From resistivity to clay thickness the SSV concept

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Arthur Nygaard
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1 From resistivity to clay thickness the SSV concept Anders V. Christiansen, Esben Auken, Flemming Jørgensen and Kurt Sørensen The HydroGeophysics Group, University of Aarhus

2 SSV -GeoStatistical Estimation of Structural Vulnerability

3 The SSV concept Boreholes Primary information (clay thickness) Poor coverage in relation to desired information Inconsistent quality (old/new, drill method etc.)

4 The SSV concept Boreholes Primary information (clay thickness) Poor coverage in relation to desired information Inconsistent quality (old/new, drill method etc.) Geophysics Secondary information (resistivity geophysical clay thickness) Better coverage Consistent known quality Till Heavy clays Sandy glacial deposits

5 The SSV concept Boreholes Primary information (clay thickness) Poor coverage in relation to desired information Inconsistent quality (old/new, drill method etc.) Geophysics Secondary information (resistivity geophysical clay thickness) Better coverage Consistent known quality Clay weight 1 0 clay transition not clay Resistivity [ohmm]

6 The SSV concept Boreholes Primary information (clay thickness) Poor coverage in relation to desired information Inconsistent quality (old/new, drill method etc.) Geophysics Secondary information (resistivity geophysical clay thickness) Better coverage Consistent known quality How do we optimally combine this information? Inversion - SSV!

7 SSV - players Translator model, geoph. to clay thickness Geophysics layered models 1 0 Consistency? Boreholes, clay thicknesses

8 SSV - players Geophysics layered models + uncertainties Translator model, geoph. to clay thickness + uncertainties 1 0 Consistency? Boreholes, clay thicknesses + uncertainties

9 SSV - what is it? SSV is a statistically based inversion algorithm for estimation of geophysical clay thickness SSV gives the (statistically) best geophysics-to-clay-thickness translator model best clay thickness map integrating borehole information and geophysics

10 Field example 25 km 2 85 line km of PACES 17,000 1D models 64 drill holes (>20 m depth)

11 Field area 1 km

12 PACES data 1 km

13 Boreholes 1 km

14 Borehole descriptions Danmarks og Grønlands Geologiske Undersøgelse BORERAPPORT Udskrevet 2/ Side 1 DGU arkivnr : Borested : Fuglrisvej Brædstrup Kommune : Brædstrup Amt : Vejle Boringsdato : 1/ Boringsdybde : 44 meter Terrænkote : 0 meter o. DNN Brøndborer : I. Knudsen, Laurbjerg MOB-nr : BB-journr : BB-bornr : Formål : Vandforsyningsboring Anvendelse : Boremetode : Tørboring/slagboring Kortblad : 1214 IINV UTM-zone : 32 UTM-koord. : , Prøver - modtaget : 9/ antal : 6 - beskrevet : 11/ antal gemt : Datum : Koordinatkilde : Koordinatmetode : Indtag 1 Ro-vandstand Pejledato Ydelse Sænkning Pumpetid (seneste) 29.8 meter u.t. 1/ m³/t 1.6 meter 1.5 time(r) ø254 ø160 meter u.t. Klimastratigrafi Dannelsesmiljø 0 ml LER, sandet, svagt gruset, gulbrun, kalkholdig, "moræneler". Prøve udtaget ved 5 m. 0 glg gl 5 7 ml LER, sandet, svagt gruset, gråbrun, kalkholdig, "moræneler". Prøve udtaget ved 12 m ml LER, sandet, svagt gruset, mørk gråbrun, kalkholdig, "moræneler". Prøve udtaget ved 20 m Sandy clay? ds 22 SAND, mest mellem, svagt gruset, lys gulbrun, kalkfri, "smeltevandssand". Prøve udtaget ved 25 m. 25 ds 30 SAND, mellem og groft, svagt gruset, lys gulbrun, kalkfri, "smeltevandssand". Prøve udtaget ved 35 m f ortsættes..

15 Borehole descriptions

16 Borehole descriptions Clay thickness [m] 1 km

17 Model grid 1 km

18 SSV inversion Iterative procedure Least squares solution: Q=(d obs -d for ) 2 /Δ 2

19 Resulting clay thickness 1 km Clay thickness [m]

20 Fit with the boreholes Clay thickness [m] 1 km

21 SSV what can it be used for? SSV provides optimized geophysical clay thickness maps integrating borehole information Important in the vulnerability mapping SSV incorporates uncertainties of geological information as well as geophysical information and interpolation SSV provides overview of Hundreds of boreholes Tens of thousands of geophysical models SSV is operational for large survey areas HydroGeophysics Group / SkyTEM booth

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