Microtremor methods applied to groundwater studies^*

Abstract

The Campaspe Deep Lead, which underlies the modern-day Campaspe River Valley in north-eastern Victoria, is an important local source of ground water. The exact location of the lead near Goornong is disputed, and it is consequently of interest to see whether a geophysical method can add information to the existing hydrological and geological data.

Two microtremor methods have been applied in this area. The horizontal to vertical spectral ratio method (HVSR) was used as a reconnaissance tool to gain information about the change in resonant frequency across the field area. Modelling of the HVSR data qualitatively estimated a basement depression below and east of the river. The passive seismic spatial autocorrelation (SPAC) method was applied at four locations in the vicinity of the aquifer to gain information about depth to bedrock and layer intervals as well as shear wave velocities. Unconstrained inversion of SPAC data resulted in similar basement topography, but overestimated by tens of metres. Constraining the basement depth to that found in nearby boreholes gives a relatively unchanged data misfit, and an approximate location of the vertical boundaries of the deep lead. Use of these methods in groundwater exploration would primarily be for cost-effective mapping of regolith and basement structure between boreholes, especially in saline overburden, land or urban environments.