A spatial and spectral analysis of the electromagnetic sensitivity in a layered earth

Abstract

Summary A sensitivity analysis of an electromagnetic (EM) technique can illustrate its theoretical resolution and is an essential step in using linearized inversion methods. Yet previous sensitivity studies in EM methods provide only a cursory examination of the complete, non-linear problem. We present a simplified analysis which illustrates the frequency characteristics and spatial variations of the EM sensitivity to buried layers in a stratified earth. The simplifications arise from ignoring the complicated geometrical effects of the sourcereceiver configuration and examining only the two fundamental response kernels which comprise the EM fields in a layered conductive earth. In this fashion, different EM prospecting methods can be compared and contrasted without dealing with specific aspects of survey methods. It is shown that although sensitivity is non-linearly related to conductivity, it is a qualitatively predictable function of frequency and source-receiver separation. Several general conclusions concerning electroprospecting can be derived in this manner. However, in all cases the realm (in space-frequency domain) of the maximum sensitivity in any layer is relatively small, suggesting that a diverse spatial and frequency sampling of the field is required in any high-resolution EM sounding survey.