::::I vote that is NOT be merged into cathodic protection and NOT be deleted.  DCVG is not CP.  Pipeline protection starts with the coating, then the CP. DCVG helps find coating problems using a test equipment similar to the test to verify adequate CP, but the test is distinctly different.  I have Peabody's 1967 edition.  In the 1967 edition of Peabody Chapter Six "Survey Methods and Techniques" describes ten survey techniques and  none of them are DCVG.  The tenth is the closest.  Measurement of Earth Currents at the surface using a probe with two reference electrodes inserted into the ground near a pipe to indicate if current is flowing to or from the pipe.