Apparatus for Physical Modeling of the Electroseismic Effect of the First Kind

Abstract

The paper describes a modified apparatus for physically modeling the electroseismic effect of the first kind in rocks. The apparatus makes it possible to simulate the effect when a rock sample is exposed to an electric field with and without current in the sample. Two methods can be used for measuring changes in the acoustic field propagation velocity. The first method excites a pulsed acoustic field and measures the time it takes for the field to propagate from the source to the receiver; the second measures the phase modulation of a sinusoidal acoustic field when the sample is exposed to an electric field. The first method has two modifications: in the first, only an electric field without an active current component is generated in a sample; in the second, both an electric field and an active current component are generated. The apparatus includes a signal generator that excites coherent electric and acoustic fields in rock samples. Field coherence makes it possible to apply an interference-immune phase method for measuring the velocity of a sinusoidal acoustic field and to decrease the sensitivity threshold for a change in velocity from 0.2 to 0.02%. The authors present results of modeling the effect using saltwater-saturated limestone and sandstone samples (four each) with a mineralization factor of 1%. When the electric field was switched on, all samples demonstrated an approximately 0.2% decrease in acoustic field velocity. In the 2–200 kHz frequency range, the velocity decrease does not depend on frequency for all limestone and sandstone samples. It is shown that the modified apparatus can reliably detect the electroseismic effect without a current in a sample, despite the fact that its value exceeds the sensitivity threshold by only 20–25 dB. Field coherence makes it possible to measure the relaxation time of the acoustic field velocity after an electric field without a current in a sample is switched on and off. The authors demonstrate that after the electric field is switched on, the relaxation time of the acoustic field velocity does not exceed 2 ms, and after it is switched off, this value is 10–20 ms. The relaxation time difference can be used to assess the nonlinearity of the electroseismic effect of the first kind.