Anatomy of a seismoelectric conversion: Measurements and conceptual modeling in boreholes penetrating a sandy aquifer

摘要

Conversions of compressional seismic waves to electric fields have been measured in two boreholes drilled in an unconfined sandy aquifer on the Gnangara Mound near Perth, Australia. The seismoelectric conversions at both field sites occurred in the vicinity of the water table at 13-m depth and yielded maximum amplitudes of 1 μV/m using a sledgehammer source on surface. Partially cemented layers, inferred from geological and geophysical logs, straddle the water table and may play a role in generating the conversion and influencing its amplitude distribution. The dense vertical sampling used in these borehole experiments reveals spatial and temporal polarity reversals of the interfacial signal which provide new evidence in support of the conceptual model for seismoelectric conversions at interfaces. We demonstrate that the growth rate of the source zone and its maximum vertical extent below the water table are encoded in the polarity of the interfacial signal. These experiments confirm that vertical seismoelectric profiling can be used to gain further insight into seismoelectric conversions and characteristics of interfaces that makes them amenable to detection.