This may refer to a ferret tracker, a two piece unit comprising a small transponder and a tracker unit which can detect the transponder and allow the user to hone-in on the transponder location when underground and estimate distance away (a bit like a metal detector).
I think the intended use is that you attach the transponder to a ferret, send it down a hole, and track where it goes.
In caving however, we take the transponder underground and shove it up as high as possible where we think the surface may be close, and a team on the surface can see if they can locate it in the estimated location.
Generally seems to work well up to 4-5m underground as long as it isn't solid rock all the way
We use it routinely to precisely locate underground passages which are close to the surface (which is great to help fix the survey), including our
very recent work to surface locate the far reaches of the Jenga Pot Covid Extensions in the next valley. We've also used it to locate the far end of Bogg Hall Cave, a few points throughout the rest of the Excalibur/Jenga system and also to track a diver underneath the River Dove where the passage is only 3m below the surface. A very useful device if you have caves with several bits that come close to the surface.
www.bellmanandflint.co.uk
