Fast imaging of TDEM data based on S-inversion

Fast S-inversion is a method of interpretation of time-domain electromagnetic (TDEM) sounding data using the thin sheet model approach. Within the framework of this method, the electromagnetic (EM) response measured at the surface of the earth at every time delay is matched with that of a thin sheet model. The conductivity change with depth is obtained using the conductance, S, and depth, cl, of the equivalent thin sheet. We analyze two different numerical techniques, the differential S-transformation and the regularized S-inversion, to determine the parameters of the thin sheet. The first technique is a direct differential transformation of the observed data into conductance and depth values. It is fast and requires no iterations or starting model. The second technique uses a regularized inversion scheme to fit the measured response with that of a thin sheet. In both techniques, the retrieved conductance values are differentiated with respect to depth to obtain the conductivity change with depth. We apply S-inversion to three-dimensional synthetic data and we successfully locate the local conductors. We also demonstrate a case history by interpreting TDEM data obtained at the Nojima fault zone in Japan. The results clearly indicate the location of the fault zone. (C) 2000 Elsevier Science B.V. All rights reserved.