Modelling gravity and magnetic gradient tensor responses for exploration within the regolith

Abstract

The regolith obscures much of Australia?s bedrock geology, posing problems for mineral exploration under cover. Gravity and magnetic gradient tensor data can provide significant improvements over established potential field techniques by producing maps showing subtle variations in field data which relate to the subsurface geology, but which are hidden to standard total or vertical field measurements. We examine the forward potential field response of a complex three-dimensional regolith model that contains targets such as palaeochannels, land mines, and mineralisation. By summing the field responses from many small elementary cubes, it is possible to represent complex structures to yield the full gradient tensor response at a specified height above the ground. The gravity gradient tensor data from the model ranges over values from ?0.12 to 0.2 Eötvös, and the magnetic gradient tensor data for the model ranges from ?1000 to 600 nanoTeslas per metre (when measured at the surface), over typical regolith-related features. When a flight height of 80 m is used, the responses diminish considerably, and only the long-wavelength features of the model are detectable. Model response values are compared to the measurable responses from existing acquisition systems, and it is shown that the magnetic gradient tensor case is most suited to regolith investigations. The resolution required for the gravity gradient tensor appears to be higher than is possible with current gravity gradiometers.