I use a Progressive Electronics Locator, was under $200 when I bought years ago, Very Similar to this Greenlee:
Greenlee Wire Finder/ Mini Locator 508S - eBay (item 350123069238 end time Aug-06-09 10:02:13 PDT)
Great for locating wires, conduits with wires, the signal goes silent when directly over the wire, then do at about a 45 degree angle, move until silent(Null), where these 2 points are tells the depth. Many uses, locate breaks in Invisible fences.