Cave large enough to detect with a pendulum?

*Ulysees*

Are you aware of any other, low-cost technologies to detect caves? I think ELF radar goes quite deep, is that so? What about sonar?

Ulysees	Cave large enough to detect with a pendulum? Are you aware of any other, low-cost technologies to detect caves? I think ELF radar goes quite deep, is that so? What about sonar?