Electrical Resistivity Tomography (ERT) is an effective method for engineering projects in atypical environments that require a detailed understanding of geologic conditions, stratigraphy and/or hazards. ERT is unique among geophysical methods because a single test can be performed crossing both land and submerged environments, such as a stream or creek crossing. A specialized marine resistivity cable can be oriented across a stream with electrodes coupled to stainless steel stakes at the ground surface, and then simply laid on the stream bed in the submerged environment. The water itself creates the electrical contact needed for the test. Other geophysical techniques would require one tool for the submerged environment and a second tool for the land-based testing.
Such testing can be beneficial for a geotechnical investigation associated with a project such as bridge replacement or improvement. For example, Pyramid Geophysical Services conducted an ERT survey for a proposed bridge replacement across a narrow stream approximately 10 feet deep. The purpose of this survey was to characterize the shallow stratigraphy below the stream bed. Pyramid utilized a marine resistivity cable set at two different electrode spacings and an Advanced Geosciences, Inc. (AGI) SuperSting earth resistivity meter for this project. This survey was effective in identifying even minor variations in the clayey soils and showed potential zones of cementation in shallow subsurface. This particular project demonstrates the versatility of ERT. This unique survey utilized a specialty cable with all the standard equipment used in a typical ERT investigation.
The other benefit of ERT in these situations is the ability to incorporate topography into the resistivity model. In this example, relatively significant topographic relief is observed across the stream channel over a short distance. If this relief is not incorporated into the model, the results would skew the relative depths of subsurface stratigraphic changes, making interpretations invalid. By incorporating elevation (either generically from field measurements or more accurately from a digital elevation map) into the model the results accurately depict soil variability with depth across the stream channel.