Seismic instruments: article

V.A. Zeigarnik
V.N. Klyuchkin
Joint Institute for High Temperatures
Journal: Seismic instruments
Tome: 54
Number: 1
Year: 2018
Pages: 5-18
UDK: 550.83
DOI: 10.21455/si2018.1-1
Keywords: geophysics, apparatus, electric-seismic effect, physical modeling, electrical field
Аnnotation: Modified facility for physical modeling of electroseismic effect of the first kind in mountain rocks is described. The apparatus provides an opportunity to model electric-seismic effect upon exposure of a mountain rock sample to the electric field either with or without electric current in a sample. The apparatus enables the use two methods of measurement of acoustic field velocity change. First one is the excitation of pulsed acoustic field and measurement of this field propagation time from acoustic source to receiver. Second one is based on the measurement of phase modulation of sinusoidal acoustic field when a sample is exposed to electric field. The first method was implemented in two versions: first version - only electric field without active electric current component is generated within a sample, second version - both ones are generated. The apparatus includes a signal generator that induces coherent electric and acoustic fields within the sample. Coherence enables the use of noise-resistant technique for measuring the sinusoidal acoustic field velocity and decreasing the sensitivity threshold for velocity change from 0.2 % to 0.02 %. The results of modeling of the effect for four limestone samples and four sandstone samples saturated by salt water with mineralization 1% are described. For all of them a decrease in the acoustic field velocity about 0.2 % was obtained under electric field. The acoustic field velocity decrease within the interval 2-200 kHz is independent on frequency for all samples of limestone and sandstone. It is shown that modified apparatus makes it possible to confidently detect electric-seismic effect without current through the sample despite the fact that its value is above the sensitivity threshold by only 20-25 dB. The field coherence permits us to measure the relaxation velocity after electric field switching on or off if there is no current through the sample. It has been shown that, after electric field is switched on, the velocity relaxation time does not exceed 2 ms, and after its switching off, is 10-20 ms. Such a difference between velocity relaxation times forms the ground for estimating nonlinearity in the electroseismic effect of the first kind.