Earth Observation – special cases

After the introduction to Earth Observation by Satellite, we move on to look at some special cases. It is said, investigated and confirmed that we tend to react more strongly to news about some rapid, violent event than to news about something changing very slowly over a long time. We do so even if we know it to be true and even though we know that small changes over a long time could affect us and our way of living more substantially. By analyzing satellite data, we can receive an overview of both slow and rapid events.

Let’s focus on rapid changes on the Earth’s surface. On April 25th, 2015 relative to each other, the Indian and Eurasian tectonic plates moved causing an earthquake. The epicenter was no more than 20 km from Katmandu, the capital of Nepal, and the rate of damage was high. An earthquake in an area with a dense population causes a need for rapid relief. With telecommunication facilities, as well as roads, damaged and with a limited airport capacity it was crucial to know where the help was needed the most – and of course – the kind of help needed. A map, based on data from ESA’s Sentinel-1 satellite, showed the earth displacements in each and every corner of the area, and gave the help organizers some first basic information about where to head on to.

he German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Earth Observation Center (EOC) has used data acquired by the ESA radar satellite Sentinel-1 over the earthquake region in Nepal to create a new regional aid map and calculate the ground displacements caused by the earthquake. The data were processed by the EOC to create an interferogram showing the surface displacements caused by the severe earthquake on 25 April 2015.
The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR); Earth Observation Center (EOC) has created a regional aid map by calculating ground displacements using data acquired by the ESA radar satellite Sentinel-1 over the earthquake region in Nepal. Credit: DLR (CC-BY 3.0)

The map above is made up from images taken before and after the earthquake. Areas, colored in blue, were moved towards the satellite (called ‘line of sight’) which indicates a vertical uplift. Areas of subsidence are here depicted in yellow, a movement that often occurs as a counter movement to the uplift in subduction zones during earthquakes. Furthermore, a horizontal shift of up to 2 m was detected.

In this Envisat Advanced Synthetic Aperture Radar, ASAR, interferogram over the Longonot volcano in the Great Rift Valley we see small surface displacements not visible to the naked eye. The Suswa volcano, to the left, was not deforming at this time. A complete set of coloured bands, called 'fringes', represents ground movement relative to the spacecraft of half a wavelength, which is 2.8 cm in the case of Envisat's ASAR. Credit: ESA
 Envisat’s ASAR interferogram over Longonot volcano 2004-2006 Credit: ESA

Other rapid natural events are volcano eruptions. But even during a quite long dormant period between eruptions there may be displacements in and around a volcano. Studying these movements increases our understanding of volcanos and may one day be an important piece in the puzzle that gives us a picture that could help us predict eruptions. In this Envisat Advanced Synthetic Aperture Radar, ASAR, interferogram over the Longonot volcano in the Great Rift Valley we see small surface displacements not visible to the naked eye. The Suswa volcano, to the left, was not deforming at this time, between 2004 and 2006. A complete set of colored bands, called ‘fringes,’ represents ground movement relative to the spacecraft of half a wavelength, which is 2.8 cm in the case of Envisat’s ASAR. In this case, some areas near the Longonot volcano were lifted approximately 9 cm. It’s also obvious that the Suswa volcano in the background stayed almost perfectly still.

During eruptions, there is, of course, more happening in a short time. Then satellite images are helpful in predicting paths of ash plumes and floating lava. Even during eruptions there’s a lot happening not visible to human eyes. On April, 23rd, 2015 an eruption in southern Chile was captured by the Suomi NPP. Read more about that here.

Total subsidence in California's San Joaquin Valley for the period May 3, 2014 to Jan. 22, 2015, as measured by Canada's Radarsat-2 satellite. Two large subsidence bowls are evident, centered on Corcoran and south of El Nido. Credit: Canadian Space Agency/NASA/JPL-Caltech
Total subsidence in California’s San Joaquin Valley for the period May 3, 2014, to Jan. 22, 2015, as measured by Canada’s Radarsat-2 satellite. Two large subsidence bowls are evident, centered on Corcoran and south of El Nido. Credit: Canadian Space Agency/ NASA/ JPL-Caltech

Humans, animals, and plants need fresh water. Ok – Plants do not need crystal clear water, but still fresh water without pollution and salt. Although we do not use ground water for every purpose we use water that in fact is potential ground water. In other words; in some areas we are taking water out of the ground at a pace faster than the water layers below our feet are being refilled. That is causing subsidence in some areas, although if not visible to the naked eye.

In response to the drought of 2015, water is being pumped out of the Californian ground at a rate that has caused parts of the San Joaquin Valley to sink nearly five centimeters per month. The groundwater levels are now nearly 30 meters lower than ever measured before. This is something that could put nearby infrastructure at risk of damage.

There are simple ways to measure groundwater levels. You don’t need a satellite to do it. On the other hand – If we measure shiftings in ground level, compare it to changes in groundwater level and from that make ourselves a model… Is it then possible to measure groundwater levels all over the globe and come up with a new “groundwater level map” every third day?

Data from several satellites are used to look at and build knowledge about the situation in California. NASA Jet Propulsion Laboratory, California Institute of Technology will guide you further.

The data collected so far, together with data collected in the future, will be important for observing changes over time. Earth Observations by satellite gives us global data. One global set of data collected in a couple of days. Now, that’s a treasure chest!

 

 


JanJan teaches mathematics and interdisciplinary science to pupils 13-16 years of age at Sursik School, Pedersöre, Finland. Space-related science often gives some sort of answer to the question “Why?”, a question quite common in math class. It also triggers curiosity, one key component in progress.

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