Exploration Geophysics - Volume 8, Issue 2, 1977

The placement of an in-hole electrode array into a mineral exploration drill hole gives rise to complicated geometric factor (G.F.) relationships. For the same arrays placed at the surface of the earth the G.F. relationships are relatively simple. This paper presents a set of nomograms giving the G.F. for three commonly used exploration logging arrays: the two (normal) array, equispaced three array and dipole-dipole array (n=1), for inclined drill holes. The nomograms may be used to compute the G.F. for drill holes of constant inclination or for deviating drill holes, by considering the deviating drill path as a series of segments of constant inclination. The disturbing influence of the air/earth interface on the G.F. for the in-hole arrays is least for the dipole dipole array and greatest for the two-array.

Currently ground pulse EM surveys using the Crone PEM system are being used throughout the world in the search for massive sulphide deposits.

Survey results from Canada, Arizona, the Sultantate of Oman and Australia demonstrate the ability of the PEM method to delineate in detail a buried conductive target.

A detailed PEM survey method using vertical loops has also been developed. This method permits accurate determination of the depth, dip and the location of the edge of a conductive target. The vertical loop PEM method has been found particularly useful when checking and defining deep conductors which produce only marginal anomalies.

Under construction at present is a borehole PEM system with a depth capability of 1000 metres. An analog model study of this system has also been recently completed. Examples from this study show how the responses of the borehole PEM system change as the borehole moves from intersection with a conductive target to well outside the target.