I have used similar, for a similar problems.

With the first the pipe was about 30cm/ 1 foot, down under concrete paving slabs and sand/soil and it worked just fine. With the second the pipe turned out to be a lot deeper than I had been told and the sensor could not pick it up. For that I just pushed the probe as far into the pipe as it would go, put a piece of tape on the probe cable before I pulled it out of the pipe to indicate the length used then laid it out on the surface so I could mark the approximate position of the other end. I was about 20cm off so no big problem.