Depth to basement and seismic velocity structure from passive seismic soundings in central Australia

Abstract

SUMMARY We constrain the depth and seismic structure of stiff sediment cover overlying a prospective basement terrane using a passive seismic technique which uses surface wave energy from microtremor (also known as ambient seismic energy or seismic noise). This may be applied to mineral exploration under cover to decrease the inherent ambiguity in modelling potential field data for exploration targeting. Data from arrays of portable broadband seismometers are used to produce vertical profiles of seismic velocity structure using Both the Multimode Spatially Averaged Coherency (MMSPAC) method which measures the azimuthal average of the coherency between sensor pairs with a common separation, and the Horizontal to Vertical Spectral Ratio (HVSR) method, to estimate the seismic velocity structure and cover thickness respectively. We have developed field protocols to ensure consistent acquisition of high quality data in a variety of ground conditions. A wavefield approaching the theoretical ideal for MMSPAC processing is 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 theoretical maximum depth of investigation of 600m can be achieved with a hexagonal sensor array with 50m radius and both MMSPAC and HVSR, although the maximum thickness of sedimentary cover in the study area limits the depth of investigation to approximately 180m. The modelling procedure we employ is sensitive to layer thicknesses of ± 5 %.