Implications of a Spectral IP Survey at Elura

A spectral IP survey was conducted over the Elura orebody by Phoenix Geoph. Ltd. during August, 1979. The results of the survey were that (1) inductive EM coupling due to layering was strongly present from high frequencies down to 1 Hz; (2) a diagnostic phase-shift anomaly pattern due to mineralisation was not evident at low frequencies; and (3) a secondary dispersion due to mineralisation was not evident within the data.

The Elura mineralisation is contained within quite barren siltstone, beneath conductive overburden and should, geometrically, constitute a good electrical (conductive and chargeable) target – as was evidenced by recent time-domain IP surveys.

The discrepancy between these two techniques was investigated to ascertain the cause of this ambiguity.

1.  The Spectra of EM coupling in the uniform background were analysed to determine the resistivity and thickness of the conductive overburden. The results of this analysis agreed with resistivity sounding data, and was confirmed by forward modelling of the EM coupling spectra.

2.  It was noticed that the EM coupling decreased over the mineralisation. Simple subtraction of observed phase responses from the background value, at frequencies from 243 Hz to 3 Hz, produces a positive phase shift anomaly pattern. This pattern is similar to the IP anomaly pattern obtained in the time-domain data.

3.  The residual anomaly is believed to be a positive phase EM response due to conductive mineralisation in a less conductive host. Similar responses, but in greater magnitude, have been observed elsewhere in Australia but only recognised in areas of no overburden and generally higher background resistivities. A model study by Hohmann (1977), of a conductive target within a less conductive half space, produced similar results.

It is concluded that care should be taken in the analysis of spectral IP data in the presence of EM coupling. Analysis using the half-space approximation is not adequate to quantify EM coupling. EM coupling due to a very conductive target can have opposite sign to that for which some theories of spectral interpretation have been developed - especially analysis in log space. It is essential to record to as low a frequency as possible, and with as high a precision as possible, to obtain data of suitable accuracy for decoupling analysis. It may not be possible to extend the theory of discriminating massive sulphides, from their spectral response, to the field case if the mineralisation is sufficiently conductive and in such a geometry as to produce a positive phase EM coupling response.