The effect of compaction on soil electrical resistivity: a laboratory investigation

Among the geophysical tools used in soil science, electrical methods are considered as potentially useful to characterize soil compaction intensity. A laboratory investigation was undertaken on agricultural and forest soils in order to study the impact of compaction on bulk soil electrical resistivity. Samples taken from four different types of loamy soils were compacted at three bulk densities (1.1, 1.3 and 1.6 g cm(-3)). Bulk soil resistivity was measured at each compacted state for gravimetric water contents ranging from 0.10 to 0.50 g g(-1). A specific experimental procedure allowed the control of the water-filling of the intra-aggregate pores and the inter-aggregate pores. Soil resistivity decreased significantly with increase in density and typically for gravimetric water contents less than 0.25 g g(-1). The decrease was more pronounced for the drier soils, indicating the strong impact of the surface conductance, especially in agricultural soils. The experimental data were in good agreement with simulated values given by the petro-physical model of Waxman-Smits (1968), at least for water saturation greater than 0.3. The analysis of the petro-physical parameters derived from the experimental data suggested that: (i) the electrical tortuosity of the loamy agricultural soil was significantly affected by compaction and (ii) the forest soil had a singular microstructure from an electrical point of view and had isolated conductive zones associated with clay embedded in a poorly conductive medium comprised mainly of soil solution and quartz grains. Our results provide the phenomenological basis for assessing, in the field, the relationship between soil electrical resistivity and compaction intensity.