Fast direct conductivity transforms for TEM systems

Abstract

In 1987, Nekut published in Geophysics a method that used the receding-image approximation of the time domain electromagnetic (TEM) response of a concentric loop system above a half-space to derive a simple, fast, direct transform that calculates resistivity as a function of depth. This method is by far the fastest of published transforms from TEM data to resistivity. Following this example, we make a further simplification that completely eliminates one intermediate step required by Nekut. His intermediate step was used to resolve differences between mirror depth (half the image depth) and the half-space diffusion depth. We simply use the half-space diffusion depth directly in Nekut?s receding image method without requiring a mirror-depth calculation and a further calculation of its associated correction. The result is an even faster direct resistivity transform method that exactly matches the published results of Nekut. A further conceptual advance is immediately clear: the fast direct resistivity transform can be expanded to other common survey geometries such as coincident squareand circular-loop TEM systems. This is achieved through use of the diffusion depth with either direct forward modelling of the half-space or the mirror approximation. We explore this conceptual advantage and give an example of direct resistivity transforms for the Slingram geometry commonly used in electromagnetic surveys.