SEISMOELECTRIC LABORATORY MEASUREMENTS IN A BOREHOLE

The seismoelectric logging method is based on measuring the electric field generated by seismic waves in a fluid-filled borehole. Two kinds of electromagnetic (EM) fields can be generated within the formation and at the interface of formations. One is a stationary or local EM wave and the other is a radiating EM wave. In this paper, we make various fractured borehole models with artificial materials or natural rocks and measure the electric field generated by a seismic source in a water-filled borehole. The experimental results show that the Stoneley wave generates both a stationary EM wave at the borehole wall and a radiating EM wave on the fracture, which propagates with light speed in the borehole. When the aperture of the fracture increas~s, the amplitude of the seismoelectric wave decreases due to the low ion concentration in the fracture. In a layered borehole model, a thin, permeable glued-sand zone is sandwiched between two nonpermeable or low-permeable layers, and the Stoneley wave generates two kinds of seismoelectric signals at the permeable zone. Compared with the acoustic waveforms in the same borehole, the seismoelectric waveforms are more effective in determining and characterizing a fracture or a fractured zone filled with a permeable layer