The Changing Earth
Earth is a planet full of energy. This can be told from its exuberant convection development and active plate movement. When the plates aggregated, the crustal rocks were thrust and created high the mountains and land, providing human a platform for the development of civilization. However, plate movements also created an undesirable by-product: geological hazards.
The towering mountains indicate that Taiwan is a young island in geologic time and still has strong plate convergence. According to the up-to-date measurement data, the Philippine Sea Plate is moving towards the Eurasian Plate, which is to the direction of Mainland China, with a rate of 8 cm/year on the island. When the rocks of convergent plate boundaries are extruded onto the surface, lifted from the soil and sunken into the landscape, many geological hazards, such as fault, earthquake, landslide, debris flow and land subsidence, also occur on the ground surface. Therefore, to protect our lives and assets, we have to understand the changes of earth of this island.
Understanding the Pulse of the Earth
To understand the surface ground deformation, the first step is to precisely measure the land. The traditional way of doing so is to adopt optical instruments such as telescope. However, this not only consumes manpower, but also gives high measurement errors particularly for the long-distance measurement. Besides, as such measurement can only be done at the site, the surveyors have to travel a long distance to reach the destination first; when such instrument is applied in large-scale cadastral survey, not only takes it years to complete the project, but also it often costs enormous manpower and money.
Although some of the surface ground deforms rather slow and has no immediate urgency in measurement, such as the land subsidence and continuous uplifting of orogenic belt, some other deformations occur rather rapid, such as instant surface rupture triggered by landslide and faults. If it is the case, the measurement shall be done within a short period of time in order to understand their characteristics.
Topographic barrier is also deemed as a big limit for traditional measurement methods: a mountain or ocean can make cadastral survey an impossible mission, not mentioning our attempt is to observe surface ground deformation.
Thanks to the rapid development of aerospace technology in recent year, human has largely used devices mounted on the airplane, satellite and other aircrafts to do many things that would be deemed as difficult tasks in the past. The monitoring of ground surface deformation also enters the age of aerospace. Among them, the widely known technology is the Global Positioning System (GPS). However, there are some limitations on carrying out a cadastral survey using GPS. For example, it is a must to dispatch personnel to selected site to establish the GPS system and to collect the GPS data. Besides, the location and density of observation points also decide the data quality. The observation points must have good air permeability in order to receive the satellite signal. Besides, the higher density the observation network is, the more complete the information we will obtain. However, this will also cost more money and manpower.
Due to these limitations, scientists hopes to develop more rapid, comprehensive and economical cadastral survey to support the current GPS measurement technology. The direction of new development is to monitor ground surface deformation using satellite images correlation techniques. Among them, the most eye-attracting one is the “radar imagery technique”.
What is Synthetic Aperture Radar (SAR)
Radar is the abbreviation of “radio detecting and ranging”. In the early development of radar technique, the reflection echoes of the ground surface are usually the undesirable noises and must be eliminated in order to observe the measurement target. As people started to install radar on the airplane and satellite, the radar imagery technique started to have major breakthrough and the reflection echoes have then become useful information that help people to understand the topographic features of earth.
There are a number of important reasons for choosing satellite radar as the cadastral tool. First, radar waves have good penetration capability. Comparing the light, radar waves can easily penetrate through the clouds, mists and smokes to reach the ground surface, and then reflect back to the space and be received by the satellite. Besides, radar measurements can also be implemented at night and the scan of radar waves can quickly provide researches the comprehensive ground surface information. This is something that other measurement tools can hardly give.
It is also important to note that as soon as the satellite enters the orbit, it can continuously provide surface radar imagery; by contrasting images captured in different time, we will be able to calculate the surface deformation rate. Besides, for specific events, we can also contrast the images before and after the event to analyze the amount of surface deformation of this event.
An enhancement on the planar resolution has always be the focus of developing radar imagery technology. Normally, the planar resolution of the radar imagery of a satellite 1000 km above the Earth's surface is about 10 km if the antenna is 10 m wide. This resolution can hardly satisfy the demand for the monitoring of ground surface. Therefore, the scientists have adopted Synthetic Aperture Radar (SAR) to enhance the planar resolution of satellite image.
The concept of SAR is to, when the satellite travels along the orbit, rapidly and repeatedly transmit radar waves and then compute these continuous and repeated echo data in order to enhance the image resolution. This is like building an enormous virtual antenna along the orbit direction of satellite in the space. This enormous virtual “SAR antenna” can largely enhance the resolution of radar imagery for more than a thousand times! This has therefore enhanced the feasibility of morning the ground surface deformation using satellite radar imagery. The remote sensing satellites that are mounted with SAR and traveling along the orbit direction include ERS-2 and ENVISAT developed by the European Space Agency, and Canada’s RADARSAT satellite.
Fifty years ago, scientists ever proposed the idea of monitoring the ground surface using SAR and the first satellite mounted with SAR, SEASAT, already launched in 2978. However, this idea only becomes reality in recent years due to the development of another important technique: differential interferometry.
The Rainbows that Tells Ground Surface Deformation
When we have two or more satellite images, it is impossible to measure the ground surface deformation with our eyes. All satellite images must be processed and computed in order to show the ground surface deformation. This processing technique is known as “differential interferometry”.
Simply saying, radar differential interferometry is a cadastral survey technique that precisely corrects the geometry of at least two sets of satellite images and analyzes their interferometry. The radar differential interferometry data enables scientists to obtain the phase difference of radar waves that is caused by the change of distance between the ground surface and satellite. Then, by computing the phase difference, scientists will obtain the change of the corresponding distance therebetween. The precision of radar differential interferometry depends on the length of used radar waves. At the moment, remote sensing satellites mainly use the X-band (the wavelength is about 3 cm), C-band (the wavelength is about 6 cm) and L band (the wavelength is about 24 cm). The shorter wavelength will give better precision to the data. We often put different colors on the change of radar wave phase. For example, phase –π to +π will be colored from red to purpose for better understanding the data. This kind of concentric ring pattern of rainbow colors, which is known as the interferograms, is formed by change of radar wave phase. The number and distribution of interferometric rings can present the line-of-sight (LOS) ground surface deformation. As the matter of fact, LOS, geomorphological effects and satellite orbit deviation can cause phase difference and form the interferometric rings. In order to reveal the true surface deformation, it is a must to eliminate these noises.
The method of processing SAR images using the said differential interferometry technique to obtain the high-quality ground surface information is known as “Interferometric Synthetic-Aperture Radar” (InSAR).
The Major Trends
While continuously developing our economy and technology, we must not forget our constructions. However, geological hazards that come one after another tell us that, if Taiwan does not take action to study earth science and reduce geological hazards, we will pay more to learn from these catastrophic events. Therefore, the monitoring of ground surface deformation will be one of the keys in preventing and mitigating geographic hazards.
With the government’s support, nationwide research units have adopted different survey and measurement tools to get involved in relevant studies. Surely, SAR will not be missing from this important task. Center for Space and Remote Sensing Research of National Central University and its resource satellite ground station started to receive satellite images since October of 1993 and has now accumulated abundant database. For example, the monitoring data of ground surface deformation before and after the 911 Earthquake; and land subsidence in coastal and metropolitan areas.
Today, our geological knowledge is much better than before. However, there are still some things that cannot be controlled. By taking the ground surface deformation as an example, although we know where the major displacements are located, it is hard for us to timely detect some small, local and non-continuous deformation. Although these deformations are inconspicuous, they are often the precursors of major geological hazards, such as earthquakes and landslide. How to rapidly, comprehensively and preciously measure ground surface deformation is the ultimate goal pursued by the scientists. The applications of SAR and other new technique can help us to better understand the impulse of earth in details.
Synthetic Aperture Radar (Source: Cheng Chung-Bai, 2005 (June). Science Development, No.390)