Using borehole electroseismic measurements to detect and characterize fractured (permeable) zones

In 1996, we measured Stoneley-wave-induced electrical fields in an uncased water well drilled in fractured granite and diorite near Hamilton, Massachusetts. Stoneley waves generated by sledgehammer blows to the surface casing produced a flow of pore fluid in permeable zones intersected by the borehole. In turn, this flow induced a streaming electrical field. Even though these electrical signals were very small (tens of microvolts), we were able to detect them using electrodes placed in the borehole, after power line and telluric signals were canceled by remote referencing and notch-filtering. Amplitude analysis of the electrical fields confirmed that they were induced by fluid flow in the fractured formation. The normalized amplitudes of these electrical fields correlate with the fracture density log and agree with the theoretical model for this electroseismic phenomenon. Our Blot-theory-based model predicts that borehole electroseismic measurements can be used to characterize permeable zones. According to this model, the normalized amplitude of the Stoneley-wave-induced electrical field is proportional to the porosity, and the amplitude-versus-frequency behavior of this electrical field depends on the permeability of a formation around a borehole.