Synthetic-aperture radar (SAR) has been widely used for Earth remote sensing for more than 30 years. It provides high-resolution, day-and-night and weather-independent images for a multitude of applications ranging from geoscience and climate change research, environmental and Earth system monitoring, 2-D and 3-D mapping, change detection, 4-D mapping (space and time), security-related applications up to planetary exploration. Radar satellites are particularly well-suited for surveillance of vast lands with limited above-ground distinct features, such as glaciers and deserts. It is also used extensively for areas optical satellites have trouble covering, such as cloud-covered areas, or areas shrouded in darkness, as radars can penetrate through clouds and do not need sunlight to illuminate the scenes they are capturing.
Andrew Hooper, Professor of Geodesy and Geophysics at Leeds and Co-Director of the Institute of Geophysics and Tectonics, started working with SAR satellite data during his PhD, using the radars’ capabilities to gain a better understanding of volcanoes.
“Radar can measure surface movement, helping understand what is going on at depth. When applied to volcanoes, you can infer something about what’s going on beneath the ground. If you get magma moving around, or any kinds of fluids, it forces the earth to bulge up or down depending on which way they are moving, and we can measure that with surprisingly good accuracy.”
Keeping an eye out
With some of the more recent SAR satellite missions, like Sentinel-1, launched into 2014, Hooper and his colleages at COMET, the Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics, are working to track the 1,500+ volcanoes above sea-level for any signs of ground deformation, a tell-tale sign of magma activity. “Now that we get regular data, it’s our plan to monitor all of the volcanoes above the sea. A lot of volcanoes are not doing much in terms of deformation but if something does happen, we can flag that, start to keep a closer eye, and alert the authorities in that country.”
Eruptions, as with earthquakes and other tectonic related processes, can be years in the making and carry subtle deformation signals over time that Hooper and his colleagues can monitor and track. There is a very good correlation between volcanoes that have deformations and volcanoes that erupt. But the timing is much harder to tie down and depends on the volcano. For example, the Icelandic volcano that erupted in 2010, disrupting air traffic for days, showed markers of activity as far back as 1994 and then again in 1999. As this volcano remained quiet for 400 years prior, scientists took it as a sign that things were happening and that the volcano should be monitored more closely by local authorities.
It’s a stretch
In a similar fashion, leading up to earthquakes, the ground is slowly straining. With regular SAR data acquisitions over longer period, the team can pick strains of a few millimetres per year over distances of more than 100 km. While they cannot predict earthquakes, they can identify areas with significant strains and can improve maps of earthquake hazards.
Past the noise
Radar technology means there is a lot of noise in the data. Hooper is currently working on increasing the signal to noise ratio. One method he favours consists of looking at time series. By looking at 50+ images of the same area, it is easier to spot areas where the signal will be good and where there will likely always be too much noise.
“The original satellites we were using could have 35 days in between passes if you were lucky. But now, you can get satellite passes every day if you integrate many different satellites, so we can see with much more details things that are happening on the time scales of days or weeks. You can see the changes and evolutions on volcanoes.”
As the number of radar satellites increases, the Centre will continue to improve their monitoring and hopefully be able to detect even more over time.