Fast imaging of TDEM data based on S-inversion

Fast S-inversion is a method of interpretation of time-domain electromagnet ic (TDEM) sounding data using the thin sheet model approach. Within the fra mework of this method, the electromagnetic (EM) response measured at the su rface 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 conductanc e, S, and depth, cl, of the equivalent thin sheet. We analyze two different numerical techniques, the differential S-transformation and the regularize d S-inversion, to determine the parameters of the thin sheet. The first tec hnique is a direct differential transformation of the observed data into co nductance and depth values. It is fast and requires no iterations or starti ng model. The second technique uses a regularized inversion scheme to fit t he measured response with that of a thin sheet. In both techniques, the ret rieved conductance values are differentiated with respect to depth to obtai n the conductivity change with depth. We apply S-inversion to three-dimensi onal synthetic data and we successfully locate the local conductors. We als o demonstrate a case history by interpreting TDEM data obtained at the Noji ma fault zone in Japan. The results clearly indicate the location of the fa ult zone. (C) 2000 Elsevier Science B.V. All rights reserved.