Electrical Resistivity Surveys

To schedule a geophysical resistivity surveying job with Subtronic Corporation, please contact us.

Multi-channel Electrical resistivity is one of the most robust, applicable geophysical methods available to near subsurface geophysicists. With modern multi-channel resistivity, thousands of data points can be collected in the time it takes older electrical resistivity equipment to collect just a fraction of the amount of data points.

Here at Subtronic we use multi-channel resistivity for the following subsurface investigations:

Structure:

  • Determine depth to bedrock/overburden thickness
  • Map stratigraphy
  • Map paleochannels
  • Locate bulk aggregate material (sand/gravel)
  • Locate voids/tunnels/caves
  • Map faults
  • Landslide investigation
  • Mineral exploration

Hydrology:

  • Characterize subsurface hydrogeology
  • Determine depth to groundwater
  • Map clay aquitards
  • Map salt-water intrusion

Contamination:

  • Map vertical extent of certain types of soil and groundwater contamination
  • Map lateral extent of conductive contaminant plumes
  • Map heavy metals soil contamination
  • Delineate disposal areas

Cultural Features:

  • Estimate landfill thickness
  • Map archaeological sites
  • Grave detection
  • Find deeply buried large utility lines

What is  Electrical Resistivity?

Electrical resistivity (also known as resistivity, specific electrical resistance, or volume resistivity) is a measure of how strongly a given material opposes the flow of electric current. Electricity does not easily flow through a highly resistive material, and a material with a low resistivity readily allows the flow of electric current. Resistivity is measured in ohms (Ω).

As a general guide, we can divide earth materials into:

  • Low Resistivity –            <100 ohm meters
  • Medium Resistivity –     100-1000 ohm meters
  • High Resistivity –           >1000 ohm meters

Common resistivity values for various earth materials is shown below:

common resistivity values

If two earth materials with the same resistivity values are in contact with each other, it will be difficult to differentiate between the two materials using resistivity, but resistivity contrasts are easily observed and much more common. Resistivity contrasts exist between dry and water-bearing soils, different rock/soil/mineral types, man-made features, and porous/non-porous earth materials.

Our Equipment

Using modern multi-channel resistivity survey equipment, such as the Advanced Geosciences, Inc. SuperSting R8™, resistivity surveys can be completed much quicker with a higher quality of data compared to an old school 4 electrode resistivity system. The SuperSting R8 can be expanded from the minimum of 28 electrodes to 112 electrodes, allowing huge survey lines to be collected in one setup. The SuperSting R8 is able to collect eight data measurements at a time, compared to most resistivity systems which can only collect one measurement at a time, therefore saving time and money on site. Once thousands of predetermined data points have been collected, the resultant large database can then be modeled, to yield plausible resistivity vs depth profiles, or if surveyed accordingly, 3D subsurface resistivity maps.

General Electrical Resistivity Information

Using resistivity survey methods, subsurface data can be collected to a depth of 15-20% of survey line length, so for a typical 56 electrode survey with a line spacing of 20 feet, and exploration depth maximum of 168 – 224 feet is possible. The longer the survey line, and therefore the greater the possible exploration depth, the greater the resolution of the resistivity data closer to the subsurface, and anomalous edge effects are reduced close to the surface.

Exploration depth examples

  • 250 ft Survey Line = 50 ft depth
  • 500 ft Survey Line = 100 ft depth
  • 1000 Survey Line = 200 ft depth

The resolution of the data collected corresponds to half the electrode spacing, so if the electrode spacing is 10 ft, the resistivity method can resolve objects at 10 ft in diameter.

Case Studies/Applications

The data for this inverted resistivity cross-section was collected using AGI’s SuperSting R8. The goal of the investigation was to find a buried 2m wide concrete aqueduct. The black arrow points to a large anomaly which corresponded to the concrete aqueduct. If certain site conditions can be met, electrical resistivity can be a great method to identify and located deeply buried utilities unlocatable via conventional utility locating techniques.

More case studies/applications of what is possible with geophysical resistivity surveys using the AGI SuperSting R8

Send an inquiry to our geophysical department to see if we can address your geotechnical needs!