A GIS Method for Analyzing Electrical Resistivity

Shallow electrical resistivity methods provide a relatively easy and cost-effective way of mapping the subsurface in wetland environments. Traditional ways of analyzing these data include curve matching, analytical methods, and pseudosections, which resemble geologic cross sections. Psuedosections and Three Dimensional (3D) results can be obtained from specialized software, such as RES3DINV or EarthImager. In recent years, ArcGIS has become very popular in environmental studies, but not as popular in electrical resistivity data analysis. In a previous study, we demonstrated that ArcGIS can be used in electrical resistivity analysis. The objective of this project was to assess the performance and accuracy of ArcGIS in analyzing 3D-wetland resistivity data. The study area used for this analysis was located at the Oconee River Greenway, in Milledgeville, Georgia. The area of focus was Alice Basin, which is one of the small basins in the Greenway wetland area. Within the wetland, we observed significant subsurface flow that transported water between basins and from the wetland into nearby Fishing Creek. This subsurface flow is typically restricted to a high permeability layer, and is responsible for most of the water loss within the wetland. To better understand how this wetland functions, electrical resistivity profiling was used to map the subsurface and the extent of the high permeability layer. The data were collected with an ULTRA MiniRes resistivity meter, using the dipole-dipole electrode configuration. We then developed an easy GIS-method for analyzing these data in 3D, and compared the results with those from the specialized resistivity software, RES3DINV. The top layer outputs for both programs were compared to visual observations and soils in the field. The spline interpolation technique in ArcGIS produced a superior match to the observed data, than results from RES3DINV. However, results for the lower layers were similar between the two programs. An additional advantage of the GIS results was that the layers can be assembled in ascending order, and then viewed in a 3D block in ArcScene viewer. The 3D block can also be saved into a video that displays different angles of view. Overall, ArcGIS can be used as a valuable, reliable, and readily available program for visualizing 3D resistivity data.