Tectonic changes, subsidence, flooding, forest fires…What happens in nature has a direct bearing on our lives. Satellite data is invaluable to our understanding of the state of nature.
Detecting the actual conditions of natural disaster from space
It isn’t easy to grasp the impact of large-scale natural disasters over a broad area from the ground. Conversely, a lot of information can be obtained using satellites that can take a comprehensive view on the situation from far away in space.
For example, ground surface movements such as subsidence and crustal deformation can be detected by measuring the distance from a satellite to the ground. With interferometry analysis using microwave sensors, it is possible to pick up ground movements accurate to a few centimeters. In particular, L-band microwave from the PALSAR, mounted on the ALOS, can see through vegetation with ease, making the ALOS particularly useful for detecting changes in land surfaces, as it is unhampered by vegetative ground cover. This is particularly useful for countries with many forests.
The extent of damage caused by landslides and flooding can be understood by looking at differences in the way microwaves reflect and radio waves scatter as a result of changes in the surface of the land due to the spread of sediment and water.
There is tremendous potential for the use of satellite data in preventing and responding to the disasters that have such a tremendous impact on our lives.
Detecting ground subsidence
From these images taken around a certain coalmine, we can calculate changes in landform in two places, which appear here as blue elliptical shapes. On the right is a coalmine where land has actually subsided. The oval on the left is subsidence due to the use of great quantities of groundwater during the digging of the coalmine. This is clearly visible from the satellite.
Subsidence have a big impact on people’s living conditions. Images made from satellite data can help us to locating such changes and thereby serve as a useful asset for decision-making about disaster prevention.
Recognizing land slides
This is a massive landslide that occurred on the Leyte island in the Philippines, as shown by PALSAR. ALOS was the first to record the disaster, overlaying an image in red taken before the event with green and blue images taken afterwards. The image clearly features the places where soil has given way. This comes from the fact that microwaves are scattered very strongly by the soil water that have fallen along with the dirt.
Even without rapid access to the location, analysis of such images allows prompt understanding of the situation on the ground.
Synthetic Aperture Radar （SAR） has characteristic to monitor the earth surface with all-weather/day-night condition. And it is useful to identify inundation area because land and water region is distinguishable by SAR based on the difference of scatter mechanism between land and water.
Figure shows flood event of the Mekong River, Viet Nam, in 2006 which observed by ALOS/PALSAR. A wide-swath observation mode of SAR like PALSAR/ScanSAR is very useful to monitor flood at large river with broad basin area like Mekong river.
Superimposed image of PALSAR/ScanSAR imagery which observed in 09/27/2006, 10/02/2009, and 10/16/2006, has highlighted inundation area of flood event of 2006.
Inundation area can be seen in the center part of image as greenish area around Mekong River in this case. In addition, occurrence of inundation in 09/27/2006 and 10/16/2006 can be estimated from relationship of color composition of each imagery.
Detecting crustal deformation
The crustal deformation caused by the Great East Japan Earthquake were also recorded clearly by PALSAR. The rainbow-colored fringes show the size and pattern of the changes to landform (uplifts, subsidence, horizontal displacement). Like contour lines, the denser the fringes, the more significant the displacement.
Data from GPS stations located in all areas are generally used for the measurement of crustal deformation. The results from PALSAR were confirmed to align very closely with the GPS results.
While GPS can only detect changes at exact location points, the advantage of satellite data is that it picks up changes in an entire area. Combining both data sets allows more precise and detailed mapping of changes.
Observing Forest Fires
Russia had suffered from an exceptional heat wave since July, 2010. Without a rain, forest fires have broken out in various places. This image shows forest fires in the south of Nizhniy Novgorod as observed on July 28, 2010. The green areas of the image are forests, grasslands, and croplands. The dark portion in the center of the image is the burned-out site.
Smoke clouds are rising from the southeast and southwest of the image.
Forests are still ablaze in these areas.
Understanding volcanic eruption
The eruption of Mt. Merapi, Indonesia in 2006 was observed by ALOS. These image are generated by optical sensor (PRISM and AVNIR-2). ALOS can take images Mt. Merapi by pointing the sensor every time it passed nearby.