what is the resistivity of sensitivity
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electrode arrays allows for the interpretation of the two- dimensional distribution of electrical conductivity in the The analytic element method is used to investigate the spatial subsurface. The electrical conductivity distribution can sensitivity of different electrical resistivity tomography (ERT) arrays. By defining the sensitivity of an array to a subsurface location we then be related to the volumetric water content, concen- were able to generate maps showing the distribution of the sensitivity tration of electrolytic solutes in the pore water, and the throughout the subsurface. This allows us to define regions of the surface conductivity of the subsurface materials. subsurface where different ERT arrays are most and least sensitive. Each ERT measurement represents some average of We compared the different arrays using the absolute value of the the heterogeneous subsurface electrical conductivities sensitivity and using its spatial distribution. Comparison is presented in the shallow subsurface. Given that the current is ap- for three commonly used arrays (Wenner, Schlumberger, and double plied and the potential is measured at the surface, all dipole) and for one atypical array (partially overlapping). Most com- ERT arrays are more sensitive to the properties of shal- mon monitoring techniques use a single measurement to measure a low subsurface materials than to deeper material prop- property at a single location. The spatial distribution of the property erties. In general, arrays with larger electrode separations is determined by interpolation of these measurements. In contrast, ERT is unique in that multiple measurements are interpreted simulta- are assumed to have sample areas that extend deeper neously to create maps of spatially distributed soil properties. We beneath the ground surface, while the sample areas of define the spatial sensitivity of an ERT survey to each location on smaller arrays are limited to the shallow subsurface. To the basis of the sum of the sensitivities of the single arrays composing form a more unique image of the subsurface electrical the survey to that location. With the goal of applying ERT for time- conductivity distribution, the apparent resistivity is mea- lapse measurements, we compared the spatial sensitivities of different sured through many electrode combinations and inter- surveys on a per measurement basis. Compared are three surveys preted simultaneously. In general, it is felt that the use based on the typical Wenner, Schlumberger, and double dipole arrays, of a large number of arrays with different array sizes one atypical survey based on the partially overlapping array, and one will lead to a more accurate representation of the spatial mixed survey built of arrays that have been shown to be optimal for structure of the subsurface electrical conductivity. a series of single perturbations. Results show the inferiority of the double dipole survey compared with other surveys. On a per measure- With modern field equipment, many electrodes can ment basis, there was almost no difference between the Wenner and be installed and measurements can be made automati- the Schlumberger surveys. The atypical partially overlapping survey cally using multiple electrode arrays. As a result, many is superior to the typical arrays. Finally, we show that a survey com- thousands of combinations of electrodes can be formed posed of a mixture of array types is superior to all of the single array using a standard set of 21 electrodes. However, caution type surveys. By analyzing the spatial sensitivity of the single array, should be used in choosing the arrays comprising a sur- and most significantly the sensitivity of the ERT survey, we set the vey to minimize soil charge time (Dahlin, 2000). The basis for quantitative measurement of subsurface properties using ERT, total time required to collect stacked measurements for with applications to both static and transient hydrologic processes. each array is typically only about 15 s. However, the total measurement time can become impractical if too many arrays are used.
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