The C-Scan survey, done with proprietary tools supplied by only one vendor worldwide, consists of applying an AC electromagnetic signal, or current, to a pipeline through a conductor attachment. In the same general area as this conductive attachment, an electrical ground is also established in soil 50 to 300 feet away from the pipeline under study.
The signal generator is then turned on and current “signal” applied to the pipeline. The C-Scan detector is then walked down the pipeline, and measurements of an electromagnetic field strength coming from the pipeline are made. C-Scan provides GPS location information on pipe centerline to what is usually sub-meter accuracy. Of even more value, it gives “depth of cover” information to the nearest inch, and can reliably identify “top of pipe” or “center of pipe” to depths exceeding 30 feet below grade.
As one surveys down the pipeline, multiple antennas in the detector provide the amount of signal loss, compared to the original signal strength at the first test point. Between a previous test point and the newest test point, average coating conductance and signal attenuation are computed. Within each pipeline interval “bracketed” by test points, the operator can compare whether the pipeline coating across that interval has shown the same characteristics as other sections, or if more signal is lost, and coating conductance “averaged across” that section is poor. If this is the case, additional measurements are taken in smaller pipe-length increments. Using this technique of multiple segments, and then using a “Current-Only Close-Interval Survey” with the same tool, a particular coating defect can be pinpointed to within feet or inches, from “top centerline” position over the pipeline.
C-Scan should be performed on a pipeline that does not have current-interrupted CP survey being done at the same time. It may also suffer from interference from high-voltage AC power line fields. Otherwise, this survey is applicable in any case where a reasonably well-coated pipeline is in use. If a pipeline has very poor coating quality, or is bare, the signal loss is so rapid that meaningful coatings anomaly information is not likely to be obtained.
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