Borehole Electroseismic Measurements in Dolomite: Identifying Fractures and Permeable Zones

Measuring the electrical field induced by a borehole Stoneley wave is a new method for characterizing a rock formation around a borehole. Our field measurements demonstrate that the Stoneley-wave-induced electrical field can be detected in sedimentary rocks (dolomite in our experiment), and that the amplitude of this electroseismic phenomenon can be used to detect isolated fractures and permeable zones. INTRODUCTION Electroseismic phenomena allow direct observation of pore fluid flow driven through a rock formation by a seismic wave. Even though these seismic-wave-driven fluid flows are very small, it is possible to detect them in field experiments by measuring the streaming electrical fields that they induce. In turn, measurements of seismic-wave-induced pore fluid flows provide geophysicists with a new way to study rock properties. Laboratory experiments (Zhu and Toksiiz, 1997) and theoretical analysis (Haartsen, 1995; Mikhailov et al., 1998) suggest that borehole electroseismic measurements can be used to detect permeable zones. Our study is aimed at determining the practical value of borehole electroseismic phenomena for geophysical exploration. We focus our investigation on the electrical fields induced by a borehole Stoneley wave. In this paper we first demonstrate that this phenomenon can be observed in sedimentary rock formations in the field. Further, we compare the results of our electroseismic measurements with the results of other geophysical measurements and show that the normalized amplitude of the Stoneley-wave-induced electrical field can be used to detect isolated fractures and permeable zones intersected by a borehole.