Abstract

A systematic investigation of self potentials (SP) shows that they vary considerably with time. To study the influencing parameters, repeated measurements were conducted every 14 days over a period of a year and a half along fixed profiles which extended over different lithological units.An analysis of the recorded SP suggests decomposition into three components of different wavelengths. Variations of the SP with wavelengths in the 0.1-1 m range and amplitudes of up to 150 mV are thought to be induced by vegetation. The mean amplitudes change with a one-year period, and there is a rough correlation with the soil temperature. A long-wavelength component (wavelengths, meters and some tens of meters; amplitudes, some 10 mV) is attributed to changes of the underground lithology. The long-wavelength SP and its pronounced variation with time are assumed to be caused by water movements in layers of different permeability. A morphology-dependent trend of the SP values which has been much disputed in the literature shows systematic variations with time, too, with gradients changing on the order of .5 mV/m difference in elevation. There is a correlation with precipitation with a one-month time lag. This effect is assumed to result from depletion and recharge of perched water aquifers. Measurements at the surface and at shallow depth establish the existence of a vertical gradient of the SP which shows variations with time of up to 50 mV/m. With a three-month time lag, the amplitude of the vertical gradient relates closely to the variation of the potential evapotranspiration. Groundwater recharge from gravitational water is considered the main cause of the vertical gradient.The results suggest that self potentials at the near-surface must be regarded primarily as being of a streaming-potential character. Their variation with time should not be neglected when the SP method is applied to problems in engineering geophysics. On the other hand, the results confirm that because of the close relation between self potentials and hydrogeologic parameters, SP measurements may become a valuable and standard technique in hydrogeology.

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