I must have described something different...Check this site which contains this other "Tunnel 8" exploration? information with a very interesting method of direct-arrival transmission imaging:
http://www.appliedgeophysics.berkeley.edu:7057/geoengineering/appliedgeophysics/papers/washb.pdf
...."just proved the viability of surface-to-borehole imaging of shallow voids". 3D VSP should have been used earlier.
If those metal detectors are not effective, could VSP data be the best to detect underground tunnels?
Hi,
When you talk of direct-arrival transimission, that ussually refers to the reflected radar pulse that bounces off the object or anomally(void),
are you talking about a borehole antena of a GPR? Anyway, given the right ground conditions,GPR should be able to detect a shallow void with a 250mhz cart pulled antena(not borehole). HH
Jose