Leak detecting and Locating plastic pipe

I had a friend call me the other day. He had a leak on a water line under his driveway. he wasnt on city water, he had a well that supplied all his water. Anyway, it was plastic pipe and it didn't have a tracer wire so we couldn't put an electronic signal onto the pipe.
We needed to trace the pipe run in order to be able to follow it above ground with the leak detector and listen for the sounds of the leak, otherwise it is just a huge hunt and peck operation with limited prospects for success especially on a large property.
Well, I had a neat little gizmo, expensive little gizmo that puts an a pulse sound frequency onto the water line. With that device, we were able to map out the basic run of the lines near the leak area. It's not as exact as an electronic signal but it definitely gets you very close. We can hear the sound pulse traveling through the pipe under the ground and use that to map the pipe run. The loundest sound level puts you on top of the pipe.
Within a half hour or so we had the run mapped and went on to search the run for the sounds of the leak. We had an area that I felt was close, very close to the leak but it just wasn't conclusive. We were on top of asphalt and didn't want to punch any unecessary holes. I could hear the telltale sounds but they were intermittent and very faint. It was a small leak. That also matched what we were seeing on the surface. A sprinkler box would fill with water but very slowly once the water was turned on.
We use a bit of compressed gas in the line and were able to pinpoint the leak precisely. I told my buddy, it's right here. It was a few feet away from where he thought the pipe run would be. Anyway, we spent the next 45 minutes sawing asphalt and digging. After a bit of work, we found pipe and a minute after that we could see our leak. It was indeed a small leak but trouble just the same. We were right on top of it with our locate.
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