Question
I am studying an induction case with HIFREQ. I compute the voltage difference between two points on the grid by reading the GPR values off a Configuration plot. The values I obtain are not what I expected. What could be wrong?
Answer
The discrepancy may be due to the fact that the configuration plots of the GPR display the magnitude of the GPR. To obtain a correct estimation of the GPR difference between two points, the difference of the real part of the GPR at those two points as well as the difference in the imaginary part of the GPR at those points must first be obtained separately; then the magnitude of the result can be computed.
The reason that the difference of the magnitudes of the GPR is not the same as the magnitude of the GPR difference is that a sizable imaginary part exists. In fact, in induction cases, the imaginary part of the voltage can often be up to ten times bigger than its real part! There are two sources for this imaginary part: the skin effect in the conductor and the external self-impedance of the conductor. The latter is due to the inductive effect of the conductor on itself and is the larger effect.
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