Strongly Magnetized Markers

A strongly magnetized iron pipe at or near the surface may provide location information that is misleading.The heavy line in Figure 3-4 represents the variation in tone frequency when the locator is moved over the pipe. When moving the instrument from A to B, the frequency of the tone increases and then suddenly decreases at B. From just beyond B the frequency of the tone increases sharply, becomes very high directly over the marker and decreases just before reaching C. From C to D the pattern is the reverse of that form A to B. It is obvious that the locator must enter the B-C region. Otherwise the marker might be assumed to be between A and B or C and D.
Figure 3-4. Signal Pattern from a Strongly Magnetized marker