Hi there, Andrew,
The following might be considered 'off topic' but it isn't really - it's evidence of the effectiveness of using water-borne pressure waves to detect sub-terranean goings-on:
First, a recap. for younger readers: way back in the early days of U.K. wireless broadcasting, home radio receivers using thermionic valves (a.k.a. 'tubes') had a high tension ('H.T.') 'dry' battery for the anode circuitry and a low tension ('L.T.') battery to power the filaments of the valves. Strictly speaking, the L.T. supply was a cell rather than a 'battery', it was a 2 Volt lead-acid accumulator in a glass jar with a carrying handle. This, being a secondary cell, needed recharging at regular intervals. The charging was usually performed by the local bicycle shop at a moderate cost.
Though this recharging cost was moderate, some householders begrudged it. There was, on the market, a low voltage dynamo driven by a Pelton wheel turbine, arranged to be connected to the kitchen cold water tap. This transferred the effort of recharging the battery from the householder's pocket to the Water Company whose pumps would now be the source of the required energy!
The Water Companies weren't at all enthusiastic about this particular piece of technology. However, it was found that the alternating passage of the Pelton 'spoons' and the intervening gaps past the jet of the turbine launched an audio frequency pressure wave back up the water main. This signal was very detectable by use of a metal rod placed one end on the bone behind the Inspector's ear and the other end placed on some accessible bit of the water main. This enabled the Water Companies' inspectors to rapidly 'home in' on the houses where these dynamos were in use. This sounding rod was/is a standard tool of the Water Companies' inspectors for leak detection.
Your intended set-up, Andrew, seems to be a modern version of the inspectors' sounding rod.
As recently as the 1960s, I remember an occasion when I was working at home (not because of Covid!) when my attention was drawn to a group of men in navy blue raincoats in the road outside my house, each of them carrying a sounding rod and taking turns to try it on the stop-cock beneath the pavement. This was a group of trainees doing their outdoor practical under the tuition of their instructor. So, it works.
Andrew, I wish you 'good hunting'.
Well, in the time that I was typing my post ......
