good point.... the problem of using infrared signals is that the tunnel I'm looking for is blocked with at least 3 or 4 meters of rocks and debries. in other words it doesn't have a surface expression or openning so that a temperature gradient cannot be detected nor measured, could it ?. I guess not. the low temperatures inside the tunnel will be damped by the heat of the ground. infrared film would have to be very sensitive to see very small temperature gradients across the surface of the ground. BUT, BUT; BUT if we measured the heat flow directly on the ground using geophysical equip. may be there's a chance... !. great, I'll check that out !.

on the other hand, using seismic waves (i.e. submarine radar) is extremely difficult. if you've checked out the vittorio peak project web site ($$$$://bozo-1.lbl.gov/users/wash/Victorio_Peak) (replace $$$$ with http) you will know what I mean. They've invested thousands of dollars and nothing yet. an additional problem with seismics is that there're lots of vegetation and full of outcrops so it's difficult to 'plant' the geophones on the ground.

I was hoping that an electromagnetic device such as WHITE'S TM808 would help 'cause it'd be easier to use...

appreciate your suggestions.