Grumpy_Mike - Thanks for the reply but I would need some convincing. As I see it the thing is that a radio wave is polarised and so a Yagi only needs to work in the direction of the polarisation but the ultrasonic is not polarised and so needs to be 2 dimentional.
GoForSomke - not really the place to discuss the merits of caving vs other activities but that stats show that caving (at least the sort of UK caving I do) is pretty safe. Driving to the caves is much more dangerous.