From resistivity to clay thickness – An inversion ...

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XVI International Conference on Computational Methods in Water Resources (CMWR-XVI) Ingeniørhuset

From resistivity to clay thickness – An inversion approach
Author:Anders Vest Christiansen <> (Post. Doc)
Kurt Sørensen <> (Associate Professor)
Esben Auken <> (Associate Professor)
Presenter:Anders Vest Christiansen <> (Post. Doc)
Date: 2006-06-18     Track: Special Sessions     Session: Hydrogeophysical data fusion

We present a concept integrating information from geophysical resistivity models with reported clay thicknesses in boreholes to produce optimized clay thickness maps. The concept use geostatistical estimation and non-linear inversion to optimize a function translating geophysical resistivity models to geophysical clay thickness. The concept minimizes the difference between reported clay thicknesses in boreholes and calculated clay thicknesses based on geophysical clay thickness models. We call it geoStatistical estimation of Structural Vulnerability (SSV). For sedimentary areas the cumulated clay thickness in the upper part of the subsurface is an important factor for the water infiltration speed and thereby the vulnerability of underlying aquifers to pesticides, nitrate etc. Borehole information contains the most detailed information on the clay thickness, but most often borehole information is too sparse for the detail level required in actual mapping situations. However, the amounts of clay are also reflected in the resistivity of the sediments and geophysical data, on the other hand, has the desired spatial coverage. In short, the concept incorporates: 1. Clay thicknesses in boreholes cumulated for some interval AND accompanying uncertainty measures. 2. Layered models obtained from inversion of geophysical data including the parameter uncertainties. 3. A spatial interpolation (kriging) taking the geophysical measurements to the locations of the boreholes for comparison. The uncertainties on the geophysical models are carried through the interpolation together with the uncertainty on the interpolation itself. 4. A non-linear inversion scheme minimizing the difference between observed clay thicknesses and calculated clay thicknesses In this way the SSV concept act as a hands-off interpolator between the boreholes translating the geophysical models to geophysical clay thickness. The presentation summarizes the central aspects of the SSV concept and discusses the various uncertainties. Finally we show an example in which SSV was used to estimate the geophysical clay thickness in a survey in Denmark.