Chih-Tien Wang

Disaster monitoring and environmental alert in Taiwan by repeat-pass spaceborne SAR

The prevailing complex geological and ecological conditions of Taiwan have drawn considerable attention from various geo-ecological communities because of their vulnerability to produce various natural hazards at different scales. Located in the tropical/subtropical zone of the Pacific Rim, its ecological and rugged mountainous properties are environmentally sensitive making monitoring and observations especially difficult because of the high population density. For example, in terms of natural hazard mitigation tectonically active regions are used for analyzing the cause of abundant risk events, such as earthquakes, landslides and land subsidence. In fact Taiwan is well suited as a test site for studying those geologically disastrous processes. Implementing novel techniques of space remote sensing has proved to be an effective means in recent years for greatly improving our understanding of these phenomena. In this paper we report on the monitoring of such events using multi-modal polarimetric and/or interferometric SAR images at C and L band from ERS, JERS-1, RADARSAT-1, ENVISAT, and from the recent ALOS satellite. For crustal and surface deformation, we used radar image pairs with long temporal baselines and large areas of coverage for investigating deformation over Western Taiwan. Pre-seismic and co-seismic deformation patterns are spatial-temporally analyzed. The other topic deals with the coastline changes observed from a sequence of ERS-1/2 SAR images within the years of 1996 to 2005. Waterlines were extracted using multi-scale procedures of edge detection and were corrected with tidal motion data. Substantial analyses were carried out in conjunction with ground surveys and lidar mapping. The topographic feature changes due to large scale landslides triggered by torrential rains were also monitored. In addition, the SAR interferograms were used to analyze the deposition changes along the riverbeds and riverbanks for short-intervals using optimal baselines. Summary and remarks on the implementation of such multi-modal polarimetric and/or interferometric SAR imagery for environmental monitoring are provided.

On the application of a spatial chaotic model for detecting landcover changes in synthetic aperture radar images.

We present a change detection method for terrain covers from multi-temporal SAR images based on a spatial chaotic model which is known to adequately characterize the coherent process of SAR imaging. The major problem of SAR change detection rises from both the presence of speckle noise and the pixel mis-registration that are commonly seen in the remote sensing image. By means of chaotic model, we first transform the images to fractal domain and then perform the CFAR detection. Simulated tests are conducted to quantitatively evaluate the impacts of these two major error sources on detection rate. Results from satellite SAR for landcover change detection clearly show that the proposed algorithm not only the speckle noise can be effectively suppressed without scarifying the spatial resolution; the excruciating mis-registration error was taken into account and removed.

Feature Enhancement of Stripmap-Mode SAR Images Based on an Optimization Scheme

Based on a nonquadratic-optimization method originally proposed for spotlight-mode SAR image reconstruction, a modification for stripmap-mode SAR images is presented in this letter. This is done by mathematically reformulating the projection kernel and numerically putting it into a form that is suitable for optimization. The performance was evaluated by measures of the target contrast and 3-dB beamwidth using Radarsat-1 data. Results were analyzed and compared with those using minimum-variance and multiple-signal-classification methods. Results demonstrate that the targets features are effectively enhanced and that the dominant scattering centers are well separated using the proposed method. In addition, the image fuzziness is greatly reduced, and the image fidelity is well preserved. The effectiveness of the modification is thus validated.

Refined filtering of interferometric phase from INSAR data

Radar interferometry has been widely applied in measuring the terrain height and its changes. The information about surface can be derived from phase interferograms. However, the inherent phase noise reduces the accuracy and reliability of that information. Hence, the minimization of phase noise is essential prior to the retrieval of surface information that is embedded in an interferometric phase. This paper presents a refined filter based on the Lee adaptive complex filter and the improved sigma filter that was originally developed for amplitude image filtering. The basic idea is to adaptively filter the interferometric phase according to the local noise level to minimize the loss of signal for a particular pattern of fringes, including such extreme cases as involving broken fringes, following the removal of undesired pixels. Ultimately, the goals are to preserve the fringe pattern, to reduce phase bias and deviation, to reduce the number of residues, and to minimize the phase error. The preservation of the fringe pattern is particularly of concern in areas of high frequency of fringe and large phase gradient corresponding to steep terrains. The proposed refined filter was validated using both simulated data and real interferometric data. Results demonstrate that the filtering performance is better than that of commonly used filters.

Monitoring tree farms and coastal environments using RADARSAT-2 PolSAR data

This paper addresses the feasibility of using RADARSAT-2 fine Quad-Pol mode to monitor coastal environment and young tree growth. It will be shown that interferometric coherence may not be high enough for the height estimation of young trees at C-band, but polarimetric sensitivity could be used for tree and crop classification. For coastal environment, we found that polarimetric signature of oyster farm reveals the effect of double bounce scattering and the orientation angle effects.

Deformation field over Western Taiwan Island using satellite InSAR

Ongoing crustal deformations in the active arc-continent collision of Taiwan generate large amount of deformation and therefore pose potential seismic hazards as well as risk to infrastructure on the island. In this paper we used radar image pairs with long time span and large area of coverage in order to investigate deformation over the western Taiwan in large spatial and temporal scale. Pre-seismic, co-seismic, post-seismic, and inter-seismic deformation patterns are spatio-temporally analyzed. These areas of interest are located in the Western Foothills of Taiwan orogeny, which the deformation front of the mountain building is propagating toward. In order to better approach the problem, we classify the crustal deformation into intense and gentle events reflecting different deformation styles. Three case studies of deformation events using DinSAR images will be reported. They are: the co-seismic deformation associated with the Chi-Chi earthquake, uplift of the Tainan area, active deformation of Chinshu fault. Correlation of location and time of these deformations is analyzed to investigate the possible precursory to a major earthquake and better understand dynamic rupture mechanisms.

Experimental study on the phase statistics of GBSAR imagery

Summary form only given. Synthetic Aperture Radar (SAR) interferometry is a well-established remote sensing technique that compares the phase of two or more complex SAR images for highly accurate measurement of displacements. Recently, the ground-based interferometric SAR (GBSAR) has proven to be a powerful tool for monitoring land deformation such as landslides, glacier movements and volcanic hazards. By using GBSAR, it is possible to detect subtle changes in the order of a fraction of the radar wavelength. However, the accuracy of the interferometric phase measurement is greatly affected by the degree of decorrelation between the SAR images that have been acquired from slightly different times or at different positions. The major contributors of phase decorrelation include radar receiver noise and temporal scene decorrelation. In this paper, the phase statistics of a typical GBSAR instrument are studied. The closed-from probability density function of the phase statistics is first derived for random media in the presence of homogeneously distributed scatterers and when strong scatterers are present within a resolution cell. The phase deviations are analyzed and compared with the experimental results obtained from a GBSAR instrument in an aneachoic chamber and at outdoor test sites.

A new client-server architecture for real-time Micro-Movement ground based SAR system

Summary form only given. Micro Movement iMager (MMI), a new solution of ground based SAR system, for multi-purpose monitoring specific region of interest. All kind of disaster including the natural hazard or human-instigated disaster can be monitored by the embedded programming functions such as the re-projection, landslide change detection, elevation change detection, real-time warning alarm and the temporal analysis for entire database. The integration of hardware and software was introduced in this paper for the purpose of operation, database access and maintenance. According to the server-client architecture, the system was split into three parts. First is the data acquisition hardware that combines a controlling system. Some characteristics such as scheduling, self-calibration and remote controlling are embraced in this system. Another is the bridge system including the database, command centre and database analysis capabilities. Because that all command and analysis were passed through this bridge system by the client, the computing capabilities, data access effectiveness and convenience of database management had be considerate. All kind of real-time and stored data can be accessed and analyzed on the fly without any redundant data transfer. The last part is the user-friendly client interface and deformation application which shown in Figure 1. All functions are built with object oriented and easy-to-use purpose reducing unnecessary operations including the hardware controlling, analysis and visualization. According to the interactive visualization toolkit, the well-analyzed information was presented with two or three dimension widget fusing DEM and aerial photo. All of operation were implemented and evaluated in chamber and real testing in-situ environment. This architecture was also verified that is suitable for the indoor and outdoor environment. The outdoor real data, which obtained at Cameron highland in Malaysia and Meihua Village in Taiwan, were demonstrated in this paper.

Estimation of average surface currents from ERS SAR images of oil-tank cleaning spills

Synthetic Aperture Radar (SAR) images from ERS-1/2 have been used to study current-induced perturbations of spill bands in the Kuroshio east of Taiwan, stemming from ballast water of several ships that had been dumped after the cleaning of tanks during the movement of the respective ship parallel to Taiwan’s coast. Under weak winds, the variable surface currents normal to a ship’s track were the main cause of the displacement of spills relative to a straight line. The currents were associated with the interaction of the Kuroshio with the island of Lutao and ocean bottom variations as well as with vortex structures of different scales, which were observed near the Kuroshio boundary. The average current velocity was determined by dividing the magnitude of the spill displacement by its residence time, which in turn was inferred from the distance of the spill from a ship and the supposed ship velocity. The current shear zones manifested themselves on the SAR images as narrow light lines. Sharp shifts in a spill were observed at its crossing lines. The estimates of current shear, based on the magnitude of the shifts, agree with published data. The width of a spill band observed on May 20th, 1994, at different distances from a ship, was found from the sections normal to it. The change in width as a function of the residence time may be well approximated by a power function with an exponent of about 0.86.

Temporal and spatial pattern of the 1997 Manyi Earthquake using differential InSAR

Since the beginning of indentation of the Indian Plate toward the Eurasia Plate at round 50 Ma, the convergence of these two continental plates had been changing the surface of our earth. The Indian plate is colliding with Eurasian plate at about 58mm/yr (Bilham et. al 1998); the high strain rate caused by this rapid convergence provides the source to the construction of the Himalaya and the Tibetan plateau. The 1997 Manyi earthquake (Mw=7.5) is one of the largest earthquake occurred in the Tibetan plateau, followed by another even greater seismic events 2001 Kokoxli earthquake (Mw=7.9) in north western part of Qinhai Province. Although the epicenters of these two events are 500 km apart, both events occurred along the Kunlun fault that is trending east-westward and spatially spanned about 1,000 km. The region along the Kunlun fault in northern Tibet serves as a natural laboratory for studying earthquakes and earthquake sequences. By InSAR with ERS radar images, we establish the time series of, to the first order, the deformation rate in the vicinity of the epicenter of 1997 Manyi earthquake. We generate interferograms associated with coseismic deformation to improve our understanding the rupture style and its underlying mechanism. Preliminary investigation of the interferogram revealed that there may be at least two surface ruptures produced during the earthquake.