We have acquired a [Formula: see text] seismoelectric section over an unconfined aquifer to demonstrate the effectiveness of interfacial signals at imaging interfaces in shallow sedimentary environments. The seismoelectric data were acquired by using a [Formula: see text] accelerated weight-drop source and a 24-channel seismoelectric recording system composed of grounded dipoles, preamplifiers, and seismographs. In the shot records, interfacial signals were remarkably clear; they arrived simultaneously at offsets as far as [Formula: see text] from the seismic source. The most prominent signal was generated at the water table at a depth of approximately [Formula: see text] and had peak amplitudes on the order of [Formula: see text]. A weaker response was generated at a shallower interface that is interpreted to be a water-retentive layer. The validity of these two laterally continuous events, and of other discontinuous events indicative of vadose-zone heterogeneity, is corroborated by the presence of reflections exhibiting similar characteristics in a ground-penetrating radar profile acquired along the same line.

[1] 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 mV/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.

[1] Seismoelectric signals have been measured as a function of depth in a borehole penetrating glaciofluvial sands, silts, and glacial till using a broadband surface seismic source, and a downhole electrode array. Transient electric field pulses, with amplitudes of 1 to 4 mV/m accompanied the arrival of seismic P-waves at the electrodes but no simultaneous interfacial signals were observed above the noise floor of approximately 0.2 mV/m. The co-seismic effect was strongest in a sand and gravel layer where its amplitude is consistent with the predictions of a simplified theoretical model. Normalization of the amplitude logs by measurements of seismic particle velocity and electrical conductivity enhanced their sensitivity to changes in lithology and porosity. The results of this experiment suggest that co-seismic seismoelectric effects show potential as a porosity/permeability logging tool in the borehole environment.

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