a locator for determining the depth of a pipeline which carries ac signals . the signals may be those induced by power lines , cathodic protection systems , or pumping equipment or other machinery connected to the pipeline . also , specifically applied ac signals from a transmitter may be employed in the absence of the other signal sources . the depth determining locator employs three directional detector coils in an elongated housing . two coils are oriented transversely to the axis of the housing and are located as far apart as a preselected depth of the pipeline . the third coil is beside a forward one of the other two and is oriented at an angle , e . g . 45 ° to the transverse . there is electronic circuit means for timing the passage of the locator over the pipe between the two coils , and to measure the ratio of a constant times the time from the forward of the two coils to alignment with the third coil , divided by the time between the two coils . that ratio represents the depth of the pipe .