I wonder if some fancy playing with ultrasonic sensors or maybe 
microwave detector might work?

Ultrasonics can be used as windspeed detectors, so maybe nulling out 
ambient wind speed would leave 'draft' speed from rock holes. 
Microwave tx and rx'ing might work the same?

Circuit Cellar had an article on using the Parallax thermopile a few 
months ago. The problem with a thermopile would be that you'd need to 
calibrate it so it didn't respond to the  background material.

Colin
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cdb, colin@btech-online.co.uk on 17/04/2009
 
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