Streaming potential in porous media 2. Theory and application to geothermal systems

Self-potential electric and magnetic anomalies are increasingly being obser ved associated with hydrothermal fields, volcanic activity, and subsurface water flow. Until now a formal theoretical basis for predicting streaming p otential of porous materials has not been available. We develop here a mode l giving both the macroscopic constitutive equations and the material prope rties entering these equations. The material properties, like the streaming potential coupling coefficient, depend on pore fluid salinity, temperature , water and gas saturations, mean grain diameter, and porosity. Some aspect s of the model are directly tested with success against laboratory data. Th e streaming potential increases with temperature, grain size, and gas satur ation, and decreases with salinity. At the scale of geological structures t he model provides an explanation for the presence of kilometer-scale dipola r self-potential anomalies in geothermal systems and volcanoes. Positive se lf-potential anomalies are associated with fluid discharge areas, whereas n egative self-potential anomalies are associated with fluid recharge areas. Self-potential anomaly maps determined at the surface of active hydrotherma l fields appear to be a powerful way of mapping the fluid recharge and disc harge areas. In the case of free convection the vorticities of the convecti on pattern generate a magnetic field. The greater these vorticities, the gr eater the associated magnetic field. It follows that hydrothermal systems a ct as natural geobatteries because of the now of pore fluids in the subsurf ace of these systems.