When DC is applied to a pipeline in the same manner as in cathodic protection, a voltage gradient is established in the ground due to the passage of current through the resistive soil to the bare steel exposed at a coating defect.

The voltage gradient becomes larger and more concentrated the greater the current flowing and the closer you are to a coating defect location. In general, the larger the defect, the greater the current flow and hence bigger the voltage gradient.

The DC voltage gradient method utilises measuring on a sensitive millivoltmeter, the out of balance between two copper/copper sulphate half cells placed in the soil in the voltage gradient at ground level. If spaced one metre apart in a voltage gradient, one half cell will adopt a more positive potential than the other, which thus enables the size of the gradient and direction of the current flow causing the voltage gradient to be established.

In order to ease interpretation and to separate what is being monitored from other DC sources such as long line cells, tellurics, other CP systems, etc., in the DC Voltage Gradient Technique, the asymmetrical DC signal impressed onto the pipeline is switched ON and OFF at the rate of 0.45 seconds ON, 0.8 seconds OFF. The DC signal can be impressed on top of existing CP systems or the pipeline CP Transformer Rectifiers (T/R) can be switched by using a special interrupter switch inserted into the negative lead from the Transformer Rectifier.

The DC signal can even be injected at a test post using batteries or a portable DC generator and temporary ground bed.

In carrying out a survey, the surveyor walks the pipeline route testing at two metre intervals with the probes in a position of one in front of the other, separated by one to two metres, parallel and preferably above the pipeline, (though not essential provided you can pick up the voltage gradient from defects in the pipeline route). As a defect is approached, the surveyor will see the millivoltmeter start to respond to the ON/OFF pulsed current, which is either a coating defect or interference to another structure. When the defect is passed, the needle deflection completely reverses and slowly decreases as the surveyor moves away from the defect. By retracing, the position of the probes can be found where the needle shows no deflection, ie: a null. The defect is then sited midway between the two copper/copper sulphate half cells.

This procedure is repeated at right angles to the first set of observations and where the two midway positions cross is the epicentre of the voltage gradient. This is directly above the coating defect.

Once located a series of electrical measurements are made that allow the severity of the defect and its corrosion status to be determined.