3D FD resistivity modelling and sensitivity analyses applied to a highly resistive phonolitic body

A highly resistive phonolitic body near Sainerholz / Westerwald in Germany has been investigated for geological mapping using vertical electrical soundings in a Schlumberger configuration. Because of its explicitly three-dimensional shape, conventional 1D and 2D interpretation techniques are not applicable. Therefore, a new 3D finite-difference forward modelling algorithm has been applied to acquire information about its subsurface structure and to explain the observed data. This investigation focuses on two exposed soundings: one located near the centre of the body and the other close to its rim. For the interpretation, data from electromagnetic measurements on the lateral extension of the body are additionally taken into account as well as geological a priori information. A possible 3D conductivity model is presented and evidence for its validity is discussed using model studies and sensitivity analyses. The latter are carried out using a newly developed 3D FD sensitivity modelling code with which the total subsurface response can be decomposed. This permits the determination of the resolution of model parameters, indicating the contribution of different parts of the model to the overall response. The results emphasize the feasibility of 3D forward modelling in practice.