Ed wrote:
"The RF on the fish line did not work."

Problem is that loss is proportional to signal frequency through the
earth, A low audio tone is much more appropriate. With a low audio tone
on an insulated conductor buried in the earth with reference to a ground
rod or ground bed, the conductor can be traced for miles. I`ve traced
4-ft. diameter poorly insulated steel pipelines for 8 miles or more and
they were buried several feet down for protection.

Reception can use a relay coil to sense the audio signal which is fed to
an audio amplifier and earphones. The coil core points toward the
conductor.

From the angles made by the core on both sides of the conductor the
depth of its cover can be closely estimated.

Best regards, Richard Harrison, KB5WZI