distance from a large object through a tunnel that is 30 feet long and 10 feet in diameter.
It was the same when I used to go caving, I never did decide if it was an indoor or outdoor sport?
Guess it is ok to use it in an indoor shower.
Does your cave have a door?
Actually yes it had one. Valley entrance in the Kingsdale Valley in the Yorkshire dales. You walk through a field and there is an oil drum on a slight mound. You take the lid off and crawl in.Also there is this one:-
You might consider an array of transducers,