Tracking the Diprotodon: microtremor passive seismic profiling as a tool for location of megafauna bone beds

Bone beds containing Pleistocene megafauna fossils dated to between 35ka and 60ka occur near Lancefield (Victoria). The bones lie within clays above gravels and extensive Quaternary basalt flows. Evidence that the bones have been subject to alluvial transportation suggests that profiling the basalt basement for paleo-channels will assist with locating further bone beds. Passive seismic (microtremor) methods, as developed variously for earthquake hazard studies and regolith studies, have been applied to this problem, using both HVSR spectral ratio methods and two-station SPAC (spatially averaged coherency) methods. Clay layers have shear-wave velocities (Vs) in the range 120-180m/s and thicknesses 1.5 to 2.5m. Microtremor data in the frequency band 5-60Hz provides excellent resolution of the Vs and thickness of the clay layers, allowing the bedrock profile to be established to an accuracy of 0.5m or better. The results indicate existence of soft clays some 40m west of an existing excavation site and thus identify a future site for excavation.