If you have the time you can turn the water on when needed and then off for a few hours and back on - - - - this will allow the water to go stright up from the leak - - -but will take about 3 days and 10 to 12 turn ons. It is amasing as the first place you see the water coming up might be several hundred feet from the leak, or you might get lucky and be right over it.
Then I dig parrell to the pipe and try to stay about 2 to 3 feet off to the side. When I think I'm close I have the water turned on briefly as I zero in on the leak.
Dig about 3 feet below the pipe with the backhoe then hand drag the mud from around the pipe into the bottom of the hole you are in.
Make what ever repair is needed and I lay a cheap tarp up the side of the hole as I back fill to mark the spot in case you ever need to find the pipe again, just cut it off at ground level. You will never see the tarp again unless needed.
I do repairs for the local farmers several times a year - - - going to repair a 15" field tile tomorrow that is running so hard we are going to use (2) 8" trash pumps to keep the hole empty and we will bed the new tile in about 12" of #4 crushed stone to give it a good bottom to rest on as I don't want to have to go back to this repair again.
This tile was cut by the local drainage board putting in another tile and they stated they will come back and repair in August when it is dry enough to work! We can't wait as it is flooding about 10 acres of some of our best ground. I am going to send the drainage board a bill but I doubt I will ever collect!