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Finding Pyramids!

Last week, I took aim at how poorly aerial thermography was being applied for more efficient use of energy in buildings. The technique just doesn’t work and never has! However, the fault does not lie within aerial thermography. Aerial thermography  is nothing short of brilliant when used properly.

Last week, I heard a BBC story (click here to read the full story) about how thermal imagery from satellites has helped archeologists locate a number of pyramids and other structures buried under the Egyptian deserts for eons!!! How? The secret is having an understanding of thermal capacitance.

I learned the power of these techniques for archeological explorations from Bob Melia almost fifteen years ago. While he has since passed on, Bob had mastered the use of thermal imaging during a career in the US Coast Guard. After he retired he combined his love of thermography with a love of history and discovered how to squeeze some fascinating stories out of the ground.

While Bob couldn’t reveal all the secrets of his successes, he made it clear they all began with a solid understanding of how materials change temperature in a thermally transient environment.

This remarkable image shows railroad tracks going into a Civil War prison camp in South Carolina. The rails were buried beneath nearly three feet of soil but Bob Melia, a pioneer in using thermal images for archeological research, found them based on a solid understanding of thermal capacitance and history. See http://articles.latimes.com/2000/may/14/news/mn-29896 for more details.

When a thermal transient occurs—in this case as the sun shines or as night falls—the affected materials change temperatures at different rates. How quickly (or slowly) they change depends on two variables:

Thermal Capacitance
The first variable is their thermal capacitance or how much energy is required (or released) when they change temperature. Typically, this is indicated as a quantity of energy for either a mass or volume of the material.

For instance, think of a pound of air and a pound of water. How much energy is required to raise their temperature by one degree? Clearly more energy is required to change the temperature of the water. Similar relationships can be measured for cubic foot samples of air and water. Once again, it is obvious water is the winner of the capacitance race, meaning it has a greater capacitance. In fact, water has the highest thermal capacitance of any common material (ammonia is slightly higher)!

Heat movement
The second variable is how the heat moves into (or out of) the material samples. Heat easily moves in fluids like water or air by convection. Let’s compare a fluid and a solid like water and granite.  The winner is not quite so obvious! While convection can be a rather fast way to move heat, conduction is typically much slower. Comparing two solids materials can be even more challenging. We need to understand not only their rates of conduction but also their capacitance values and, even then, the heat flow dynamics in the real world can be quite complex.

Now back to those pyramids hidden beneath the sand.

The stone used in the structures has a higher thermal capacitance than the undisturbed soil around them. They also have different conductivity values with the stone being roughly three times more conductive. As the sun warms the surface of the land and heat transfers into the subsurface regions, the soil, with a lower capacitance, warms more rapidly than the buried stone structures. However, once the stone is warmed, heat flows within it more rapidly. At night, the process reverses as heat transfers out of the ground to the cooler clear sky, and the undisturbed soil cools more rapidly than the stone.

At some point in the early evening of a clear night after a sunny day, the temperature difference between the two areas is at a maximum, enough to be detected by the thermal imaging system in most cases. If conditions are good and if the stone is not buried too deeply and more importantly, if the person interpreting the image understands all the variables, the subsurface structures are revealed! How deeply can these subsurface structures be and still create detectable thermal signatures? Some reports indicate success at twenty feet or more!

I realize most of you will not be looking for lost pyramids or other buried treasures, but next week I’ll talk about some of the practical uses of these techniques used either from the air or while being hand-held. We still can’t actually see through the soil, but once we learn the tricks of capacitance, a whole new world opens up to reveal what lurks beneath it!

Thinking Thermally,
John Snell—The Snell Group, a
Fluke Thermal Imaging Blog content partner

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