Alex Hay-Man Ng

Assessment of Radar Interferometry Performance for Ground Subsidence Monitoring due to Underground Mining

This paper describes the results from the recently launched SAR satellites for the purpose of subsidence monitoring over underground coal mine sites in the state of New South Wales, Australia, using differential interferometric synthetic aperture radar (DInSAR) technique. The quality of the mine subsidence monitoring results is mainly constrained by noise due to the spatial and temporal decorrelation between the interferometric pair and the phase discontinuities in the interferogram. This paper reports on the analysis of the impact of these two factors on the performance of DInSAR for monitoring ground deformation. Simulations were carried out prior to real data analyses. SAR data acquired using different operating frequencies, for example, X-, C- and L-band, from the TerraSAR-X, ERS-1/2, ENVISAT, JERS-1 and ALOS satellite missions, were examined. The simulation results showed that the new satellites ALOS, TerraSAR-X and COSMO-SkyMed perform much better than the satellites launched before 2006. ALOS and ENVISAT satellite SAR images with similar temporal coverage were searched for the test site. The ALOS PALSAR DInSAR results have been compared to DInSAR results obtained from ENVISAT ASAR data to investigate the performance of both satellites for ground subsidence monitoring. Strong phase discontinuities and decorrelation have been observed in almost all ENVISAT interferograms and hence it is not possible to generate the displacement maps without errors. However these problems are minimal in ALOS PALSAR interferograms due to its spatial resolution and longer wavelength. Hence ALOS PALSAR is preferred for ground subsidence monitoring in areas covered by vegetation and where there is a high rate ground deformation.

Deformation mapping in three dimensions for underground mining using InSAR – Southern highland coalfield in New South Wales, Australia

This article presents 3D surface deformation mapping results derived from satellite synthetic aperture radar (SAR) data acquired over underground coal mines. Both ENVISAT Advanced Synthetic Aperture Radar (ASAR) and Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) data were used in this study. The quality of the 3D deformation mapping results due to underground mining is mainly limited by two factors. (1) Differential interferometric synthetic aperture radar (DInSAR) is less sensitive to displacement along the north–south direction in the case of the current SAR satellite configurations. (2) The mining-induced displacement is continuous and nonlinear; and the accuracy of the 3D DInSAR measurement is severely affected by the similar but non-identical temporal overlaps of the InSAR pairs. The simulation and real data analyzes have shown that it would be more practical to use DInSAR pairs with the assumption of negligible northing displacement to derive the displacements in the easting and vertical directions. The northing displacement could then be estimated from the residuals. This limitation could be overcome in the future with the launch of more radar satellites, which would provide better viewing geometry.

Preliminary Results of Satellite Radar Differential Interferometry for the Co-seismic Deformation of the 12 May 2008 Ms8.0 Wenchuan Earthquake

Abstract Satellite differential SAR interferometry has been widely accepted as a powerful tool to map co-, post- and inter-seismic deformation since its successful application to the 1992 Landers Earthquake. As soon as the Ms8.0 Wenchuan Earthquake occurred on 12 May 2008 in the Sichuan Province of southwestern China, the Japan Aerospace Exploration Agency tasked its Advanced Land Observing Satellite (ALOS) to respond to the disaster by collecting images. This paper presents the preliminary DInSAR results of co-seismic deformation of the quake observed from two satellite paths of the onboard ALOS/PALSAR sensor with post-seismic images acquired on 19 and 24 May. Results from pixel offset analysis and difference of coherence will also be discussed. The radar mapping is still ongoing because the ruptured seismic fault is more than 300km in length. Each swath of the PALSAR fine beam covers only about a 75km segment of the fault, and it takes 46 days for ALOS to revisit the same site.

Multi-path PALSAR interferometric observations of the 2008 magnitude 8.0 Wenchuan Earthquake

The Satellite Differential Interferometric Synthetic Aperture Radar (DInSAR) has already demonstrated its potential to map co-, post- and inter-seismic deformation. This paper presents an.alysis of the surface-displacement field of the 12 May 2008 magnitude 8.0 Wenchuan Earthquake using the Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) DInSAR. Six PALSAR interferograms of paths 471 to 476 are generated. A new approach, which uses the information of overlapping interferograms, has been developed to compensate for orbit error. The main advantage of this approach is that the calculation is performed without the need for phase unwrapping. Once the orbit error has been compensated for, the interferograms are then merged to form a single interferogram. The final mosaic of interferograms demonstrates a much more consistent co-seismic surface-displacement field than the original interferograms.

Phase Unwrapping for Very Large Interferometric Data Sets

Phase unwrapping is one of the most challenging steps in synthetic aperture radar (SAR) interferometry processing. With the rapid advancement of SAR technologies, the interferometric data sets from newly launched satellites are becoming larger and larger. When the computing resources required for unwrapping an input data set exceed computer hardware capabilities, phase unwrapping becomes even more problematic. In this paper, a new method is proposed in order to address the problem of unwrapping large data sets. The proposed method separates the unwrapping procedure into two stages. First, an approximate L1-norm phase unwrapping solution is efficiently obtained from a simplified minimum-cost flow network. Then, the blocks partitioned from the input data set are unwrapped by solving the corresponding independent network optimization problems directed by the approximate solution, either in parallel or in series. By then simply aligning the unwrapped blocks, a full-size unwrapped result is obtained. A significant advantage of the proposed method is that the globality of phase unwrapping solutions can be guaranteed. Using an interferogram with a size of 120 000 × 9274 pixels, the authors demonstrate that the proposed method is able to efficiently unwrap very large interferometric data sets using limited computing resources.

Subsidence Monitoring over the Southern Coalfield, Australia Using both L-Band and C-Band SAR Time Series Analysis

Land subsidence is a global issue and researchers from all over the world are keen to know the causes of deformation and its further influences. This paper reports the findings from time series InSAR (TS-InSAR) results over the Southern Coalfield, Australia using both ALOS-1 PALSAR (Phased Array type L-band Synthetic Aperture Radar) and ENVISAT ASAR (Advanced Synthetic Aperture Radar) datasets. TS-InSAR has been applied to both rural and urban areas with great success, but very few of them have been applied to regions affected by underground mining activities. The TS-InSAR analysis exploited in this paper is based on GEOS-ATSA, and Measurement Point (MP) pixels are selected according to different geophysical features. Three experiment sites with different geological settings within the study zone are analysed: (1) Wollongong city, which is a relatively stable area; (2) Tahmoor town, a small town affected by underground mining activities; and (3) the Appin underground mining site, a region containing multiple underground mining activities. The TS-InSAR results show that the performance of both C-band and L-band is equally good over Wollongong, where the subsidence gradient is not significant and most subsidence rates are between −10 mm∙yr−1 to 10 mm∙yr−1. However, over the Tahmoor and Appin sites, difference in performances has been observed. Since the maximum displacement gradients that can be detected are different for L-band and C-band-based TS-InSAR methods, some rapid changes could cause the TS-InSAR to fail to estimate the correct displacements. It is well known that L-band can perform better than C-band, especially in underground mining regions and mining-affected regions where the deformation rate is much higher than city areas because of its wavelength. Statistical analyses are also conducted to further prove the above statement.

Crustal deformation in Australia measured by satellite radar interferometry using ALOS/PALSAR imagery

Abstract The Advanced Land Observing Satellite (ALOS), launched on 24 January 2006, is a Japanese satellite carrying an L-band SAR sensor, namely the PALSAR, which is expected to demonstrate good performance in applications such as crustal deformation measurement, subsidence detection and landslide monitoring. This paper describes a case study of Differential Synthetic Aperture Radar Interferometry (DInSAR) using ALOS/PALSAR data to detect crustal deformation caused by a recent small earthquake in Western Australia. Single Look Complex (SLC) images acquired by the ALOS/PALSAR sensor were used to measure the co-seismic deformation of the 9 October 2007 earthquake that occurred south of the town of Katanning, in the state of Western Australia. Three images with dual polarizations (HH & HV) were used in this study; two acquired before the earthquake event and one after. The two-pass DInSAR processing method was applied to generate differential interferograms. The peak-to-peak surface displacement is up to 32 centimetres in the radar line-of-sight direction. The interferograms were used to constrain the fault modelling. The co-seismic displacements were modelled using a two-segment uniform slip model in a homogeneous isotropic half-space. A genetic algorithm was used to determine the optimal source parameters for the nonlinear inversion. The resultant maximum slip is about 0.4 metres on an almost pure reverse fault striking ~NE55° and dipping at ~40°S. The scalar moment was estimated to be 1:84 × 1016 Nm (Mw4.8), which is in good agreement with the seismological results. The root-mean-square difference between the DInSAR observed and modelled displacements is 1.6 cm.

Near real-time satellite mapping of the 2015 Gorkha earthquake, Nepal

This article discusses the near real-time (NRT) satellite mapping activities in response to the recent Gorkha earthquake in Nepal by UNSW as well as other institutions around the globe. This study demonstrates that data from current SAR satellites can already be processed and delivered in near real-time to support post-disaster response and emergency management. Three ALOS-2 PALSAR-2 interferometric pairs were used by the GEOS team at UNSW (2 Stripmap pairs and 1 ScanSAR pair) to deliver a suite of satellite remote sensing products, such as the differential interferometric SAR (DInSAR) interferogram, DInSAR ground displacement map, contour map of ground deformation, horizontal ground displacement based on the pixel offset tracking analysis, and damage map based on coherence difference analysis. This study shows that the mapping products can be released 6–8 hours after the post-event image is acquired using international ground receiving stations, with the direct mapping activities such as DInSAR and GIS processing typically taking 3–4 hours only. This study also discusses the urgent need for internationally coordinated development and deployment of SAR satellite constellations in order to greatly reduce the latency in NRT mapping of disasters, which will benefit a range of other satellite remote sensing applications as well. Moreover, it is suggested that the near real-time responses be coordinated across the globe in order to improve the effectiveness of rapid disaster mapping in order to mitigate the effects of earthquakes and other natural disasters.

Subsidence monitoring in the Ordos basin using integrated SAR differential and time-series interferometry techniques

ABSTRACT Recent researches have illustrated with the image tracking method that Ordos, China is suffering from a significant drop in earth surface level. However, such method can lead to bias in terms of its accuracy. In this paper, land displacement in Ordos between 8 January 2007 and 19 January 2011 was mapped using L-band ALOS Phased Array type L-band Synthetic Aperture Radar (PALSAR) data. Twenty PALSAR images were utilized to generate both Differential Synthetic Aperture Radar Interferometry (DInSAR) and time-series InSAR (TS-InSAR) results. Several locations in the eastern Ordos experiencing rapid land subsidence were identified. The subsidence rates ranging from −30 mm year−1 to 30 mm year−1 were measured in line-of-sight direction. The comparison between TS-InSAR and DInSAR results, although showing good agreement in general, reveals some gaps in time-series map near Qu Jia Liang coalmine mainly due to sudden changes within the four-year period. DInSAR result was exploited to fill these gaps after removing the tropospheric stratification phase delay and the verification step was conducted over the relatively stable region identified by TS-InSAR analysis. At last, the refined DInSAR result was converted into time-series velocity map and superimposed to TS-InSAR outcome to generate a final product.

Impact of ground subsidence on the Beijing-Tianjin high-speed railway as mapped by radar interferometry

The construction of the Beijing–Tianjin high-speed railway line started in July 2005. The railway has been completed and opened to traffic in August 2008, just before the Beijing Olympic Games. The 113 km railway costing 13.3 billion Yuan (US