The Role of Electrical Interface Conduction in Geothermal Exploration

In order to investigate the interface conduction in the basaltic rocks of the high temperature fields in Iceland we measured the electrical conductivity for the frequency range 0.1 – 100000 Hz versus pore fluid salinity of 12 selected samples of basaltic material from Iceland. These included 2 fresh and completely unaltered samples of recent basaltic lava, 5 samples of basalt and hyaloclastite from the smectite alteration zone and 5 from the chlorite zone. About 2-5% reduction in conductivity is observed per decade in frequency. For the unaltered samples a linear relationship is found between the bulk conductivity and the pore fluid conductivity over almost the whole range of salinities, showing that the pore fluid conduction is always dominant and the interface conduction is negligible. The samples from the smectite zone show almost no dependence on the pore fluid salinity but considerable interface conduction as predicted, the value being from 20-300 µS/cm with the isoelectrical point at fluid conductivity in the range of 4000 – 6000 µS/cm. In contrast to previous hypothesis, the samples from the chlorite zone show also significant interface conduction, 4 out of 5 samples show value in the range of 10-30 µS/cm but the isoelectrical point is lower than in the smectite zone, usually at fluid conductivity in the range 1000-3000 µS/cm. Since the temperature dependence of conductivity is at least twice as high for the interface conduction as for the pore fluid conduction, our results imply that interface conduction is the dominant conduction mechanism for most high temperature geothermal fields regardless of their pore fluid salinity. Furthermore, the observed change in conductivity at the top of the chlorite zone is not due to change in dominant conduction mechanism, i.e. from interface conduction to pore fluid conduction, as has previously suggested, but probably rather due to reduced degree of interface conduction in the chlorite zone associated with the lower cation exchange capacity of chlorite compared to smectite. As a consequence of this we present a revised version of the model for the electrical resistivity of the basaltic upper crust in Iceland.