Self-Potential Changes Induced by Geotheiimal Fluid Production

Abstract

When a sink or source of fluid is presenl wilhin a reservoir as a resull of production of geothermal fluids, a surface electric potential anomaly can be prodoced through electrokinetic coupling if the following conditions are satisfied. First. there must be a houndary separating reF,ions of differing streaming potential coefficient C; second. ther'e mllst be a nonzero component of pressure gradient parallel to this boundary (Nourhehecht,1963: Filterman,1979).

A temperature boundary, a boundary between regions of different pore water chemistry. and/or a contact of different rock formations are considered as the most prohable candidates ror discontinuities in the value of C in a geothermal reservoir (Figure 1)(lshido and Mizutani 1981).

When a pressure change induced by production and/or inj~ction of fluids propagates to a houndary between regions of differing C in a reservoir (Figure 1). an electric putenlial anoma Iy appears at the surface through eleclroki net ic coup I ing. We can model the polential generating mechanism quantitatively using the formulation derived by Nourbehecht (1963) and Filterman (1979). If the boundary is nearly vertical. the anomaly will be dipolar In waveform (Fitterman,1979).