A model study of a thin plate in free space for the EM37 transient electromagnetic system

Abstract

The computer program PLATE, developed at the University of Toronto, models the electromagnetic (EM) response of an inductively thin plate in free space. We used PLATE to compute two components of the time derivative of the magnetic field for a range of models for the EM37 fixed‐source transient system ([Formula: see text] loop). Analysis of the response curves produced methods of interpretation for obtaining plate geometry and conductance. The overall width of an anomaly, the distance between peaks and the width of the updip lobes, can provide an estimate of depth. Dip has the dominant effect on the ratio of the peak amplitudes. A rough estimate of plate size and the position in time (early or late) of the currents is essential before proceeding with interpretation. Strike length is not obviously reflected in the shape of the curves, but depth extent is indicated by the rate at which the downdip tail returns to the baseline, except for vertical plates. For vertical plates, curve matching may be the only method of obtaining an estimate of depth extent. Varying conductance for a particular model in free space affects whether a channel represents an early, intermediate, or late time response. The shape of a profile varies with the time of measurement. The estimated time constant can be used to calculate the conductance, provided an estimate of the shortest dimension of the plate is available. Extinction angles appear frequently for plates of small depth extent but do not occur for plates which are of infinite strike and depth extent with respect to the size of the transmitting loop.