Preliminary Understanding of Near-Field Effect of CSAMT in Electric Azimuthally Anisotropic Half-Space

Abstract

This paper reviews the spatial characteristics of electromagnetic fields induced by a horizontal electric dipole on the surface of a horizontal layered earth with azimuthal anisotropy. It is well known that depth of investigation becomes independent of frequency and dependent upon array geometry in the near-field zone, thus limiting the interpretation of data by conventional MT methods. On the basis of existing theory, we estimate the far-field distance (FFD) of a homogeneous half-space with azimuthal anisotropy through numerical modeling. To define FFD, there are two criteria suggested in the study. If the anisotropic parameter is constant, the FFD is linearly increasing with resistivity increasing in strike direction for all operating frequencies. From comprehensive analysis, the FFD along the strike direction should be estimated by comparing a relative difference between the responses of CSAMT and MT in TM-mode; the FFD along the direction perpendicular to the strike should be estimated by comparing relative difference between responses of CSAMT and MT in TE-mode. In an azimuthally anisotropic half-space, the near-field effect is more severe in the strike direction than in its orthogonal direction. These simple but quantitative analyses aim to understand the near-field effect before inverting data using a far field approximation for CSAMT exploration in azimuthally anisotropic conductive media.