ISSN:
1365-2478
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
,
Physics
Notes:
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: 〈displayedItem type="mathematics" xml:id="mu1" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:00168025:GPR105:GPR_105_mu1"/〉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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-2478.1991.tb00304.x
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