ion concentration) (Bryson, 2005).įigure 1: Equipotential lines and current lines for electrode pairs in a homogeneous half-space (from EPA, 2016b).įigure 2: Example of resistivity inversion using AGI software (EPA, 2016b AGI, 2017). The resistivity of a soil is affected by a multitude of properties that can be assessed in situ and include soil type(s), moisture content, temperature, porosity and pore structure, soil mineralogy, double layer thickness, and pore fluid composition ( i.e. An example of software used to perform the inversion computations is shown in Figure 2. The assumed resistivity profile is iteratively updated until the calculated and measured apparent resistivity profiles sufficiently match. In general, an assumed resistivity profile is generated and used to calculate an apparent resistivity profile which is compared to the recorded field data. Since the resistivity profile cannot be directly computed from apparent resistivity measurements, an inversion method must be used to estimate the resistivity profile ( e.g. Electrical resistivity surveys directly provide an estimate of the apparent resistivity of the surveyed material. More recent innovations use arrays of many probes for measuring cross-sections, or probes in grid patterns for tomography methods which allow 2D and 3D subsurface imaging. An example of this class of array is shown in Figure 1. Based on the applied current and the measured voltage, the apparent electrical resistance of the subsurface can be measured. Wenner, Schlumberger, and dipole-dipole arrays). Two sets of probes are typically inserted vertically into the soil one set delivers a current into the soil and the second set measure the voltage differential over some distance ( i.e. Electrical resistivity surveys involve inserting electrodes directly into the ground surface and transmitting an electrical current into the soil which acts as a resistive medium.
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