Benchmarking Passive Seismic Cover Depth Assessments

Abstract

Passive seismic techniques utilise the properties of ambient seismic waves to infer information about the structure of the subsurface, namely the depth(s) at which significant impedance contrasts occur. Geoscience Australia has recently conducted three passive seismic surveys to assess the suitability of two passive seismic methods, the horizontal over vertical spectral ratio (HVSR) and spatial autocorrelation (SPAC) techniques, for estimating cover depth over crystalline basement. Both techniques rely on ambient seismic noise in the form of surface waves. The HVSR technique involves measurement of the horizontal and vertical components of ambient seismic noise at an individual site. Where an impedance contrast exists, a maximum is observed in the HVSR value at a frequency directly dependent on the interface depth. The SPAC technique utilises dispersion observed in surface waves, which is also dependent on interface depth. Shear wave subsurface velocity profiles are constructed through inversion of the HVSR and/or SPAC curves.

The logistically simpler HVSR method, requiring only one seismometer, was found to produce estimates with significantly lower error than estimates from SPAC for depths up to 300 m in the Murray Basin, where unconsolidated to semi-consolidated Cenozoic sediments overly Paleozoic crystalline basement. Both HVSR and SPAC methods failed to resolve the target interface with the exception of one site in the Gawler Craton, located at a depth of 900m. Further work is planned for these data with different processing techniques. The HVSR technique produced estimates consistent with other geophysical techniques (airborne electromagnetic, refraction seismic, and magnetotelluric methods) for the majority of sites in the Thomson Orogen, where the Mesozoic Eromanga Basin, Cenozoic cover, and regolith associated with both these stratigraphic groups overly crystalline basement to a depth of up to 550 m. The accuracy of these profiles will be verified with stratigraphic drilling.

Results from these three surveys provide strong support for the passive seismic technique, in particular the single station HVSR method, as a highly effective and logistically low-cost and simple tool suitable for mapping cover depth in many regions of interest across Australia.