Transient electromagnetic responses of high-contrast prisms in a layered earth

Abstract

A numerical solution has been developed for transient electromagnetic (TEM) responses of prisms in a layered host. The solution is based on the method of integral equations, where the prisms are replaced with an equivalent scattering current. This scattering current is approximated with pulse and divergence-free basis functions. The divergence-free functions model eddy currents closed in the prisms, and hence simulate the inductive responses of the prisms in a very resistive host. Checks on the solution show that it is valid for conductive and resistive hosts, including a free-space host. The masking effect of conductive overburden is shown to delay and suppress the three-dimensional (3-D) TEM response of a conductor. After the overburden response is removed, an interpretation of the conductor with free-space models is a poor approximation when the basement rock is conductive. Instead of an exponential decay at late times, the conductor's response decays as an inverse power law. When the basement resistivity is increased, the conductor exhibits an exponential decay at late times. We have used the solution to gain interpretational insight into the lateral resolution of multiple conductors with different TEM surveys. The resolution of multiple conductors is very poor in a fixed-loop survey, but in a central-loop survey the resolution is better, provided the data are interpreted at early times. At later times, multiple conductors may not be resolvable and interpretational ambiguities could arise in a central-loop survey.