samme sandlag skal kunne ses i to boringer Der opnås et geometrisk netværk og ikke den fulde (sande) rumlige opbygning Trusler Dæmpning af signal også ved gennemgående sandlinser Kan påvirkes af uheldig boringsplacering SWOT analyse af S-bølge seismik metoden Styrker Udstyr og software eksisterer Teknikken kan umiddelbart testes Vil kunne anvendes i områder, hvor E- og EM-metoder forstyrres Muligheder Forbedring af udstyr mht højere frekvenser Implementering af overfladekilde Full waveform inversion Svagheder Uvist om opløselighed er tilstrækkelig Trusler Manglende energiudbredelse i mediet

140 * 1 ) / &

141 Proposal: S wave crosshole seismic tomography Personal Egon Nørmark (EN), Institute of Geoscience, Aarhus university (IG) Per Trinhammer (PT), Institute of Geoscience, Aarhus university (IG) Giulio Vignoli (GV), GEUS Introduction Crosshole S-wave tomography will be carried out in order to estimate S-wave velocities. Results will be used for the interpretation of geological structures. - In this case sand lenses embedded in till. Sand is expected to have lower velocity compared to till. This is judged from the knowledge about P-wave velocities, but we cannot be sure if it also applies to S-wave velocities. However, generally significant variations in S-wave velocities have been observed in the unsaturated zone. The velocity contrast is important for the success of the tomographic experiment. If there are no velocity contrasts the experiment will only have limited value. Crosshole tomography will be carried out by using one borehole for source signal and another for the acquiring the seismic signals. Only first arrival s-waves will be used for traveltime tomography. Equipment The equipment to be used consists of an S-wave borehole sparker and two 3 component (3C) geophones. Both source and receivers will be clamped to borehole sides by use of pneumatic clamping. Existing recording equipment from IG will be used for acquisition. Power supply for borehole sparker will be rented. Minor modifications of the power supply are needed, in order to make the equipment work for the present application. Beside borehole equipment 3 component geophones will be used along the surface between the boreholes. Geophones and geophone cables from IG will be used. Boreholes diameter The Borehole equipment will fit into 3 inch boreholes. If bigger the borehole diameters are used, a spacer must be mounted on borehole equipment. It is not clear if this procedure will give rise to some practical problems. Thus 3 inch boreholes must be considered to be optimal, but bigger borehole diameter will probably work as well.

142 It is expected that PVC casing is present. It is important that the distance between the casing to the undisturbed sediments is as small as possible. If there are unsaturated conditions, it is crucial that the gap between the casing and formation is backfilled to ensure a good acoustic connection between source and formation. Crosshole measurements The crosshole measurements will be carried out by placing borehole source in one of the borehole receivers in another borehole. A string of 3 component geophones (most likely 24 geophones) will be placed on the surface between the boreholes. It will be attempted to cover all combinations of source and receiver depths. Sample distance depends on how long time it will take to acquire data, but a sample distance of 0.5 m 1.0 m is probably realistic. This defines one set of crosshole measurement. Beside the borehole source, hammer seismics will also be attempted. - That is the source will be on the surface and receivers in the borehole. It would be natural that this gives lower frequencies compared to the borehole source. If this is the case these surface source measurements might not be so valuable. With 3 or 4 boreholes it will be attempted to cover all combinations of measurements between source and receiver boreholes. It is not expected to give additional information to shot in both directions (that is to swap source receiver boreholes). It might be attempted but only to verify the measurements. The number of crosshole measurements will increase significantly by the number of boreholes, if all combinations of crosshole geometries should be covered. If 3 boreholes are made 3 sets of these measurements are relevant. With 4 boreholes 6 sets of measurements is relevant. For 4 boreholes 10 sets of measurements can be made. If more than 4 boreholes are made there will probably so many mutual crosshole combinations that not all of them will be carried out. If 4 or 5 boreholes are made it is possible to make measurements with different mutual distance between the boreholes. This might be a beneficial. Since each set of measurements give information along the plane spanned be the boreholes, the 3D coverage can be reasonably well be predicted from the geometry of the boreholes. The main concern in placing the boreholes for the crosshole seismic experiment, is the mutual distance between the boreholes. It is of course crucial that signals can be transmitted between the boreholes with sufficiently high frequency. The optimal distance between the boreholes is hard to predict in advance but crosshole tests is expected to give a much better idea of how the equipment perform. Suggested borehole geometries: 3 boreholes

143 4 boreholes: Or 5 boreholes: It is not expected that signals passing a borehole will disturb the measurements if the borehole diameter is 3 inch. The choice of borehole geometry is not expected to be of major importance for the results but will of course have effect on the 3D definitions of the geological structures. With a distance of 10 m between boreholes (distance between source and receiver borehole) it is expected that a distance of 20 m the rim of the pit is sufficient. Test of equipment It is planned to test equipment in Klim, Northern Jutland, where a number of boreholes are available. The mutable distances in between them are less than 15 m. Tests will reveal if the equipment works as expected and if signals can be acquired with sufficiently high frequencies. Results will be used to suggest distance between boreholes in the target area. The initial guess of distance between boreholes: 8-10 m. Documenting the geological setting When all experiments are done the material will to our knowledge be excavated. Here it is important that the geological setting is documented. For instance by a lot of photos which later might be composed into cross section photos.

144 Time schedule Test in Klim Evaluation of test measurements Measurements in target area Medio/Ultimo April Ultimo April May/June Interpretation June/September * Results Primo October * *) EN is in field work until from Ultimo July to Primo September. Thus in this time span it is not possible for EN work on the interpretation.

145 INSTITUT FOR GEOSCIENCE AARHUS UNIVERSITET Projektforslag Kortlægning af sandlinser med overflade og borehuls DCIP Institut for Geoscience Esben Auken Professor Dato: 2. marts 2015 Direkte tlf.: Mobiltlf.: Afs. CVR-nr.: Side 1/9 Indhold 1. Projektmål 1 Projektforslagsstillere og økonomi 1 Problemstilling 2 2. Metodebeskrivelse 2 Nødvendige udviklinger for at kunne gennemføre forsøget 3 Krav til målelokalitet og borehuller 4 Dataprocessering og tolkning 4 3. Afrapportering og evaluering 5 Afrapportering 5 Tidsplan 5 Udviklinger efter initialt forsøg 7 Bemanding 8 1. Projektmål Målet med dette projekt er at afprøve og evaluere metoderne direct current (DC) og induceret polarisation (IP) potentiale til at kortlægge sandlinser af forskellig størrelse i moræneler. Det skal via felt- og modelleringsforsøg evalueres, om metoderne er egnet samt hvilke udviklingsomkostninger, der skal påregnes, hvis metoderne skal bruges i egentlig produktion. Projektforslagsstillere og økonomi Projektforslaget er udarbejdet af Professor Esben Auken og Lektor Anders Vest Christiansen, HydroGeophysics Group, Institut for Geoscience, Aarhus Universitet. Økonomi er angivet i Bilag A. Institut for Geoscience Aarhus Universitet Høegh-Guldbergs Gade Aarhus C

146 INSTITUT FOR GEOSCIENCE AARHUS UNIVERSITET Side 2/9 Problemstilling I forbindelse med kortlægning og oprensning af forurenede lokaliteter er det af kritisk betydning at kende den detaljerede geologiske og hydrogeologiske opbygning af jordvolumenet omkring forurenings hotspots. Med afsæt i den dominerende geologi i hovedstadsområdet skal fokus rettes modkortlægning af sandlinser i moræneler indenfor et jordvolumen i den umættede zone på 10x10 meter i overfladen og 10 meter i dybden. Indenfor denne firkant skal den/de udpegede metode(r) kunne kortlægge pladelignende strukturer med en størrelse på (tykkelse x bredde x længde) på 10 x 100 x 100 cm, herunder deres placering, udstrækning og rumlige orientering. DC og IP metodernes evne til at opløse sådanne strukturer er helt afhængig af modstandskontrasten imellem sand og moræneler, samt at der kan måles mellem borehuller og fra overfladen til borehuller. Elektroder skal placeres med lille afstand, og der skal måles data med et meget højt signal/støj forhold. 2. Metodebeskrivelse Med DC metoden sendes der elektrisk strøm i jorden ved hjælp af to stålspyd, også kaldet elektroder, og derefter måles det resulterende spændingsfald imellem to andre elektroder, som er placeret i nærheden af strømelektroderne. Spændingsfaldet følger Ohms lov, der relaterer spændingsfald med jordens elektriske modstand og den udsendte strøm. Jordens elektriske modstand er tæt forbundet med litologien, f.eks. er sand kendetegnet ved en høj modstand, hvorimod ler typisk har en lav modstand. Med DC metoden kan forskellige litologier derfor kortlægges med stor detaljegrad. Pga. komplekse processer på overfladen af lermineraler eller i porerummene på sandlag, kan der ske en forsinkelse, og spændingsfaldet vokser derfor først op til sit maksimum efter nogle sekunder. Denne proces kaldes induceret polarisation, og selve opvoksningen af spændingen eller et tilsvarende henfald, efter strømmen er slukket, måles med IP metoden. Målinger af DC og IP data foregår samtidigt og der måles således ét DC punkt og IP datapunkter for at beskrive spændingshenfaldet præcist. IP datasættet komplementerer DC datasættet, da IP metoden er særdeles velegnet til at kortlægge moræneler. Kortlægning af tynde sandlinser i moræneler er en meget kompliceret problemstilling, da linserne er forholdsvis små i forhold til deres rummelige afstand til strøm- og potentialeelektroder. For at kunne opløse linserne skal der anvendes tætsiddende elektroder i borehuller samt på overfladen. Det skønnes nødvendigt både at måle potentialer imellem borehullerne og fra overfladen til borehuller. Det forventes, at der både skal måles DC og IP data. Hvor DC data er lige følsomme overfor høje som lave modstande (sand og moræneler), vil IP data udelukkende have følsomhed overfor moræneleret. Traditionelt modelleres et

147 INSTITUT FOR GEOSCIENCE AARHUS UNIVERSITET Side 3/9 IP henfald med bare ét data punkt, som udgør en integration af et helt henfald. I dette projekt vil vi anvende en helt ny in-house udviklet modelleringsteknik, hvor hele IP henfaldet modelleres, da dette har vist sig at forøge opløseligheden væsentligt for overflademålinger. Da koden er in-house, skal det understreges, at der internationalt ikke findes tilsvarende forsøg, hvor det er forsøgt at måle med DC og IP metoderne i borehuller samt at modellere fulde IP henfaldskurver. Nødvendige udviklinger for at kunne gennemføre forsøget Forsøget indebærer, at der laves en række udviklinger, og at der fremstilles hardware og softwarekomponenter, som vi pt. ikke har til rådighed. Disse udviklinger er beskrevet i det følgende: Filterrør med påmonterede elektroder: For at kunne lave målinger imellem borehuller, skal der fremstilles filterrør, hvor der er påmonteret elektroder. Dette gøres ved, at der udenpå filterrøret pålimes kobbertape, som forbindes til en ledning, der føres indeni røret. Det forventes, at disse filterrør er 10 m lange og at elektrodeafstanden er 20 cm. Det betyder, at der på hvert filterrør vil være ca. 50 elektroder. For at rørene kan transporteres, laves de i længder af ca. 3.3 m. Breakoutbokse: Til hvert elektroderør skal der fremstilles en breakoutboks, der samler elektrodeledningerne til et stik, som passer til det måleinstrument, der anvendes til forsøget. Måleinstrumentet kan håndtere op til 128 elektroder, når der påmonteres en forlænger boks. Der kan således måles imellem to borehuller med hver 50 elektroder samt med 64 elektroder fra overfladen til hvert borehul. Kontrolsoftware og måleprotokoller: En vigtig del af forsøget vil være at kvantificere, hvilke målekonfigurationer der bedst løser opgaven. En målekonfiguration består af to strømelektroder og op til 12 potentiale elektroder, og det er deres indbyrdes placering, der er afgørende for den opnåede opløselighed. Der skal derfor udvikles en række forskellige målekonfigurationer, der alle skal eksekveres under selve feltforsøget. Da det forventes, at der skal opnås stor datatæthed, vil det kunne forventes, at der skal måles i mange timer, og at der ved hver konfiguration opnås vel over DC data punkter med tilsvarende gange så mange IP punkter. Dataprocessering: Vi råder ikke over automatiseret data processeringssoftware til at håndtere data fra elektroder placeret i jorden, og derfor skal der skrives en række matlab rutiner, der kan processere data samt forberede input til selve den kode, der inverterer data. Det forventes, at der kan anvendes samme filterteknikker som for overfladedata. Datainversion: Vores eksisterende inversionskode kan håndtere elektroder på jordoverfladen og i princippet også i borehuller. Det forventes dog, at der skal

148 INSTITUT FOR GEOSCIENCE AARHUS UNIVERSITET Side 4/9 laves en række tilretninger i koden, da elektroder i jorden ikke har været afprøvet før. Visualisering af inversionsresultater: Der skal skrives en række matlab rutiner, der kan forberede input til 3D visualisering af inversionsresultater. Disse rutiner vil i høj grad kunne baseres på eksisterende koder. Visualisering foretages i ParaView programmet. Krav til målelokalitet og borehuller Feltlokaliteten er Kallerup Grusgrav nord for Hedehusene. Denne lokalitet er velbeskrevet, og efter opmålinger vil det opmålte område blive udgravet for kontrol af resultater. Som udgangspunkt vil vi ikke i forsøget foretage målinger i tre dimensioner (3D). Dette skyldes, at vi råder over en 2D kode, mens en tilsvarende 3D koden er under udvikling i et andet forskningsprojekt. Pga. den stramme tidsplan, vil denne kode dog ikke være færdigudviklet til anvendelse. Vi forventer dog, at man fra 2D resultaterne vil kunne kvantificere, hvad der kan opnås i 3D. Vi forventer, at der bores tre borehuller på række med en indbyrdes afstand på 5 m. Vi ønsker det midterste borehul til kontrol af resultater. Hvis den ønskede opløselighed ikke kan opnås imellem borehullerne med 10 m afstand, kan vi kvantificere, hvad der kan opnås med en afstand på 5 m i et og samme forsøg. Der vil således både blive målt sekventielt imellem alle borehuller og fra hvert borehul til overfladeelektroderne. Borehullerne placeres mindst 20 meter fra randen af grusgraven, gerne mere. Dimensionen af borehuller afgøres ud fra, hvad der praktisk kan lade sig gøre. Dog skal borediameteren afstemmes med størrelsen på filterrøret, da det er vigtigt, at de gennemborede jordlag pakker tæt til filterrøret, så der opnås optimal elektrodekontakt og dermed et højt signal i forhold til støj samt lav overgangsmodstand imellem elektroder og jorden. Man skal kunne bevæge sig til fods på lokaliteten og køre til den med et almindeligt køretøj. Dataprocessering og tolkning Den meget store mængde data processeres på tilsvarende måde som for overfladedata. Dvs. de enkelte IP henfald undersøges for støj og støjede data fjernes. Hvis kontakten imellem elektroder og jordlagene er god, forventes det, at der kan opnås næsten støjfri data. Data inverteres med en meget høj diskretisering og lav grad af regularisering (hvor meget de indbyrdes celler i modellen må afvige fra hinanden - også kal-

149 INSTITUT FOR GEOSCIENCE AARHUS UNIVERSITET Side 5/9 det modellens glathed). Det bliver nødvendigt at udføre en lang række tolkningsforsøg for at opnå optimale modeller. Dette vil være yderst krævende rent beregningsmæssigt, og derfor vil adgang til en vores store servere med 64 CPU er være afgørende. Hvis der ikke kan opnås tilfredsstillende resultater med de inversionsrutiner, vi normalt anvender, vil vi forsøge at lave det, der hedder nul space Monte Carlo inversion. Med denne teknik opnår man ikke en enkelt model, men en række modeller, der er lige sandsynlige og tilpasser data. Denne teknik er dog endnu mere beregningstung end den normale teknik, og den har ikke været afprøvet på DC-IP data før. 3. Afrapportering og evaluering Afrapportering Projektet afrapporteres i to dele. Den første del omhandler selve forsøget og de opnåede resultater. Den anden del udføres, hvis resultaterne er positive (vi kan opløse sandlinserne), samt at de overordnede mål for projekt er nået. Den vil indeholde en gennemgang af de udviklinger, der skal udføres for at gøre metoden operativ og omkostningseffektiv ved anvendelse i større stil i forbindelse med undersøgelser af forurenede grunde. Den vil også indeholde forslag til finansiering af disse udviklinger samt anbefalinger, hvis flere forsøg er nødvendige. Tidsplan Forsøget vil kunne afvikles fra april og til udgangen af Måledelen med tilhørende fremstilling af hardware er relativt let at tidsfastsætte, mens tolkningsdelen er sværere, da et tilfredsstillende resultat med stor sikkerhed ikke kommer ved at vælge standardopsætninger.

150 INSTITUT FOR GEOSCIENCE AARHUS UNIVERSITET Side 6/9 Ultimo marts: Koordinationsmøde med henblik på endelig udformning af metodebeskrivelser og mål for udvikling samt udformning af en tentativ tidsplan for feltforsøgene. Ultimo april Ultimo maj: Tilretning af plan for indretning af feltlokalitet herunder indbyrdes placering af borehuller samt specifikation af dimension, dybde, udbygning m.m., der kan danne grundlag for arbejdsbeskrivelse til boreentreprenør. Nødvendige udviklinger i inversionskoden for at kunne håndtere elektroder i jorden udføres. Udvikling af måleprotokoller og målerækkefølge. Fremstilling af elektroderør og breakoutbokse. Ultimo maj: Udførelse af borehuller og montering af elektroderør. Primo juni: Udførelse af målinger. Efter udførsel af målinger kan lagserien på feltlokaliteten dokumenteres, og borehullerne sløjfes. Primo juni medio september: Dataprocessering og inversion (der er sommerferie i juli). Primo september: Koordinationsmøde med henblik på deling af resultater og fastlæggelse af formen i rapportering og dokumentation. Primo september primo oktober: Afrapportering samt anbefalinger. Ultimo oktober: Afslutningsmøde/workshop med fremlægning af resultater og diskussion af evt. yderligere potentiale for metoderne.

151 INSTITUT FOR GEOSCIENCE AARHUS UNIVERSITET Side 7/9 Det vil være meget svært for os at håndtere en strammere tidsplan end den ovenfor skitserede, da vi har en række andre projekter, der afvikles sideløbende. Udviklinger efter initialt forsøg Det første forsøg er tænkt som et proof of concept, og skal teknikken udrulles til egentlig produktion, skal der udvikles en række ting, der gør den omkostningseffektiv. Dette vil blive diskuteret i afrapporteringen og vil indebære: Der skal udvikles elektroderør, der let kan sættes ned i et borehul og efter måling trækkes op og anvendes igen. Dette kan gøres med strømper, hvorpå der er monteret elektroder, og som kan pustes op mod borevægen. Ved at fjerne trykket kan de trækkes op af borehullet og anvendes igen. Der skal vælges én og kun én målekonfiguration, som giver optimal opløsning, og som det ikke tager for lang tid at afvikle. Der skal udvikles switchbokse, der automatisk kan switche imellem hele elektrodesæt i borehullerne. Vi råder allerede over sådanne bokse, der dog ikke helt har de egenskaber, der er brug for til borehuller. Dataprocesseringen skal automatiseres således, at data effektivt kan processeres og forberedes til inversionsrutinen. Yderligere skal inversionen kunne foretages i 3D og ikke kun 2D. Der skal udvikles en række standardvisualiseringer, som gør resultaterne umiddelbart forståelige og klare. I en produktionssituation skal målingerne kunne foretages af de rådgivende firmaer, der arbejder med jordforurening i for Region Hovedstaden. Dette indebærer en kapacitetsopbygning hos deres medarbejdere, så de kan anvende den nye teknik.

152 INSTITUT FOR GEOSCIENCE AARHUS UNIVERSITET Side 8/9 Bemanding Et projekt som dette kræver en indsats fra en række personer hver med deres ekspertise. Vi forventer at følgende personer deltager: Overordnet projektledelse: Lektor Anders Vest Christiansen Ansvarlig for hardware samt faglig sparring: Professor Esben Auken Teknik: Elektroniktekniker Simon Ejlertsen Tilretninger i inversionskode, design af elektrodekonfigurationer: Adjunkt Gianluca Fiandaca Feltarbejde: Geofysiker Nikolaj Foged, Simon Ejlertsen, Gianluca Fiandaca Data processering og tolkning: Nikolaj Foged, Gianluca Fiandaca Afrapportering: Anders Vest Christiansen og Nikolaj Foged.

153 PROJEKT DOKUMENT: GPR TOMOGRAFI Lars Nielsen og Majken Caroline Looms Zibar, Københavns Universitet Kort metodebeskrivelse Ground penetrating radar (GPR) er en elektromagnetisk metode hvor en elektromagnetisk bølge udsendes fra en transmitter antenne. En receiver antenne bruges til at registrere hvorledes bølgen udbredes i det undersøgte medie. GPR kan bruges i to konfigurationer, refleksions GPR og transmissions GPR. Ved refleksions GPR placeres de to antenner ved siden ad hinanden og den elektromagnetiske energi der optages ved receiveren skyldes hovedsagligt refleksioner der opstår på grænseflader mellem medier af forskellig dielektriske egenskaber. Refleksions GPR bruges derfor især til kortlægning af geologiske aflejringsstrukturer. Ved transmissions GPR, eller cross-borehole GPR, placeres antennerne i to nærtliggende borehuller således at den energi der optages ved receiveren hovedsagligt stammer fra den direkte bølge der har passeret i gennem mediet mellem borehullerne. Fordelen ved cross-borehole GPR er at mediets dielektriske egenskaber og elektriske egenskaber kan estimeres ud fra en viden om antennernes eksakte indbyrdes placering og antagelser omkring bølgetype og udbredelsesmønster. Cross-borehole GPR er i de sidste årtier blevet brugt til at bestemme vandindhold i umættede medier eller porøsitet i de mættede medier grundet den store forskel i vands og lufts/sediments dielektrisk egenskaber. Langt de fleste studier har beskæftiget sig med ucementeret sandede aflejringer, men der er også foretaget undersøgelser i kalkaflejringer samt sandsten. Materialer, med høj elektrisk ledningsevne, såsom ler, undgås som regel, eftersom det elektromagnetiske signal dæmpes med en reduceret indtrængningsevne som følge. Af samme årsag er det uklart hvorvidt cross-borehole GPR kan bruges til at kortlægge sandlinser i moræneler. Indledende undersøgelser For at afklare dette har vi lavet nogle indledende syntetiske test i MATLAB for at undersøge dæmpningen af den elektromagnetiske bølge gennem en sandlinse af forskellige tykkelser ( 5 cm). Disse værdier er sammenholdt med dæmpningen i homogent umættet sand, hvor vi har erfaring med at kunne måle med acceptabel signal-til-støjforhold mellem borehuller placeret med op til 7-10 m afstand. Seks forskellige scenarier med forskellige antagelser vedrørende sandets og morænelerets dielektriske og elektriske egenskaber er blevet undersøgt, se Tabel 1. Øvrige antagelser inkluderer 1 m afstand mellem de to antenner, en horisontal gennemgående sandlinse og 100 MHz antenner. Den konceptuelle model er vist i Figur 1. Tabel 1: De dielektriske og elektriske egenskaber af de seks undersøgte scenarier. Sand Moræneler ε r [-] σ [S/m] μ r [-] ε r [-] σ [S/m] μ r [-] Scenarie Scenarie Scenarie Scenarie Scenarie Scenarie

154 Figur 1: Den konceptuelle model brugt i de syntetiske analyser. Sandets dielektriske permittivitet og elektriske ledningsevne blev i alle scenarier holdt konstant på henholdsvis 5 og S/m, svarende til et vandindhold og elektrisk modstand på henholdsvis 0.08 cm 3 /cm 3 og 500 Ωm. Morænelerets dielektriske permittivitet blev varieret mellem 5 og 40 ifølge tabelværdier i Davis & Annan (1989), og den elektriske ledningsevne blev varieret mellem og 0.05 S/m, svarende til henholdsvis 40 og 20 Ωm. Sidst-nævnte værdi må antages af være absolut worst-casescenario. Følgende konklusioner kunne drages af de indledende analyser: 1) Hvis moræneler har ε = 5 (scenarie 1) bliver det elektromagnetiske (EM) signal igennem en 5 cm sandlinse dæmpet med 42% i forhold til hvis undergrunden var homogen sand. Hvis undergrunden udgøres fuldstændigt af moræneler dæmpes signalet med 81%. 2) Hvis morænleret har ε = 40 (scenarie 2) forstærkes EM signalet igennem sandlinsen med 30% fordi sandlinsen fungere som en waveguide. En dæmpning på 81% i forhold til homogen sand burde stadig kunne måles med vores udstyr specielt hvis borehulsafstanden ikke er for stor. Det er dog mest sandsynligt at ε ler > ε sand, idet vandindholdet i ler må forventes at være højere end i sand grundet de mindre porer og derfor bedre retentionsegenskaber. I dette tilfælde forstærkes EM signalet og ud fra disse indledende syntetiske undersøgelser virker det derfor meget muligt at man kan detektere sandlinser i moræneler ved brug af cross-borehole GPR under forudsætningen af at sandlinserne er gennemgående. Tidsplan I forhold til den overordnet tidsplan har vi følgende kommentarer: Aktivitet Lars Nielsen Majken Zibar Koordinationsmøde, ultimo marts Ingen problemer Ingen problemer Feltforsøg ultimo maj/primo juni Meget undervisning Ingen problemer Koordinationsmøde, medio august Grønland: 18/8-30/8 Ferie: frem til 17/8 Afslutningsmøde, medio september Underviser tirsdag og torsdag Underviser tirsdag og torsdag Derudover har vi ingen problemer vedrørende den foreslåede tidsplan.

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