ELECTROMAGNETIC COUPLING IN FREQUENCY AND TIME‐DOMAIN INDUCED‐POLARIZATION SURVEYS OVER A MULTILAYERED EARTH

Abstract

Electromagnetic coupling responses in frequency and time‐domain induced‐polarization measurements over a multilayered earth are evaluated. For collinear dipole‐dipole and pole‐dipole configurations over a dissipative layered subsurface, the percent frequency effects of electromagnetic coupling are seen to be as high as 60 percent for large [Formula: see text] values, where L is the length of the receiving dipole, [Formula: see text] is the conductivity of the top layer of the half‐space, and f is the higher frequency of excitation used. In both frequency and time‐domain analyses, the distinctive effects of layering compared to that of a homogeneous half‐space response are shown for different electrode configurations, layer geometry, and electrical parameters of the subsurface. The pole‐dipole configuration of electrodes, in general, exhibits higher coupling compared to the dipole‐dipole configuration. In time‐domain measurements, the late off‐time transient decays reflect almost entirely the normal polarizability of the layered subsurface, in that the coupling responses are significant only during the early off‐time of the transient. The mutual impedance between grounded dipoles of arbitrary length is computed by extension of the complete solution of the boundary‐value problem of a horizontal electric dipole situated over a multilayered half‐space. A number of nomograms are presented for various layered structures to eliminate the electromagnetic coupling response in the induced‐polarization measurements in order to obtain the true polarization effect of the subsurface.