Resistivity Mapping
Resistivity Mapping for Dam Seepage
Earthen structures are very commonplace and found in many varying locales, and have many different uses. Earthen dams and impoundments may be employed to store water for municipal or recreational use, or as retention ponds in industrial settings. Levees are another type of earthen structure that provides for the safe transport of water in urban settings or used for flood defense. Despite the type of structure or location, in virtually all cases it is vital that the integrity of the structure is understood, monitored, and maintained to prevent failures causing the uncontrolled release of water or other stored fluids. Over time, the fluids stored in an earthen structure will begin, and are typically designed, to seep into the structure, and as the seepage continues there can be an increased susceptibility to piping and slope failure. Catastrophic failure can have immense costs, not just financially but to the environment and even the potential for loss of human life.
Failure of earthen structures can have immense social and financial costs. A resistivity mapping investigation can help mitigate risks before they become failures.
Electrical resistivity mapping profiles can easily identify areas of low resistivity (or high conductivity) that could indicate adverse conditions from uncontrolled seepage.
Electrical Resistivity mapping can be tailored to any size, in both the lateral sense (along the ground surface) and vertically (depth), and cope with a variety of terrain (slopes, abutment areas, rip-rap, etc.). This method is equally suitable for investigating known problems (leaks, seepage) or to confirm that a functioning earthen structure is of sound construction. Time-lapse resistivity is another useful application of electrical resistivity mapping whereby successive survey measurements are compared through long-term monitoring to determine whether there have been any structural or hydrological changes over time.
In the example below, we used resistivity mapping along the crest and downstream slope to determine the origin of an uncontrolled seep along the dam. We collected several lines of data to form a 3D representation of the electrical structure; the plot shows several slices through the 3D model. The data shows a clear low resistivity / high conductivity path that leads from the crest to the seepage area, which explains the seep formation.