Caver Bob maintains a list of long sea caves. I'm looking for instruments to measure the length of sea caves.
So far, I've used a laser to measure two small caves, 33 and 55 m long.
But I think the best solution might be sonar with an active transponder. The master sends a ping, the transponder answers pong - on another frequency, to avoid confusion with all the echoes in a cave.
The master then divides the delay by two, and multiplies with the speed of sound.
This solution should work, even around corners.
A simpler one-frequency set-up could be a conventional depth sounder turned horizontally, and a submerged radar reflector. But I think false echoes would be a problem.
Also see Faroese caves on Facebook and Google Maps.
Google update: JW Fishers make this stuff.
