Thermal image

This high resolution airborne thermal infrared image shows a portion of Norris Geyser Basin. The imagery was acquired at night in March 2015. Pixels are about 1 meter (3 feet) on a side. Bright areas are warmer, dark areas are cooler.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from R. Greg Vaughan, research scientist with the U.S. Geological Survey.

In a previous Yellowstone Caldera Chronicles article we discussed using satellite thermal infrared remote sensing to study the thousands of thermal features that are spread out across Yellowstone National Park. One of the advantages of using satellites is that you can cover large areas with a single image. A disadvantage, however, is that small spatial details cannot be resolved in images that have a wide field of view and large pixels (which are generally the size of a soccer field). How can this problem be solved?

Stop us if you've heard this one. A pair of photons check into a hotel. The hotel clerk asks if they need help carrying luggage. The photons reply, "No thanks. We're traveling light." Get it? Photons are traveling light! But different types (or wavelengths) of light can have different numbers of photons. One of the reasons we cannot get high spatial resolution thermal infrared surface images from Earth orbit is because there just aren't as many photons coming from the surface at thermal infrared wavelengths compared to the number of photons at visible wavelengths. In other words, we can get high resolution images from orbit if we are looking at visible light, but this is not possible using thermal infrared wavelengths. Thermal infrared instruments have to be very sensitive to detect the few photons that are emitted by merely warm objects. One of the ways to increase sensitivity is to make the pixels bigger, so that satellite sensors can collect more photons. The trade-off is lower spatial resolution. This why we need airborne platforms.

It turns out that thermal infrared cameras mounted on aircraft, such as airplanes, helicopters, or unoccupied aerial systems (UAS), can capture images with pixels that are less than or equal to 1 meter (3 feet) on a side. The smaller the pixels, the higher the spatial resolution, and the more detail you can see on the surface. Although these higher-resolution airborne images have a smaller spatial footprint and would not be used to map the entire park, they can be used to see important features that cannot be detected by satellites.

As an example, we constructed a temperature map of the Back Basin area of the Norris Geyser Basin using a series of images acquired by a thermal infrared camera on an airplane. In the image, bright areas are warmer and dark areas are cooler. Individual thermal features and the warm waters that drain them are thermally distinct. Boardwalks that are raised off the ground are cooler than the surroundings, and overland trails are warmer. When these images were acquired, at night in the winter time (March 2015), there was snow on the roofs of buildings, on the boardwalks, and on the ground in areas away from the thermal features, which is why these areas have below freezing temperatures.

The highest temperatures in the mosaic are about 70C (158 degrees) and are located at Echinus Geyser, Monarch Geyser, and Cistern Spring. On the ground, these thermal features display boiling temperatures, around 93C (200 degrees), but infrared imagery almost always retrieves slightly cooler temperatures due to a combination of steam interference and the fact that all pixels contain a mixture of boiling water and cooler background.

Of course, there are trade-offs with airborne platforms, as well. Airborne image collection missions are expensive, so they are not conducted very often. Someday unoccupied aerial systems (also called drones) may provide platforms for inexpensive, routine data collection over thermal areas. The National Park Service prohibits the use of drones in national parks, except for search and rescue or during a crisis (such as the recent eruption of Kīlauea Volcano). In the future, with proper flight safety protocols, drone platforms operated by trained and certified pilots could provide high-resolution images over thermal areas at a frequency that could be used for regular monitoring and change detection.

Images from airborne platforms fill an important niche between satellite-based and ground-based data. Satellite data can provide information about the distribution of thermal areas across the entire park, including remote and inaccessible areas; airborne data can provide detailed information about local-scale thermal areas; and ground-based data can provide extremely detailed and accurate information about individual thermal features. The combination of all three is an ideal strategy for a robust thermal monitoring plan in Yellowstone.

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