SEISMO‐ELECTRIC EXPLORATION: EXPECTED SIGNAL AMPLITUDES¹

Abstract

For more than 20 years, Soviet scientists have published papers and registered patents describing the conversion of seismic to electromagnetic energy in geological environments and the detection of the electric or magnetic signals as a method of geophysical exploration. Because of the potential importance of a reliable geophysical technique for locating quartz veins, we have been conducting extensive laboratory and field tests of the phenomena. For the purposes of designing appropriate field tests we need to know approximate signal magnitudes, but little has been published on them. The present paper describes a simplified model from which order‐of‐magnitude estimates of expected electric and magnetic signal strengths can be made with sufficient accuracy for such purposes. For mathematical convenience we model the target as a homogeneous sphere in which the seismic input induces uniform, time‐varying electric polarization. More realistic configurations can be described by linear superposition of the potentials of appropriate sub‐elements. True piezo‐electricity is, by definition, linear. Therefore, responses should have the same frequency content as the seismic input. Combining the low‐frequency form of our results with the assumption that the entire thickness of a target vein responds in phase to a seismic excitation, we obtain the following estimates of the maximum electric and magnetic fields at a distance r from the target: where p is the resistivity and μ0 the dielectric constant of the ground in which E is measured, V is the effective volume of the target, P is the polarization of the vein, s is the seismic stress at the target, and a is the effective piezo‐electric coefficient. Signals observed in experiments at the Erickson Mine, British Columbia, gave electric fields in acceptable agreement with our theoretical predictions. We conclude by considering plausible relationships for the high‐frequency signals observed from sulfide minerals, assuming that they represent the release of stored stress triggered by the seismic arrival.

Temporal variations in magnetic signals generated by the piezomagnetic effect for dislocation sources in a uniform medium

“…Pride & Haartsen 1996;Haartsen & Pride 1997;Garambois & Dietrich 2001, 2002Ren et al 2012;Gao et al 2013a,b), motional induction (e.g. Gershenzon et al 1993 & Barker 1991;Ogawa & Utada 2000a,b). However, these studies have not covered the entire possible range of EM sources in the Earth's crust; thus, studies of unconsidered mechanisms are still required to fully characterize elastic-EM conversions due to seismic waves.…”

Experimental studies of electrokinetic conversions in fluid‐saturated porous medium

“…Theoretical studies have demonstrated electrokinetic effects in porous media (Kozak and Davis, 1987;Neev and Yeatts, 1989;Thompson and Gist, 1991;Pride and Morgan, 1991;Pride et al, 1993;Haartsen, 1995). Field tests have also observed seismoelectric effects (Martner and Sparks, 1959;Sobolev et aI., 1984;Russell and Barker, 1991;Thompson and Gist, 1993). Because of the low signalto-noise ratio in field tests, intensive data processing is needed to extract the electric signals; it is difficult to compare the field results to the theoretical calculations.…”

Electromagnetic signals related to incidence of a teleseismic body wave into a subsurface piezoelectric body

“…As for the prospecting of mineral piezoelectric crystals, Kepic et al (1995) estimated the distribution of sulfide minerals by simultaneous measurement of the acoustic wave and the electric field during an explosion experiment, which indicated that in-situ electric signal detection is possible under a special condition. Russell and Barker (1991) also numerically estimated amplitude of seismo-electric signal presuming a piezoelectric sphere embedded under the ground, expected in seismo-electric exploration. It showed consistency between amplitudes of the observed and expected piezoelectric signals.…”