Cave/void detecting-I need solution for up to 500euro...

[Flareproof]

Hi I'm new to here, so basically I run with my garrett250 on weekly basis but for some time now I'm more and more into caves detecting, I've done some research but can't decide which way should I go. I found echo sound method quite cheap but is it reliable?

[jimrocks]

I have spent much money with traditional GPR and come away empty due to time/dollar restraints, terrain issues and highly crystallized rock composition.
The property I'm working on has very consistent riolitic rock exposures and contains myorolitic air cavities with amethyst crystals inside. The...