Spectral analysis of the horizontal loop electromagnetic profiling data over a conductive half-plane

Abstract

Most interpretive methods in geophysical prospecting involve the analysis of a single signal which may be a spatial or time domain record of a given deterministic geophysical variable representing the anomaly of interest. In the present development, the electromagnetic (EM) response resulting from the interaction of the fields due to a thin plate-like conductor and the horizontal loop EM prospecting (HLEM) system is the pertinent geophysical variable. The interpretation of ground EM response due to a conductive thin plate is carried out through Fourier analysis. The introductory EM response 1S based on the rudimentary model 1n which a thin conductive plate is simulated by a single current filament which lies at or near the top edge of the plate. The am~litude spectrum 1S derived from analytical expressions of the response of the above model through Fourier transformation. In the transform domain, highly simplified expressions result from which the depth and the quality of the conductive target can be estimated. In fact, depth can be determined directly from the slope of the amplitude spectrum which constitutes the first method of depth estimation herein described. A second method for estimating depth using harmonic.;..> analysis is described . This method is based on the value of the wave number at which the real component of the Fourier transform drops off to zero. It is demonstrated that this value depends on the depth of the wire model if the coil separation 1S kept constant. Spectral information on depth is available at zero or higher harmonics. This method yields two depths (making it hard to choose the correct one) and may be effectively use 1n areas where depth of occurrence of the orebody is roughly known. The use of the slope method to get a rough estimate of depth and the final determination thereof using the harmonic method are described.