ACVG is similar to DCVG with the exception that the applied current is an alternating signal.
The theoretical voltage gradient at a single coating flaw (valid for both DCVG and ACVG) could be described by the radial equal potential gradient lines in an ideal situation (with uniform soil resistivity).
A linear anode was used as the current source for GIS and DCVG testing and the five vertical anodes in the southwest corner of the facility were used as the current source for the signal in the PCM and ACVG testing.
Table 1 is a summary or the ACVG and DCVG indication data by approximate location against those found in the baseline-isolated survey.
It is likely that the same relative current splits occurred during the ACVG surveys.
The ACVG and PCM surveys were also affected by the parallel cable in close proximity to the buried pipe by what appeared to be a self-canceling of the signal along the parallel cable.
This is the principle and method used for ACVG surveys; two soil-contact probes (metal or half cells can be used) are placed on the ground making soil contact and measuring for pressure spikes (increased voltage) along the length of the pipe.
Where there are leaks or holes in the pipe coating, the AC current flows out into the soil and the locator measures the current loss from measurements taken before and after the defect, while the ACVG survey measures the resulting pressure increase in the soil next to the pipe-coating defect.
This is as simple as moving the ACVG probes around to pinpoint the highest pressure or greatest voltage, and if it is not directly over the pipe, we can safely rule it as a foreign metal object or interference for the survey.
For example, the ACVG
has the two ground-contacting electrodes mounted on an A frame instead of two surveyors, and uses a meter indication instead of an audible signal.