On the website of italian club CGEB you can read two interesting articles about the use of ARVAs  (avalanche transceivers) on detecting cave wall thickness, or for example, where is the cave passage closest to the surface? With a known survey of the cave, they checked around where should be the a second entrance made to Grotte Impossible. At the point where the cave should be closest to the surface, they double checked it with the Arva device, and they could se the planimetric errors of the survey, and the rock thickness to the cave. So they were able to know where should they dig in reality (in contranst of the point “showed” by the survey). this was quite important, because they had to dig more than a dozen meters in solid rock, artifically. Here and here are the two articles.

You can read about a tweak with ‘clinometers here. It can help you to read more easier the instrument.