Electrical resistivity (ER) testing is an effective geophysical method to identify subsurface geologic hazards, stratigraphic variability, karst features, and hydrogeologic conditions. A typical single ER test utilizes an array of electrodes placed in a straight line at a consistent spacing across a target area. This test results in a 2D cross-sectional profile of resistivity values with depth, providing information about the subsurface directly beneath the location of the electrode array.
For some situations, it can be helpful to incorporate 3D resistivity models to more comprehensively understand resistivity anomalies. Karst features (possible voids, caverns, fracture zones, and solution channels) are good examples of geologic hazards that can connect laterally and vertically in the subsurface and can be more accurately modeled in a 3D space. There are two typical approaches to modeling resistivity data in three dimensions. The first is to set up a true 3D grid of electrodes in the field and collect data in three dimensions. This approach can be extremely effective across smaller areas, because the total size of the grid is limited to the length and spacing of the electrode cables. However, this type of grid cannot be established across larger areas that exceed the electrode cable lengths.
For larger areas, a “pseudo-3D” modeling approach can be utilized. Using Advanced Geosciences, Inc. (AGI) EarthImager 3D processing and inversion software, it is possible to combine a series of parallel 2D resistivity lines that were performed individually in the field and generate a 3D model. The inversion software interpolates between the results of each parallel 2D test and generates a final 3D model. This approach has many benefits over a true 3D grid: 1) Significantly larger areas can be modeled in 3D, 2) The field testing time for each 2D resistivity test is significantly less than a true 3D command file, and 3) The post-processing time to combine parallel 2D lines is significantly less than post-processing a true 3D data set.
Pyramid Geophysical Services has implemented this pseudo-3D approach of combining parallel 2D lines to generate 3D models at a variety of project locations across the United States. As discussed above, most of these projects were associated with karst hazards such as possible voids, caverns, fracture zones, and solution channels. A single 2D test can sometimes make a minor hazard appear more significant than it may be in the actual subsurface. By performing several parallel 2D tests around such an anomaly, the resulting 3D model can help to show the true significance of the feature as well as the lateral and vertical extents in the subsurface.
Click here to view a Pyramid case study specifically analyzing a sinkhole.
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