Mapping the Regolith using Seismic Refraction and Magnetic Data: Results from the Southern Cross Greenstone Belt, Western Australia

Seismic and magnetic data were collected along a 4-km traverse which crosses a contact between greenstone and granitoid rocks about 1 km north of the Nevoria Gold Mine in the Southern Cross Greenstone Belt, Western Australia. The locations of magnetic sources were determined from the magnetic data by Werner deconvolution. First-arrival traveltime data allow the recognition of two time-distance branches, indicative of two layers. The depth to the refractor (top of the second layer) was determined using the plus-minus method. In the greenstones, the refractor coincides with a zone of magnetic sources, about 30 m wide, which occurs at depths of between 5 and 50 m. In the granitoids, the refractor coincides with the base of a broad zone of magnetic sources that extends from the surface to depths of between 20 and 100 m. These results are provisionally interpreted in terms of contrasts in magnetic susceptibility and seismic velocity at the base of the regolith, suggesting that the protolith-regolith interface can be mapped using the magnetic and seismic refraction methods.

A section through the regolith and into the protolith is exposed in the Bottle Dump Pit of the Nevoria Gold Mine. Measurements of magnetic susceptibility and density have been made on samples of amphibolite and banded-iron formation from different depths in the pit. These data yield evidence for the postulated increase in magnetic susceptibility and density, and therefore probably seismic velocity, at the interface between the regolith and protolith.

However, the contrasts in physical properties are small because the regolith in the pit is unusually fresh for the area.

These data imply that magnetic profiling may provide a cheaper alternative to seismic refraction studies as a method to map the extent of sub-surface weathering.