Influence of natural time-dependent variations of electrical conductivity on DC resistivity measurements

Long-term direct current (DC) resistivity monitoring surveys are carried out to investigate structures and processes in the subsurface which are coupled to changes in electrical conductivity. Examples would be leaching tests in the unsaturated zone or the observation of a salt tracer spreading in the saturated zone. As these investigations usually take some hours to weeks, natural time-dependent background variations of electrical conductivity have to be considered. Parameters influencing the electrical conductivity are temperature changes, variable water contents in soil due to precipitation or changing groundwater levels, and natural fluctuations of the ionar content in groundwater. Measurements over different time periods (one month to one year) at different test sites demonstrated daily and seasonal trends of electrical conductivity and temperature of groundwater, surface temperatures and groundwater levels. Variable water saturation and changing soil temperatures (in the uppermost soil) are estimated to influence the electrical conductivity most significantly, followed by changing water temperatures and varying ionar content. To verify these conclusions, multi-electrode DC resistivity monitoring has been performed at two test sites for 16 and 22 days. The results reflect natural characteristics and processes in the subsurface. With mean values and standard deviations of apparent resistivities plotted in pseudosections, different areas of the subsurface with different ranges of resistivity variations have been specified. In the area of variable groundwater level (varying water saturation) and in the uppermost soil (strong temperature and moisture changes), especially high resistivity variations are obvious (up to 26%). In the saturated zone, variations are low (predominately below 1%). Concerning resistivity measurements, we conclude that electrodes positioned below the groundwater level (e.g. for salt tracer tests) or between the uppermost meter of subsurface and the area of variable groundwater level (e.g. for leaching tests in the unsaturated zone) should be used to avoid strong influences of background variations. (C) 2003 Elsevier B.V. All rights reserved.