3D FD RESISTIVITY MODELING AND SENSITIVITY ANALYSES APPLIED TO A HIGHLY RESISTIVE PHONOLITIC BODY

A highly resistive phonolitic body near Sainerholz/Westerwald in Germa ny has been investigated for geological mapping using vertical electri cal soundings in a Schlumberger configuration. Because of its explicit ly three-dimensional shape, conventional 1D and 2D interpretation tech niques are not applicable. Therefore, a new 3D finite-difference forwa rd modelling algorithm has been applied to acquire information about i ts subsurface structure and to explain the observed data. This investi gation focuses on two exposed soundings: one located near the centre o f the body and the other close to its rim. For the interpretation, dat a from electromagnetic measurements on the lateral extension of the bo dy are additionally taken into account as well as geological a priori information. A possible 3D conductivity model is presented and evidenc e for its validity is discussed using model studies and sensitivity an alyses. The latter are carried out using a newly developed 3D FD sensi tivity modelling code with which the total subsurface response can be decomposed. This permits the determination of the resolution of model parameters, indica ting the contribution of different parts of the mod el to the overall response. The results emphasize the feasibility of 3 D forward modelling in practice.