Message Board Thread - "Geology and IR?"

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Geology and IR? Dave 1/7/2004
I will be embarking on a project this summer in search of some archeological under ground tunnels. These tunnels or caverns may be up to 100' below the surface of the ground and possibly filled with sea water. Some may be much closer to the surface and air filled. I was wondering how effective IR scanning would be in detecting their location from a few low level passes from a light plane. I will be using a FLIR P20 camera with standard lens. Thanks for any info. Dave
Geology and IR Bernie Lyon 1/8/2004
In order to detect underground voids (air) or chambers filled with water, there must be a temperature signature that is produced on the surface. If the cave is filled with water, the thermal signature may be provided by transient heating or cooling of the surface above. If the surface is exposed to solar loading all day, followed by a clear night, the water will cool off slower than the rock (provided that the rock is not an aquifer), due to the high specific heat of water. The presence of water would produce a warm signature, similar to that in a roof moisture survey. If the cave was filled with air, you might see a cooler signature due to the increase in thermal mass where there is more rock. I suspect that this signature would be difficult to detect. If the air or water is too far below the surface, it will not be detected due to increased insulating properties and lateral heat conduction. I don't believe that you will detect anything at a depth of 100 feet. There are many variables involved in this application. If possible, you may try to determine how feasible this is by conducting a test with caverns of known depth and interior. I would recommend starting with caverns close to the surface. If these are detectable, you could proceed to deeper caverns to evaluate the limitations of the method.

Below are two web links and a publication that may provide additional information. Good luck with your investigation.

Hook, et. al., Use of Multispectral Thermal Infrared Data in Geological Studies: in Remote Sensing for the Earth Sciences;
Manual of Remote Sensing, 3rd Ed., Vol 3, edited by Andrew N. Rencz, John Wiley and Sons, New York.
Re:Geology and IR? JKEngineer 2/4/2004
An alternative to Bernie's suggestion of an intensive experimental field program to determine the ability to find deeply buried caverns is to do the bulk of the work through heat transfer analysis and simulation. While such an approach would need some validation using field data or experiment, it would not need the level of field work that a purely experimental program would entail. As a result, it would be faster, less expensive, and generally more feasible. It would not require finding examples of the cavern type structure to fit each proposed experimental condition. Experimental validation of the results could be done with an existing data set, if such could be found.

The approach I am describing is much as Bernie describes: look at a range of conditions to estimate what produces a detectable signal. The primary difference is that it would be done on a computer, not in the field. Using, for example, a finite element analysis package to perform the heat transfer analysis would enable a variety of conditions to be examined. As I like to remind thermographers, we measure temperature; heat transfer is how the temperature got there.

I have published several papers with example calculations of this type, i.e., using FEA based heat transfer analysis to model processes of interest to thermographers. Let me know if you would like to review them.

I would be glad to work with you on your project.

Jack M. Kleinfeld, P.E.
Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

4011 Hillman Ave
Bronx, NY 10463

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