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Space sentinel

Infrared satellite images may be useful for predicting volcanic eruptions

Kilauea, in Hawaii, has been erupting ontinuously for years

UNITED STATES GEOLOGICAL SURVEY (USGS)Kilauea, in Hawaii, has been erupting ontinuously for years UNITED STATES GEOLOGICAL SURVEY (USGS)

Of the hundreds of active volcanoes on Earth (the exact number is the subject of debate among specialists), few are being monitored by fixed instruments or field researchers, who risk death from asphyxiation, burial or incineration. Although a number of satellites are continuously scanning the Earth’s surface, there is no global system that can alert people living near volcanoes that an eruption is about to occur. That goal is still far from being achieved, but alternatives have been proposed for the study of volcanic activity on a global scale. With that concern in mind, researchers at Brazil’s University of Campinas (Unicamp) and England’s University of Cambridge have developed a method for analyzing images of volcanoes obtained from different satellite-borne infrared radiation sensors and, in this way, perhaps anticipate eruptions months in advance.

In separate studies, the team of scientists applied the method to study the activity of five active volcanoes over the course of the past decade in different parts of the world. Their findings indicate that it is possible to implement an automatic system through which subtle changes in the thermal activity of volcanoes can be detected from space. Such changes serve as an indicator that a volcano is about to erupt. “By using algorithms developed in our research to investigate these anomalies, we can identify signals that precede lava flows in certain volcanoes,” says Samuel Murphy, an Englishman who defended his doctoral thesis on the topic at Unicamp in early 2013, under the guidance of Carlos Roberto de Souza Filho, a geologist specializing in remote sensing at that São Paulo State university, and vulcanologist Clive Oppenheimer of the University of Cambridge. The hotter an object, the more infrared radiation it emits. “The changes in thermal energy emitted by a volcano over time can be observed from space,” explains Souza Filho. After completing his master’s degree in geology in 2007 at the University of Bristol in the United Kingdom, Murphy decided to meet some of his Brazilian mother’s family in Campinas, and he ended up enrolling at Unicamp. There he became a doctoral candidate under Souza Filho, who had worked with Oppenheimer during his own doctoral studies in England.

In an article published in April 2013 in Remote Sensing of Environment, the journal with the highest impact factor in that field, the English and Brazilian geologists and vulcanologists showed the potential of the methodology. They compared the activity of Láscar, in northern Chile, between 2000 and 2012, with that of three other volcanoes—Erta‘Ale in Ethiopia, which has a more or less continuous lava lake; Kilauea in Hawaii, which has been discharging lava almost non-stop since 1983; and the explosive Kliuchevskoi, the largest volcano on Russia’s Kamchatka Peninsula. The thermal images used in the study were obtained by two infrared sensors, ASTER and MODIS, both of which were installed aboard NASA’s Terra satellite in 1999. Its orbit, at an altitude of about 700 kilometers, passes over the North and South poles and brings the satellite over the Equator at the same time each day and night.

ASTER captures infrared radiation at several wavelengths and obtains images with a spatial resolution of 30 to 90 meters, which is considered relatively high. Although its spatial resolution of one kilometer is much lower than ASTER’s, MODIS records two images of a single location on Earth every 24 hours, while ASTER does so only once every 16 days on average. The researchers’ idea was to analyze the images from the two sensors in combination. The data from ASTER were used to distinguish regions of the volcanoes that had thermal anomalies, while the data from MODIS helped them monitor changes in these areas over time.

The map shows the five volcanoes studied.

The map shows the five volcanoes studied.

This method enabled the researchers to monitor the evolution of the lava lake inside the crater of Kliuchevskoi on two occasions. Images in February 2007 indicated abnormal activity in the volcano, but the eruptions did not begin until June of that year. The effects of that eruption caused air traffic disruptions in parts of Asia and the United States. Signs of a smaller eruption in 2008 were also identified months beforehand. The researchers suggest that the lava spills from the Kliuchevskoi eruptions could have been predicted two weeks ahead of their occurrence. This would be possible with an automatic monitoring system capable of issuing an alert when an area inside the crater equivalent to more than five pixels out of a total of 500 million contained in the satellite images is 40ºC hotter than the surrounding area.

The MODIS data also served to identify subtle weekly, monthly and annual oscillations in the size and intensity of volcanic regions with thermal anomalies. “Faster oscillations are associated with surface activity, such as lava spills,” Murphy explains. “Slower oscillations are related to deeper activity, such as an increase in the amount of magma in the magma chamber.” Another participant in the research was vulcanologist Robert Wright of the University of Hawaii at Manoa, who proposed a pioneering global volcano monitoring system in 2000 using images from the MODIS sensor.

In an earlier study published in 2011 in the Journal of Volcanology and Geothermal Research, Souza Filho, Oppenheimer and Murphy analyzed images obtained by ASTER between 2000 and 2009 from two very different volcanoes. Mount Erebus is located in a frozen polar region on Ross Island in Antarctica. It has a permanent lava lake that is continuously active, while Chile’s Láscar volcano is in a broiling desert and has intermittent explosive eruptions. The challenge was to identify and distinguish, on infrared satellite images, portions of the volcanoes that are only a few tens of degrees hotter or cooler than the surrounding areas, and interpret them with the help of reports from field expeditions to the volcanoes.

The researchers were able to identify a subtle increase in temperature inside the Erebus crater, associated with an unusual water plume very rich in volatile compounds that issued from the volcano in January 2001. Of equal interest was warming detected around Láscar nine months prior to a period of eruptions in October 2002, probably owing to an increase in the emission of gases. Three months before the beginning of another period of eruptions in April 2006, the researchers observed a gradual decrease followed by an increase in temperature, associated with the emission of gases or the formation of a lava dome.

Mount Erebus, in Antarctica: largest active volcano on the continent

NICK POWELL / NATIONAL SCIENCE FOUNDATIONMount Erebus, in Antarctica: largest active volcano on the continentNICK POWELL / NATIONAL SCIENCE FOUNDATION

Volcanoes are points on the Earth where magma—molten rock that in some places lies just below the Earth’s crust and whose temperatures range between 600ºC and 1,300ºC—is able to rise and accumulate near the surface in underground chambers. During an eruption, the magma may simply overflow the volcano’s crater, or it may escape through fissures and flow out in lava spills. It may also solidify before it reaches the surface and accumulate in domes (see figure at left). “Domes are dangerous because the magma inside them contains volatile compounds, mainly water, that are dissolved and incorporated within it; the high temperatures and concentrations of these volatiles within the magma can cause extremely violent eruptions,” Murphy explains.

It all depends on the chemical composition of the magma. The less silica contained in the magma, the more fluid the lava, and eruptions tend to be gentler and continuous, as in the case of the Hawaiian volcanoes. A richer silica content produces more viscous magma, which often accumulates in domes until it explodes. The explosions can create a plume of ash, steam, carbon dioxide and sulphur dioxide that rises tens of kilometers skyward, or streams of rock and ash flowing around the volcano at a speed of up to 300 meters per second—a phenomenon known as a pyroclastic flow.

Each year there are 50 to 70 eruptions around the globe, lasting anywhere from hours to months. Atmospheric currents spread ash from major eruptions over thousands of kilometers. The ash can accumulate, melt and fuse inside airplane turbines. To date, no aircraft has actually been downed by this phenomenon (although flights have been temporarily affected), but losses incurred from turbine repairs and air traffic bans in several countries for a number of days can reach billions of dollars.

The Brazilian state of Rio Grande do Sul is affected periodically by plumes from Andean volcanic eruptions. On the morning of April 21, 1993, residents of that state who left home early to vote in a national referendum on the country’s system of government were surprised with a light drizzle of ash. In the city of Porto Alegre, streets, car hoods and roofs were covered with a fine layer of dark-colored dust that had been launched into the air by an eruption of the Láscar volcano in northern Chile, 1,800 km away. More recently, air space over the state was shut down due to the eruption of Puyehue, another Chilean volcano, in 2011.

Although volcanoes are nearly impossible to predict, they usually show signs of an impending eruption, sometimes hours or even months beforehand. The most studied indication is an increase in earth tremors detected by seismological stations. Other common clues are changes of a few centimeters in topographic relief, which can be detected, for example, by interferometric radar systems; and an increase in the emission of gases as well as changes in their composition. The Unicamp-Cambridge group is counting on another approach, namely, satellite monitoring of changes in thermal activity throughout the volcano and its surrounding area. According to Souza Filho, most of the previous studies used satellite images to focus on quantifying the region of a volcano that had the highest temperature or the average temperature of a volcano as a whole. However, what often makes it possible to distinguish a dormant volcano from one about to explode are temperature variations in specific portions of the volcano.

“Challenges remain for turning these satellite observations into a routine operation,” Oppenheimer comments via email directly from Ross Island in Antarctica, where he is observing the continent’s largest active volcano, Mount Erebus. “But Murphy’s research not only identified fascinating trends in volcanic heat emissions, it also analyzed the details of techniques for automating the processing of enormous volumes of data and quickly extracting the thermal signals we are interested in.” Temperature oscillations can help predict the duration and intensity of future eruptions. “Each volcano has its own very individual activity,” explains Brazilian vulcanologist Rosaly Lopes of NASA’s Jet Propulsion Laboratory. “It is important to understand that individuality because it will probably behave similarly in the future.”

Murphy is continuing his research on remote sensing of active volcanoes at Unicamp, through a post-doctoral research grant from FAPESP. He and Souza Filho are analyzing images from NASA’s newest Earth observing satellite, Landsat 8. In 2014, the European Space Agency is expected to launch two new satellites, Sentinels 2 and 3. Taken together, the data from these satellites will enable the monitoring of volcanoes via images with a spatial resolution of up to 10 meters, obtained every five days. In the meantime, Murphy has concluded in his thesis that even the new instruments will not be able to provide reliable temperatures for volcanoes. In an article accepted for publication in November 2013 in the journal Remote Sensing of Environment, the researchers have proposed an even more refined method for quantifying the thermal energy radiated from each point of a volcano. “We are somewhat anxious as we await the outcome of this particular article. It’s a controversial finding, because it could change the way thermal anomalies have been measured from space since at least the 1970s,” Souza Filho says.

Global volcano monitoring using the next generation of orbital sensors with an emphasis on South America (No. 2013/03711-5); Grant mechanism: Post-doctoral research grant; Coord.: Samuel Murphy – Unicamp; Investment: R$163,082.88 (FAPESP).

Scientific articles
MURPHY, S.W. et al. Modis and Aster synergy for characterizing thermal volcanic activity. Remote Sensing of Environment. No. 131. April 15, 2013.
MURPHY, S.W.; OPPENHEIMER, C.; SOUZA FILHO, C.R., Calculating radiant flux from thermally mixed pixels using a spectral library. Remote Sensing of Environment. No.132 (accepted November 21, 2013).