Diffusion coupled (electrochemical) self-potential effects in geothermal areas

Electrochemical effects caused by the diffusion of geothermal brines are investigated as a possible source mechanism for the generation of self-potential. One of the difficulties associated with the modeling of the coupled salt flow and electric current problem is that the coupled equations are nonlinear because the electrical conductivity depends on the ambient salt concentration. The linearized solutions correspond to the case where the concentration variations are small compared to the background concentration. The linear case is not of much interest in geothermal areas where the brines may be much more concentrated than the normal groundwater. The investigation of the coupled transport problem in this paper shows that the solution of the salt flow problem gives a concentration distribution that is very close to that derived from the solution of the ambipolar diffusion problem. in pure ambipolar diffusion, the local electric field is such that the cations and anions drift with the same velocity and the total electric current is zero. When ambipolar diffusion is an adequate description of the transport of solute ions, the salt flow and electric current problems can be solved in a manner analogous to the coupled fluid flow and heat flow problems as described in Sill (1981, 1982a). The results of model calculations indicate that this mechanism can generate only small self-potentials ({approx} 10 mv)