Saturday, November 22, 2008

Taking a volcano's temperature

While running through my RSS feed, I came across this National Geographic article about using thermal infrared imaging to monitor and forecast volcanic eruptions. I'm currently working on a project that involves using satellite imagery to detect and map hydrothermal alteration products in a volcanic dome, so I was definitely interested, especially because the scientists involved are using data from the same instrument that I am.

First, I have to give them props for using the term "forecast" rather than "predict". I know a number of volcanologists who are touchy about using "predict", because any conclusions about what a volcano may or may not do are necessarily based on probabilities, just like a weather forecast. When people start thinking that we can say for certain when a volcano will erupt, we get the blame when it doesn't, and they have to deal with the consequences of precautionary evacuations - or if it erupts sooner and takes everyone by surprise. It's very important that people know that no scientist can be 100% sure when a volcano will erupt and what it will do - voclanoes are simply too complicated.

I also like the bit about volcanologists being "courageous scientists". I suppose that even though we're very aware of the danger involved in our work, we don't really see ourselves as courageous. It's just another aspect of a job, and for the most part a calculated risk when we venture onto an active volcano to observe eruptions. That said, if there are alternative ways to get the same information with less risk of getting injured or killed (or having to spend an inordinate amount of time and effort getting to hard-to-reach places), I'm all for them.

Anyway, the article discusses how Michael Ramsey and Adam Carter of the University of Pittsburgh are using a combination of ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) satellite images and FLIR (Forward-Looking Infrared Radiometer) camera pictures to monitor active volcanoes on the Kamchatka peninsula. This area of Russia is of major concern to both volcanologists and the aviation industry, because there a number of airplane flight routes go directly over this area. Erupting stratovolcanoes can create ash plumes that rise up to 50 km, far above the normal crusing altitute of a jet. The worldwide Volcanic Ash Advisory Centers already keeps a watch on these volcanoes using the MODIS (Moderate Resolution Imaging Spectroradiometer) and TOMS (Total Ozone Mapping Spectrometer). MODIS detects the thermal signatures of eruptions, and TOMS detects the gases - usually SO2 - associated with plumes.

(ASTER image of Bezymianny volcano lava flow from NASA Visible Earth image archive)

Ramsay and Carter (together with U.S. scientists at the University of Alaska-Fairbanks and Russian experts at Kamchatka's Institute of Volcanology and Seismology) worked at Bezymianny volcano, using the FLIR to record temperature increases in the lava dome just days prior to an eruption. They were able to correlate their ground data with data from NASA's ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) instrument, which records several bands of thermal infrared data. This is particularly significant because it means that for volcanoes with fairly well defined thermal precursors to explosive eruptions, scientists can use satellites to monitor them, rather than traveling to remote locations with expensive, cumbersome equipment.

There are a few caveats. Any thermal satellite image records not only the temperature at any one time in a location, but the temperature history. Thermal emissivity varies with the amount of energy a surface absorbs, and with the rate that the surface re-emits that energy. Some surfaces absorb lots of energy but lose it quickly; some absorb energy and emit it slowly; others don't absorb much at all. All of this shows up in a thermal image, which should be treated more as a time exposure than a single snapshot. The article sums it up pretty well:
"Because the satellite images capture an average temperature reading for the entire volcano at a given moment, the scientists knew the reading in some areas was probably many times higher."
This must be taken into account when analyzing satellite data. A single "bright" ASTER image can mean that temperature increased suddenly, or that temperatures increased steadily over the course of hours or even days, depending on how quickly the lava's surface loses heat. Several images, taken hours or days apart, however, would prove very useful. Unfortunately, obtaining even one line of ASTER data - or data from any satellite that records in the thermal band - is expensive, and requires a special requisition process. Additionally, most thermal data comes from instruments on satellites that also serve other purposes; finding a way to dedicate any satellite entirely to recording thermal imagery of volcanoes would be difficult and expensive (again). Still, it would be extremely useful - and it's exciting research even without that.

The research is being partially supported by the National Geographic Society's Committee for Research and Exploration. A Bulletin of Volcanology article about their work can be found here.

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