oa Crosshole Seismoelectric Measurements in Borehole Models with Fractures

Laboratory experiments are performed in cross-borehole models with fractures to investigate seismoelectric conversions in the fractures. A seismic wave propagating in a fluid-saturated porous medium moves ions in the double layer between the fluid and solid and induces an electric field. When there is discoutinuity (such as a fracture), the seismic wave induces a radiating electromagnetic (EM) wave. We measure the electric field with an electrode in a borehole, which is induced by an acoustic wave, and investigate the relationship between the electric signal and the fracture aperture with ultrasonic borehole models with a fracture. The experimental results confirm that a radiating EM wave is induced by a guide wave at a fracture between an acoustic source and electric receiver boreholes. The position of a vertical or inclined fracture between two boreholes can be determined with the arrival times of the EM wave and the formation velocity by placing the acoustic and electric receivers in the second borehole. Because the seismoelectric measurement is based on the seismoelectric conversion induced by acoustic waves, crosshole seismoelectric measurements might be a new means to investigate a fracture between two boreholes more directly than traditional acoustic measurements. It may also be used to construct a tomography with the traveltime extracted from the electric signal in a borehole similar to traditional crosshole acoustic tomography.