Hager-Richter - Consultants in Geology & Geophysics

Boone County, KY: Resistivity survey for new tunnel construction project.

Resistivity Case Study: Construct Bedrock Topography/Fracture Maps for DNAPL Contamination

Problem
DNAPL contamination at a large industrial complex had the potential for migrating off site in an area of karst terrain. Hager-Richter was hired to perform seismic refraction and electrical resistivity surveys to determine the depth to bedrock and detect openings in the bedrock, such as karst solution channels and former mine workings.

Solution
In order to provide our client with the best possible information, we collaborated with the Earth Resources Laboratory of the Massachusetts Institute of Technology (MIT) to obtain 2-D apparent resistivity images along profiles. MIT's proprietary Resistivity Array Imaging (RAI) and concomitant inversion code were employed to yield highly detailed resistivity data. Several new voids were discovered and a bedrock topography map was constructed.

General Applications

Environmental audits and site assessment
Water resource management
Groundwater resource studies: mapping and plume delineation
Public/Private remedial investigations and feasibility studies
Landfill closures geological mapping

Specific Applications

Determine electricalcharacteristics of surrounding area
Determine depth of water table
Locate and define contaminant plumes
Determine extent of saltwater intrusion
Locate fractures and faults
Identify areas of potentially unstable ground, e.g. mine shafts, voids
Assist in dam stability analysis
Determine overburden depth
Locate buried mafic dikes
Corrosion control design
Determine lithology/structure

The resistivity method measures the apparent resistivity of the subsurface, including effects of any or all of the following: soil type, bedrock fractures, contaminants and ground water. Variations in electrical resistivity may indicate changes in composition, layer thickness or contaminant levels. The resistivity method is useful for simultaneously detecting lateral and vertical changes in subsurface electrical properties. Electrical resistivity is also used to delineate 3-dimensional bodies with anomalous electrical conductivities. Depending on the client's need, Hager-Richter uses traditional Vertical Electrical Sounding (VES) and lateral profiling methods as well as state of the art 2-D imaging techniques to produce the best results for any situation. The use of rapid, automated data acquisition systems used to create 2-D "tomograms" has increased the flexibility of resistivity as a tool for environmental and geotechnical applications.

Since no two projects are alike, the depth and diversity of our experience provide distinct advantages for our clients.

Electrical resistivity surveys have certain limitations. They may be unsuitable for examining highly industrialized and/or urbanized areas where cultural features such as buildings, fences and power lines may interfere with the collection of accurate data. The target depth, size and of course, resistivity contrast may pose limitations.

We can provide you with an accurate assessment of your requirements to determine whether or not the use of the resistivity method is appropriate at a particular site.