The use of electromagnetic (EM) geophysical methods to evaluate ground conductivity variations is an effective tool to resolve environmental issues. However, the EM profiling instruments commonly used for such applications provide depth-averaged conductivity values at point locations. Additionally, a variety of factors can contribute to conductivity variance, including soil type, moisture content, and the thickness of stratigraphic units. For many projects, it may be necessary to resolve the depth or thickness of such soil units, and to verify the dominant factor that is causing the observed conductivity changes. This month we provide you with two recent case studies where geophysical conductivity mapping was combined with either secondary geophysical methods and/or physical sampling to better understand site conditions.
Our first case study involves a site that was known to contain isolated zones of shallow clayey soils and organic deposits. An EM survey was conducted to identify the locations of these deposits based on conductivity variance. Multiple areas of conductive soils were identified that were interpreted to correspond to shallow clay or organic soil deposits. However, the depths of these deposits were still unknown.
Subsequent to the EM survey, electrical resistivity tomography (ERT) was performed across specific locations to further constrain the zones of high conductivity identified by the EM, and to examine the depths of the high conductivity soils. Resistivity profiles were performed across the site at locations of conductive soils as well as in zones of lower conductivity to provide background information.
The second project site was thought to contain a significant volume of buried fly ash that had been deposited from a coal combustion plant. The suspected ash deposit was located in an undeveloped parcel of land with fill material covering the suspected dumping area. The EM survey was performed to delineate the extents of the buried ash deposit. A clear zone of high conductivity was observed in the EM results; however, the specific boundary between ash and native soil was unclear due to a lack of physical sampling.
Shallow soil borings were performed to ground-truth the EM data, allowing for a well-constrained ash/soil boundary to be established using the conductivity measurements. The photo on the left is from a sample taken in the zone of highest conductivity, where the ash deposit was thought to be thickest. This sample recorded up to 19 feet of ash material, in contrast to thinner ash deposits observed moving outward toward the zones of lower conductivity. The sampling allowed for an accurate boundary to be established using the geophysics, and also drastically reducing the number of samples that may have otherwise been required.