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Things may have changed since I last fooled with IR imagery, but my
recollection is that it doesn't take a whole lot of insulation to
mask temperature differentials.  A layer of snow, or a couple of feed
oops, feet of earth are going to cover any subsurface warm spots.  What
you might pick up is cave entrances, assuming that the cave is exhaling,
and that the ambient air temperature is cooler than the subsurface air
being exhaled-- which is 54 farenheit if memory serves.  It also seems
to me that not all caves exhale-- there is one in Virginia called
Breathing Cave, but beyond that I don't remember any as being particularly
breathy.  I don't know what kind of money you have to throw around, but
aerial photography isn't exactly cheap...  You probably need to talk to
someone in the IR business-- it's possible USGS Reston, or the nearest
ESIC would have a handle on someone who could give you a better answer.
 
charley seavey
aka DOCMAPS