Cover depth and seismic velocity structure along a transect in central Australia from passive seismic soundings.

Abstract

The depth and seismic structure of stiff sediment cover overlying a prospective basement terrane were constrained with a passive seismic technique which uses surface wave energy from microtremor (also known as ambient seismic energy or seismic noise). Such constraints are very useful for mineral exploration under cover to decrease the inherent ambiguity in modeling potential field data for exploration targeting. We use data from small arrays of portable broadband seismometers, processed using both the Multimode Spatially Averaged Coherency (MMSPAC) method and the Horizontal to Vertical Spectral Ratio (HVSR) method, to produce profiles of seismic velocity structure along a 12 km transect. Wavefield conditions approaching the theoretical ideal for MMSPAC processing are created by combining the energy content of an off-road vehicle, driven around the seismometer array, and ambient sources. We find that this combination results in significantly higher quality MMSPAC waveforms in comparison to that obtained using ambient energy alone. Under ideal conditions a maximum depth of investigation of 500 m with a sensitivity to layer thicknesses of ± 5 % can be achieved with a hexagonal sensor array with 50 m radius. A high velocity layer, if present in the sediment package, causes a reduction in the quality of the observed waveform frequency range which is sensitive to deeper structure. This can limit the model sensitivity for underlying layers but may be addressed by detailed analysis of the HVSR peaks. Microtremor recordings including off-road vehicle noise, combined with the MMSPAC and HVSR processing techniques, may therefore be used to constrain sediment structure and depth to basement in a cost effective and efficient method which could contribute greatly to future mineral exploration under cover.