What Are DCVG and ACVG Surveys?
DCVG (Direct Current Voltage Gradient) and ACVG (Alternating Current Voltage Gradient) are
above-ground pipeline survey techniques used to detect coating defects — known as "holidays"
— in the protective coating of buried pipelines, and to assess the effectiveness of the cathodic
protection (CP) system that guards the pipeline against external corrosion where coating breaks
occur.
Both techniques work on the same fundamental principle: a signal is applied to the pipeline (DC
in DCVG, AC in ACVG) and the surveyor walks above the buried pipeline with two reference
electrodes placed in the soil. Where the pipeline coating is intact, the signal remains within the
pipe. Where a coating holiday exists, current escapes into the surrounding soil — creating a
voltage gradient in the soil above the defect that the surveyor's instruments detect.
DCVG: A DC signal is applied to the pipeline — typically by interrupting the existing impressed
current cathodic protection (ICCP) system to create on/off potential cycling. The surveyor
measures the voltage gradient in the soil and identifies the direction and magnitude of current
f
low to locate and characterise each coating holiday. DCVG is the only voltage gradient technique
that can quantify the percentage of current drain at a holiday — providing a severity rating (%IR)
that prioritises coating defects for repair.
ACVG: An AC signal at a defined frequency is applied to the pipeline via a signal transmitter. The
surveyor uses a receiver to detect and locate the increased signal attenuation above coating
holidays. ACVG is generally faster to deploy over long distances and is effective in environments
where DCVG suffers from interference — but it does not provide the quantitative severity rating
that DCVG delivers.