Resistivity Mapping

Resistivity Mapping for Dam Seepage

Electrical Resistivity Mapping is an essential tool for the investigation of earthen structures such as levees, impoundments, and earthen dams to investigate problem areas related to seepage.  The electrical resistivity method is a particularly suitable type of geophysical investigation for mapping dam seepage, given its sensitivity to moisture content and saturation of the subsurface.  As fluid infiltrates into the earthen structure, areas of the structure may become saturated and the conductivity will increase.  A high contrast will exist between dry and saturated materials, providing an excellent target for subsurface electrical resistivity mapping.

 

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.

 

Joe Cain sets up a resistivity survey line at a dam site in Colorado. Resistivity cabling is deployed across the crest of the dam and electrodes are installed at three meter intervals and connected to the cable. Resistivity data is acquired and an image is generated from the data. One type of example is presented below. hydroGEOPHYSICS Inc.
An Electrical Resistivity Mapping investigation is a cost-effective and non-invasive technique that can identify problem areas associated with seepage in and around earthen dams or levees.  This type of investigation is safe for the structure and can provide actionable information almost immediately.  Electrical Resistivity measurements are typically made using an array of small, stainless steel electrodes placed on the surface.  A small current is injected into the soil, and a voltage is measured along the remainder of the array. In this manner, a profile of the measured resistivities can be modeled to provide a map of the interior of a dam or levee structure. Depending on the survey design, results can be provided as two-dimensional (2D) cross-sections or in some cases as three-dimensional (3D) model plots.  These profiles can easily identify areas of low resistivity (or high conductivity) that could indicate adverse conditions from uncontrolled seepage.  Seepage can also lead to the transport of finer grained materials within the earthen structure, potentially creating void spaces that can weaken the structure or provide preferential pathways for continued seepage.  Electrical resistivity mapping can identify these void spaces, or other defects, as they will provide contrasting electrical properties relative to a sound earthen structure.

 

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.

 

A 3D representation of electrical resistivity structure of an earthen dam. The dam experienced uncontrolled seepage, with the source clearly outlined