AN ELECTROMAGNETIC METHOD FOR RAPID BASIN EVALUATION AND SOME SPECIAL PROBLEMS

Abstract

The magneto-telluric method uses natural electro-magnetic fields to show the resistivity structure of the earth. Resistivity is then related to porosity as in well-log interpretation. The magneto-telluric (MT) method is widely used for rapid basin evaluation and for a range of special problems.

Two of these applications are illustrated by Australian examples. One is rapid inexpensive reconnaissance of a porous section overlying basement. The other is penetrating overthurst material and other situations that give rise to surface problems in reflection seismic surveys.

In an example from the Murray Basin, a –40 mgal gravity anomaly was easily shown to involve no more than 200-300 m of porous section over resistive basement.

A Cooper Basin example is cited to emphasise the flexibility of the method. Good data were obtained at a time of widespread flooding, when readings could only be done on high ground. It was neither possible nor necessary to have sites on continuous traverse lines.

Recent reflection work on the northern margin of the eastern Officer Basin has been able to resolve very little detail beneath the Musgrave Block overthrust. MT work disclosed the thin-skinned nature and a number of other structural features of this thrust, improving on an earlier gravity picture. However, south of the thrust the seismic section had much higher resolution. The combination appears to show a thick evaporite-carbonate zone.

MT research is continuing, with a survey in the Eromanga Basin late in 1980. However, for the technique to contribute to Australian exploration it will be necessary for commercial crews to begin work. Contract crews typically consist of five or six men with two or three vehicles. Overall monthly cost of such surveys is $10 000-$50 000, or roughly 10 per cent that of reflection survey and processing.