Seismoelectric experiments in a natural laboratory: comparisons of theoretical and measured transfer functions for co-seismic arrivals.

摘要

The verification of transfer functions, describing the frequency dependence of seismoelectric effects generated by P-waves in homogenous poroelastic media, and their dependence on various physical properties, is of fundamental importance in developing possible applications for seismoelectric phenomena. To test theoretical transfer functions at frequencies achievable in the field, co-seismic seismoelectric effects were measured in two boreholes at the Boise Hydrogeophysical Research Site (BHRS).;Transfer functions calculated from co-located measurements of seismic and seismoelectric arrivals were compared to two independently derived theoretical transfer functions reported in the geophysical literature. Analyses presented in this study show that the two theoretical formulations are similar at low frequencies. They differ, however, in that the transfer function based on a more complete set of governing equations includes components to account for inertial effects on relative fluid flow and to account for frequency dependence in the electrokinetic coupling coefficient.;Both of the theoretical transfer functions predicted seismoelectric to seismic amplitude ratios that exceeded observed ratios by less than an order of magnitude. Predictions of the more sophisticated model were superior---differing from observed ratios, on average, by a factor of 2.5, which is considered to lie within the range of experimental error given uncertainties in some of the physical properties. Curiously, observed ratios were less sensitive to changes in porosity and electrical conductivity than predicted by theory; it is unclear whether this discrepancy is a result of inadequacies in the theoretical models or in our knowledge of the physical property variations at the site. In agreement with theory, transfer functions calculated as ratios of observed electric field to observed particle acceleration display relatively flat profiles over the frequency bandwidth of approximately 100 - 500 Hz achieved during this experiment. However, observed bandwidths were not sufficiently broad to reveal the permeability dependence that is expected to occur at slightly higher frequencies.