Case studies on the application of passive seismic horizontal to vertical spectral ratio (HVSR) surveying for heavy mineral sand exploration

Abstract

Summary This study presents the results from several case studies on the application of passive seismic Horizontal to Vertical Spectral Ratio (HVSR) surveying methods for Heavy Mineral Sand (HMS) deposit subsurface layer detection for exploration and mining. The results from these case studies demonstrate the usefulness of this rapid and low cost survey method to complement HMS deposit mapping and its ability to provide additional stratigraphic information in gaps between drillholes. HMS deposits typically occur in geological settings that are ideal for the application of the passive seismic HVSR method, because HMS deposits are typically shallow and may demonstrate acoustic impedance contrasts relative to surrounding sedimentary deposits or underlying acoustic bedrock. Trial HVSR survey results vary between different styles of heavy mineral sand deposits, from providing a direct estimate of the depth to the top of known HMS mineralisation based on a positive HVSR response from more dense and higher velocity HMS lenses, to detecting parallel silt and clay horizons, sometimes producing an inverted HVSR response, to be used as a bounding marker horizons for HMS deposits, and in many cases detecting the acoustic hard rock basement forming the base to the unconsolidated, young sedimentary deposits and basin fill containing HMS layers. In each case study, the use of a lightweight, self-contained and simple to use seismometer has allowed HMS explorers to carry out surveys quickly and cost effectively, in some remote areas with difficult access, mostly using company field staff following a short training session. The techniques and approaches to process and model HVSR data for shallow stratigraphic mapping during these trial surveys have contributed to advancing the passive seismic HVSR surveying method to become more commonly used for large production surveys.