[Snowtractor]

There are all kinds of inexpensive metal pin finders for locating metal. YOu could buy one, maybe off ebay, extend the wires from the sensor head to the buzzer or meter and water proof them. Then lower it down and make grid pattern searches for something metallic. In atmosphere it starts to buzz louder and faster as it approaches metal. At 20 feet from a chain link fence our pin finder would start to sense it.
It may be tough to do good grid search at 120 feet with such a light head though. Maybe a couple of bricks in a bag and taep the wires to the rope holding the bricks. Remember to mark the depth on the rope.
As for hooking up, driver is the only way to go. Advertise fro someone they might do it for the bragging rites.
Sean