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, Massachu setts. Stoneley waves generated by sledgehammer blows to the surface casing produced a flow of pore fluid in permeable zones intersected by the boreho le. In turn, this flow induced a streaming electrical field. Even though th ese electrical signals were very small (tens of microvolts), we were able t o 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 induce d by fluid flow in the fractured formation. The normalized amplitudes of th ese electrical fields correlate with the fracture density log and agree wit h the theoretical model for this electroseismic phenomenon. Our Blot-theory-based model predicts that borehole electroseismic measureme nts can be used to characterize permeable zones. According to this model, t he normalized amplitude of the Stoneley-wave-induced electrical field is pr oportional to the porosity, and the amplitude-versus-frequency behavior of this electrical field depends on the permeability of a formation around a b orehole.