Abstract: At least one string of several spaced measuring probes for the measurement of surface potentials is laid on the surface of the earth in such a manner that several probes are located outside and several probes are located inside the area of the surface over a reservoir. The potentials measured by all probes are polled in a sequence of periodic cycles. The signals from each probe are, during each cycle, filtered to separate higher-frequency changes in the signals from substantially lower-frequency changes in the signals. For the higher-frequency changes in the signals from each probe, an average and/or a sum are determined as a measure of the higher-frequency amplitudes for each probe. The higher-frequency amplitudes for the probes of a string of measuring probes are compared and the location at which the higher-frequency amplitudes for two adjacent probes change abruptly is taken to be the hydrocarbon/water interface in the underground reservoir.