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.
- 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
- 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.
- 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.
- 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.
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