A Novel yet Simple Approach to the Interpretation of HVSR Data in Australia - A Data Rich Case Study from the Pilbara

Two passive seismic surveys were acquired at a location within the Hamersley Basin in the Pilbara region of Western Australia, to assist BHP with optimisation of both drill planning for managed aquifer recharge studies, and for iron ore exploration drilling. The geology of the target area consists of stratigraphic metasediments, which have been intruded, faulted, folded, eroded, and then mostly covered by thin to moderate thickness tertiary detrital cover. Following review of various airborne geophysical datasets, two passive seismic surveys were planned across the target location with the primary objective of better defining the thickness of detrital cover. The passive seismic surveys consisted of three-component nodal geophones being deployed in a rolling manner at ~3,400 stations. The data were processed using the empirical horizontal-to-vertical spectral ratio (HVSR) method. Interpretations from various apriori unpublished passive seismic surveys and trials across the Pilbara using this method have returned mixed results, with the consistency and continuity of HVSR horizons along and between survey lines being one of the primary concerns. When reviewing the processed HVSR data, it was observed that a HVSR trough/ resonance minimum typically produced a vertically more discrete and laterally consistent horizon, with more depth variability than the traditionally picked HVSR peaks. The trough horizon also produced steeper dipping and paleochannel-like features, which are known to exist in the survey area and were largely absent from the HVSR peak/ maximum horizons. The HVSR trough horizon was picked for all passive seismic stations, where present, and when gridded into a surface using half the picked frequency, it correlated well with various geological features previously interpreted from the airborne magnetic, gravity gradiometry and electromagnetic data. This surface is being used to inform managed aquifer recharge modelling and drill planning and optimise future exploration drill planning.