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Capacitively-Coupled Resistivity

Capacitively-coupled resistivity (CCR) is a method in which resistivity measurements are made by using a set of cables or capacitive plates instead of metal stakes in the ground (galvanic). In a traditional resistivity array two stakes are used as a transmitter dipole to inject current into the ground and two stakes are used as a receiver dipole to measure the voltage generated by the current. In a CCR system two cables are used for the transmitter dipole and two cables are used for the receiver dipole. The cable acts as half of a capacitor, the earth acts as the other half of the capacitor, and the insulating jacket around the cable acts as the dielectric separating the two halves of the cable-earth capacitor. An AC current will pass through a capacitor and a DC current will be blocked. In a CCR resistivity meter an AC current is applied to the cable which then passes through the earth-cable capacitor into the ground. At the receiver side the AC voltage is measured on the receiver cables. This means there is no requirement for there to be a direct galvanic contact between the transmitter or receiver and the ground. This in turn means the measurements can be made as the cables are actively towed along the ground.

Common applications

  • Soil electrical resistivity-conductivity mapping for soil types
  • Topsoil thickness
  • Detection of subsurface voids and cavities
  • Location of sand and gravel channels
  • Structural investigation of dikes and levees
  • Depth to shallow bedrock
  • Detection and areal extent of pollution plumes from landfills, containment ponds, mining leachates
  • Detection of shallow kimberlite structures for diamond exploration
  • Mapping of permafrost features such as depth to permafrost, ice lenses, thaw zones
  • Large archaeological feature identification

Considerations

  • As with all resistivity meters the data quality can be compromised when working near long linear conductors such as buried gas metal pipelines, railroad tracks, rebar, etc.

Benefits/Limitations

  • The use of towable cables for CCR measurements eliminates the need to hammer stakes into the ground saving time for survey setup.
  • Since the resistivity array can be towed along the ground while measurements are made, the survey can be done up to 10 times faster than a tradition galvanic resistivity survey using metal stakes in the ground.
  • CCR does not have problems with contact resistance, so it can be used on dry soil, ice, frozen ground, pavement, asphalt, bedrock, etc.
  • The amount of current coupled to the ground capacitively is in the low milliamp range. This limits the depth of investigation to less than 15 meters in most applications.
  • The CCR frequency is band limited so it does not have problems with interference from power lines, industrial noise, natural telluric noise in the ground, or most other noise sources.

Deliverables

  • CCR can use industry-standard inversion software once the measured voltages and currents have been processed to apparent resistivity. Apparent resistivity, true resistivity, N-space, and depth are the processed parameters.
  • Both horizontal plan-view and vertical depth sections can be generated by the 2-D and 3-D inversion software.

Below is an inverted section showing soil thickness and faulted bedrock.

click to enlarge

Best Tool

Ohmmapper

Further Reading:

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