Magnetotelluric Probing for Buried Geologic Structures

Abstract

Few subsurface geometric configurations have been studied in electromagnetic-induction prospecting. In particular, geophysicists have usually considered only models of the earth which are flat and which consist of parallel regions of contrasting electrical conductivity. In order to evaluate the electric and magnetic field over subsurface geologic structure, the earth is modeled by two conductive layers which are joined at an irregular interface. Maxwell's equations are then applied to obtain a set of integral equations for the electromagnetic field which are solved by using a perturbation technique. Horizontal electric field and apparent resistivity master curves are calculated for the case of H-polarization. These curves demonstrate that for a given subsurface structure and resistivity contrast, the horizontal electric-field anomaly is greater when the first layer is more conductive than the second layer than when the first layer is more resistive than the second layer. When the wavelength in the more conductive layer of the electromagnetic field is less than the width of a resistant anticlinal ridge, the horizontal electric field decreases over the ridge. On the other hand, when the wavelength is much greater than the width of a resistant ridge, the horizontal electric field increases over that ridge. Given the resistivity contrast and electric-field anomaly, the depth to the crest of a buried anticlinal ridge may be ascertained. Similarly, overburden thickness determinations accurate to within 10 percent can be made from electric-field anomalies provided that the width of the subsurface structure is small relative to the depth of the structure.