I'm assuming that the 3/4 is rigid metallic. Sounds like a collapsed pipe maybe? This is pretty common here in southern cali, especially near the ocean where soil is extremely corrosive.
You could use an inexpensive in-ground tracer tied to one of the existing wires. I have one I picked up for about $180 and I've had very good luck with it, granted the conduit is not too deep. Otherwise, I agree with homer - get a locator service and avoid all the headaches and wasted labor hours.
Good Luck!
