The DCVG survey is used to locate and approximately size coating defects on buried pipelines or structures.  When a pipeline is under cathodic protection, voltage gradients will naturally occur in the soil in the vicinity of coating defects. This results from the cathodic protection current causing gradients as it passes through the ground in the vicinity of the coating holidays.  These gradients can be detected in the ground by measuring the voltage drop between two reference electrodes on the ground surface.

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For practical application of the DCVG survey technique, a pulsed d.c. signal is applied to the pipeline at a normal rate of 300mS “on” and 700mS “off”.   Using two reference electrodes and a voltmeter (normally analog), the voltage gradient at a coating holiday can be detected, located, and sized.

 

For a pipeline with a standard depth of cover, the accuracy of the coating defect location is generally within 12 inches.  Greater depths of cover result in lower location accuracy.  Sub-meter GPS is recorded at all defect locations as a standard practice.