Polarisation and slowness of seismoelectric signals: a case study

In saturated porous media, seismic compressional waves can generate electric and electromagnetic signals of observable amplitudes. These signals can be recorded with dipoles consisting of two electrodes each. As the generation of seismoelectric signals is connected with properties, such as hydraulic permeability, porosity, and fluid salinity, it is possible that the seismoelectric method could be used in hydrogeophysics for determining these parameters. However, the seismoelectric method still remains in the experimental stage. This is mainly due to the difficulties involved in seismoelectric measurements. We present repeatable seismoelectric field data with exceptionally high signal-to-noise ratios and prove that the assumed seismoelectric signals are not artefacts but the desired signal converted from seismic compressional waves at subsurface boundaries. Their amplitude distribution, arrival times and slownesses, as well as their polarisations, are checked against theoretical predictions and comparisons with other methods, such as refraction seismics, borehole logging data and downhole seismics and seismoelectrics. Conventional velocity filters in the frequency–wavenumber domain accentuate the seismoelectric signals converted at subsurface boundaries from incident seismic compressional waves, differentiating them from the usually less informative seismoelectric waves travelling inside compressional waves and being confined to these waves. The recording of three-component seismoelectric data enabled us to perform a polarisation study showing that the polarisations of the seismoelectric signals encountered are mainly found on the radial and the vertical components and thus follow the predictions. The three-component data suggest the existence of a composite signal from a thin layer which would have remained undetected with one-component measurements. Repeatable signals were also encountered on the transverse component which we interpret as 3D effects of a layer dipping in the transverse direction.