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WARD, S. H. Et al. Multispectral electromagnetic exploration for sulfides. Geophysics, v. 39, n. 5, p. 666-682, Oct. 1974.

RESUMO

A 14 frequency electromagnetic system has been developed and applied to continuous sounding-profiling over the two sulfide deposits of the Cavendish Test Site in Ontario, Canada. The tilt of the major axis, till angle, and the ratio of minor to major axis, elliplicity, of the ellipse of magnetic field polarization are measured over the frequency range 10.5 hz to 86, 016 hz. A dual coil receiver employing a phase-sensitive detector has been used to measure tilot angle and ellipticity with three transmitter configurations along a single traverse line of the Cavendish test site. These different transmitting coil configurations are referred to as horizontal coil, vertical axial coil, and vertical rotating coil. In the horizontal coil configuration, tilt angle and ellipticity are measured in the vertical plane passing through the axis of a horizontal transmitting coil. In the vertical axial coil configuration, tilt angle and ellipticity are measured in the vertical plane which passes through the axis of a vertical transmitting coil; the axis of the transmitting coil is oriented normal to the strike of the subsurface conductivity inhomogeneity. Finally, in the vertical rotating coil configuration, the transmitting coil is located along strike from the traverse over the conductivity inhomogeneity and rotated to contain each point of observation in turn; measurements of tilt angle and ellipticity are made in a vertical plane normal to plane of the transmitting coil. The limitations of single-body interpretation schemes are illustrated by the data. Recourse  to calculation of scattering by two – and three – dimensional complex environments will be required to provide any sort of full interpretation of these data from one simple Precambrian geologic setting. A comparison of the results for the three transmitting coil configurations indicates that the vertical axial coil configuration is least sensitive to overburden and host rock, while adequately responsive to concentrated and disseminated sulfides.

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