As illustrated in Figure 3-13, cast-iron pipes produce the strongest magnetic signals at their joints.
The initial search should be performed as follows:
1. Set the Gain control to HI.
2. Hold the locator vertically 1 to 1-1/2 feet above the surface
3. Walk along without turning or tilting the locator.
4. Mark the locations where the maximum signals occur.
5. Return to an area of maximum signal strength and hold the locator several inches above the surface. The gain setting will probably have to be reduced during this second pass.
Four-inch, cast-iron pipes can be located at depth up to 10 feet.