Exploration Geophysics - Volume 18, Issue 4, 1987

VLF-EM prospecting equipment usually measure the tilt angle of the major axis of polarization from horizontal either in degrees or in percent and the ellipticity (in percent) of the polarization ellipse of magnetic field. However, the structure of the elliptically polarized field in the presence of local conductor allows the establishment of inter-relationship of the space components (amplitudes of the vertical and horizontal magnetic field components) and the tilt angle with the phase components (ellipticity or imaginary component), providing an opportunity to obtain the ellipticity without actually measuring with a phase shifting circuitary provision in the equipment. The validity of the inter-relationship under real field conditions thus obtained is demonstrated by field examples. Further, the field results show the applicability of the VLF-EM technique in detecting resistive targets also when electrical field components alone or the impedance measurements are made. However, the magnetic field components alone do not reflect the resistive target well.

This paper compares the usefulness of several types of detailed airborne and ground magnetic surveys, and a detailed gravity survey, in mapping the geology of the Corsair area, about 10 km east of Kalgoorlie. The gravity and magnetic surveys when interpreted together map out the distribution of mafics, ultramafics and sediments. The mafic rocks are only weakly magnetic, but they generate an easily observable gravity anomaly of about 20 µm.s ^-2. Surficial deposits are associated with both elongate and high-amplitude short-wavelength magnetic anomalies; their distribution can best be mapped as the area of short-wavelength anomalies in the unsmoothed observations (dappled on pixel maps). The magnetic field due to basement is best derived by attenuating the high-amplitude short-wavelength anomalies; this is most reliably done by estimating the mode, rather than the mean, of the observations. The airborne gradiometer survey gave superior results to both the aeromagnetic total intensity survey, and the ground magnetic survey, in that it contained the short wavelength information required to map the surficial deposits, and it had better detail of the anomalies due to the basement. The image of the gravity gradient provided the best representation of linears. Many of these linears are in part coincident with workings, so they are gold carriers. Known gold mineralization is preferentially developed along a sheared zone abuting the boundary between the two major basement rock associations; this zone corresponds with the major gravity gradient.

Thermal conductivity measurements have been made on core samples from the Eromanga/Cooper Basin sequence in northern South Australia. A divided bar apparatus was used and the measurements were made with the samples dry and saturated with water. The results are compared with predictions made from theoretical models of thermal conductivity in porous rocks although they do not unequivocally support any particular model. The samples are divided into sandstones, siltstones and shales with effective matrix thermal conductivities of 6.0, 3.5 and 2.5 WtrHK-¹ respectively and it is suggested that a suitable predictive model is the average of the Maxwell and geometric mean models. These two are effectively the upper and lower bounds for the models considered.

Clinoptilolite and mordenite and other zeolite bearing altered ignimbritic rocks have been subject to petrophysical laboratory studies in which textural, magnetic, and electrical properties were investigated. Zeolite-poor and barren rocks also were included in the study. Preliminary results indicate that zeolite-rich rocks manifest: temperature dependent apparent porosities, low grain and dry bulk densities, generally low magnetic susceptibilities, a range of electrical resistivities — possibly reflecting degrees of silicification, and a tendency to low phase angles. The density and porosity data characterise zeolitic rock quite well; magnetic and electrical data are not diagnostic but do provide useful supplementary information on the likely or possible degree of zeolitisation.

Procedures have been developed for the calibration of a SIROTEM down-hole probe, and for determining the correct polarity of connections to the probe in the field. An exponentially decaying signal derived from the calibration unit and driven into the transmitter loop in the same direction as required for field operations induces a similar signal in the probe placed inside this loop. By connecting the probe so as to obtain a positive response, the correct polarity of the connections to the probe is ensured before the beginning of a down-hole survey. Such a check may be applied equally well to any sensor other than a down-hole probe, e.g. to check the polarity of the connections to a roving vector receiver.

The frequency and phase response of the slimline down-hole probe has also been measured. The band-width is -20 kHz.