But they had huge advantage of no surrounding civilization. Power line above you can easily "echo" in the picture that should come below you. I've used one myself to see some plumbing.
But the resolution gets poor quickly as you go deeper. Most common applications seem to be inspecting rebars from reinforced concrete and inspecting roads for subsurface problems. Both in range of just few meters.
http://earthzine.org/2008/09/26/a-brief-history-of-radio-ech...
But they had huge advantage of no surrounding civilization. Power line above you can easily "echo" in the picture that should come below you. I've used one myself to see some plumbing.
But the resolution gets poor quickly as you go deeper. Most common applications seem to be inspecting rebars from reinforced concrete and inspecting roads for subsurface problems. Both in range of just few meters.