Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

It should be noted that absolute probe signals, which are close to 180° apart,
such as a shallow inside surface flaw and a magnetic deposit at the outside
surface, will give very similar differential probe signals. They may be
distinguished only by observing the initial direction of movement of the spot,
which is the same direction as the absolute signal. It should also be noted that if
the probe is pushed rather than pulled, there is no change to absolute signals,
but differential signals are traced out in reverse. That is, the initial direction of
the spot movement is the opposite to when the probe is pulled, and opposite to
the direction of the corresponding absolute probe signal. One other property of
differential probe signals is that when a condition persists for some length along
the tube, like a seam, the first half of the signal is produced when it detects the
beginning of the condition, and the second half of the signal is produced when it
detects the end of the condition. In between, when both coils are sensing the
condition, there is no signal.
Charlie Chong/ Fion Zhang