ELECTROMAGNETIC MODE CONVERSION BY SURFACE-CONDUCTIVITY ANOMALIES - APPLICATIONS FOR CONDUCTIVITY SOUNDINGS

A natural-source electromagnetic sounding of the earth made near a sur face-conductivity anomaly will resolve different features of the under lying conductivity structure than a sounding in a more uniform region. The surface-conductivity anomaly deflects horizontal electric current s induced by an external source into a vertical plane converting trans verse-electric (TE) mode currents into the transverse (TM) mode. The r esulting current distribution involves both vertical current flow and spatial variations with shorter wavelengths than the external field, p roviding increased resolution of resistive layers and of the conductiv ity structure at shallow depths. We examine the sensitivity of the con verted-mode response for the vertical-gradient sounding (VGS) method i n order to plan electromagnetic soundings in a narrow ocean strait suc h as the Strait of Georgia between Vancouver Island and the Canadian m ainland. An integral-equation method is used to model the current syst em induced by a mode converter, consisting of a known conductivity str ucture, such as a body of ocean water. It is shown that the depth of p enetration of the secondary current distribution produced by the mode converter depends on both the horizontal scale of the feature and the distance from its edge. Within this depth range the current system is strongly perturbed by the existence of either conductive or resistive layers. The sensitivity of the VGS response (the ratio of the horizont al magnetic field at the base and surface of the mode converter) is ex amined using forward modelling of layered conductivity structures. The response is found to be dependent on both the TE and TM current syste ms. For a narrow ocean strait such as the Strait of Georgia, a measure ment of the converted-mode VGS response along a line of sites on the f loor of the strait, will provide resolution of conductive and resistiv e layers in the upper 10 km. The appropriate frequency range over whic h the VGS response should be measured in the strait is 10(-2) Hz to 10 Hz. In our investigation of mode conversion we examine both the frequ ency- and time-domain response. Snap shots showing the current system evolving in the earth after a step or impulse illustrate the interacti on of the EM signals with resistive and conductive layers. We show tha t the time-domain response can be used in a 'geometrical sounding' ana logous to seismic refraction to determine the conductivity structure. Finally we examine the limitations on the accuracy of the frequency an d time-domain VGS response imposed by natural signal levels and instru ment sensitivity.