Layered model inversion of central-loop EM soundings near a geological contact

Abstract

Central-loop time-domain EM is a widely used method for depth sounding due to its relative insensitivity to lateral changes in conductivity. Near a geological contact, however, this method produces distortions which are not easily distinguished from the layered-model response that is sought by inversion. In this paper we examine this problem for a simple two-dimensional model. Central-loop TDEM data were collected over a vertically dipping quarter-space model using a scale model system developed at U C Berkeley. Both vertical and horizontal magnetic field measurements were made within 250m radius loop transmitters with soundings spaced from 50 to 200m apart along a profile orthogonal to the contact. The sounding data from individual stations were then fitted to layered models using a standard least squares algorithm and the resulting layered models were plotted as a pieced-together cross-section. For the vertical field data the contact effect appears as a loss in field strength at lag times that depend on the distance from the contact and the conductivity of the quarter-space. At stations more distant from the edge the fields are reduced only at later times, whereas closer to the contact the effect shifts to earlier times and increases in magnitude. The layered model inversions seem to interpret this distortion as a fictitious resistive layer that grows more shallow and more resistive as the contact is approached. For stations within one loop radius of the contact the entire sounding is affected and the layered model inversions show little similarity to the true section. The scale model data also indicate that horizontal magnetic fields in the centre of the loop are diagnostic of the contact effect. These data may be used to determine which parts of a sounding may be confidently used for inversions. We have empirically found that for values of the TDEM tipper (Hx/Hz) greater than 0.5 the layered-models produce unreliable results.