Reduction of equivalence in layer-model interpretation by combination of electrical resistivity soundings and electromagnetic conductivity measurements; some case histories in groundwater survey

An interactive programme for microcomputers has been developed and has been used for rapid interpretation of electrical resistivity soundings (conventional Schlumberger configuration) and frequency domain electromagnetic conductivity measurements at low induction numbers (vertical and horizontal loop configuration; Geonics EM 343).

Layer model interpretation can be improved by simultaneously calculating the electrical resistivity sounding response and the electromagnetic conductivity response. Whenever discrepancies occur between the measured electromagnetic data and the calculated data the resistivity layer-model interpretation has to be matched, until a model has been found that fits both the electrical and the electromagnetic measurements. In this way, equivalence in interpretation of electrical soundings can be reduced.

Calculation of ‘imaginary EM 34-3 sounding curves’ makes it possible for the surveyor to predetermine the optimal configuration of the electromagnetic system of measuring variations in thickness or conductivity of a target layer. An example is given.

Two recent case histories are discussed in which equivalence was eliminated: one concerning the delineation of a pollution plume (The Netherlands), the second concerning the determination of depth to basement (Sudan). For groundwater surveys in basement areas of Africa the combined application of electrical resistivity soundings and an electromagnetic system operating at low induction numbers can be very useful; the survey target in general is shallow and the electrical resistivities of the crystalline bedrock are high.