One 6 volt battery powering door bell or buzzer is pictured as 6 volt generator and lamp at top. Usually two connections can be found to metal underground pipe to make hidden pipe part of audio frequency pulsing circuit.
Lower right, 32 ohm resistor represents headset connected to coil ( 1 or 2 foot diameter pickup coil of bell wire of 50 to 100 turns. To calculate, π times 100 = 314 feet of bell wire at 1 foot diameter.
A fine tuning idea, to sound test near a connection end about how best to hold coil to get most LOUDNESS OF SOUND in headphones and the range of rig in case stronger signal, stronger pickup coil, or different headset may be needed to give more distance range that can be detected.
LOUDNESS OF SOUND shows coil closest to metal. Moving coil across ground surface to detect buried sound signal source.
Might be adjusted for walls as well?
