A SHORT REVIEW ON PERSISTENT SCATTERER INTERFEROMETRY TECHNIQUES FOR SURFACE DEFORMATION MONITORING

Technology advancement has urged the development of Interferometric Synthetic Aperture Radar (InSAR) to be upgraded and transformed. The main contribution of the InSAR technique is that the surface deformation changes measurements can achieve up to millimetre level precision. Environmental problems such as landslides, volcanoes, earthquakes, excessive underground water production, and other phenomena can cause the earth's surface deformation. Deformation monitoring of a surface is vital as unexpected movement, and future behaviour can be detected and predicted. InSAR time series analysis, known as Persistent Scatterer Interferometry (PSI), has become an essential tool for measuring surface deformation. Therefore, this study provides a review of the PSI techniques used to measure surface deformation changes. An overview of surface deformation and the basic principles of the four techniques that have been developed from the improvement of Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR), which is Small Baseline Subset (SBAS), Stanford Method for Persistent Scatterers (StaMPS), SqueeSAR and Quasi Persistent Scatterer (QPS) were summarised to perceive the ability of these techniques in monitoring surface deformation. This study also emphasises the effectiveness and restrictions of each developed technique and how they suit Malaysia conditions and environment. The future outlook for Malaysia in realising the PSI techniques for structural monitoring also discussed in this review. Finally, this review will lead to the implementation of appropriate techniques and better preparation for the country's structural development.

Polarimetric Persistent Scatterer Interferometry for Ground Deformation Monitoring with VV-VH Sentinel-1 Data

With the launch of the Sentinel-1 satellites, it becomes easy to obtain long time-series dual-pol (i.e., VV and VH channels) SAR images over most areas of the world. By combining the information from both VV and VH channels, the polarimetric persistent scatterer interferometry (PolPSI) techniques is supposed to achieve better ground deformation monitoring results than conventional PSI techniques (using only VV channel) with Sentinel-1 data. According to the quality metric used for polarimetric optimizations, the most commonly used PolPSI techniques can be categorized into three main categories. They are PolPSI-ADI (amplitude dispersion index as the phase quality metric), PolPSI-COH (coherence as the phase quality metric), and PolPSI-AOS (taking adaptive optimization strategies). Different categories of PolPSI techniques are suitable for different study areas and with different performances. However, the study that simultaneously applies all the three types of PolPSI techniques on Sentinel-1 PolSAR images is rare. Moreover, there has been little discussion about different characteristics of the three types of PolPSI techniques and how to use them with Sentinel-1 data. To this end, in this study, three data sets in China have been used to evaluate the three types of PolPSI techniques’ performances. Based on results obtained, the different characteristics of PolPSI techniques have been discussed. The results show that all three PolPSI techniques can improve the phase quality of interferograms. Thus, more qualified pixels can be used for ground deformation estimation by PolPSI methods with respect to the PSI technique. Specifically, this pixel density improvement is 50%, 12%, and 348% for the PolPSI-ADI, PolPSI-COH, and POlPSI-AOS, respectively. PolPSI-ADI is the most efficient method, and it is the first choice for the area with abundant deterministic scatterers (e.g., urban areas). Benefitting from its adaptive optimization strategy, PolPSI-AOS has the best performances at the price of highest computation cost, which is suitable for rural area applications. On the other hand, limited by the medium resolution of Sentinel-1 PolSAR images, PolPSI-COH’s improvement with respect to conventional PSI is relatively insignificant.

A Most-Unfavorable-Condition Method for Bridge-Damage Detection and Analysis Using PSP-InSAR

The main contribution of this study is to provide a new idea to detect bridge damage by using PSP-InSAR technology. A most-unfavorable-condition method is proposed for bridge-damage detection and analysis. The method can determine the specific damaged location and occurrence time by using the differential deformation values of persistent scatterer (PS) points on bridge piers. Taking Beijing Suzhou Bridge as an experimental area, 96 COSMO-SkyMed time-series SAR images were used from September 2011 to November 2017. Deformation values of PS points around Suzhou Bridge were acquired and analyzed. Experimental results show that in July 2017, the unusual maximum differential deformation value was 25.73 mm. It occurred between piers D3 and D4 of Suzhou Bridge, and it was deduced that the main girder between piers D3 and D4 may have been damaged in July 2017. As a validation, taking the differential deformation value between piers D3 and D4 as an input, the maximum tensile stress, and the maximum compressive stress were calculated as 2.1 MPa and 8.4 MPa, respectively, through a finite element model. The tensile stress exceeded the design value of the concrete, further confirming the damage of the girder between piers D3 and D4. Moreover, all results are consistent with the Suzhou Bridge damage information shown in existing records, which verify the accuracy and reliability of the proposed method.

Pemanfaatan Persistent Scatterer Interferometry Synthetic Aperture Radar (PSInSAR) Untuk Mengidentifikasi Laju Deformasi Permukaan di Lapangan Panas Bumi Ulubelu

Lapangan panas bumi Ulubelu telah diekstraksi sejak tahun 2012 dengan menghasilkan 2 x 55 MW dari PLTP unit 1 & 2 dan meningkat menjadi 4 x 55 MW sejak tahun 2016 dengan beroperasinya unit 3 dan unit 4. Peningkatan eksploitasi energi panas bumi di Ulubelu berpotensi menimbulkan perubahan kondisi geologi dan lingkungan yang salah satunya adalah subsiden. Penelitian ini bertujuan untuk mengidentifikasi potensi laju deformasi permukaan memanfaatkan metode Persistent Scatterer Interferometry Synthetic Aperture Radar (PSInSAR) di lapangan panas bumi Ulubelu. Sebanyak 49 data Sentinel-1 periode Oktober 2014 hingga Maret 2020 dengan mode descending telah diolah dan dianalisis menggunakan tiga software utama yaitu SNAP, StaMPS dan StaMPS-Visualizer. Pembentukan interferogram pada setiap pasangan data (image pair) antara master dengan seluruh slave dilakukan menggunakan SNAP. Seluruh data interferogram kemudian diexport sebagai input data StaMPS untuk mendapatkan nilai piksel yang memiliki koherensi terbaik dan persistent. Hasil pengolahan menunjukkan laju deformasi per titik persistent scatterer (PS) berkisar antara -7,3 hingga +7,5 mm/tahun relatif pada arah Line of Sight (LOS) tanpa validasi lapangan. Pola deformasi berupa penurunan muka tanah berada di sekitar area eksploitasi panas bumi, sedangkan kenaikan muka tanah (uplift) terdeteksi di luar area eksploitasi. Hasil analisis menunjukkan bahwa kesamaan laju deformasi pada PLTP unit 1 & 2 dengan PLTP unit 3 & 4 mengindikasikan proses subsiden di area Ulubelu didominasi oleh proses ekstraksi fluida panas bumi. Temuan ini juga memperkuat penelitian sebelumnya yang menunjukkan bahwa proses subsiden di area panas bumi Ulubelu disebabkan oleh pemadatan batuan alterasi.

Mining-induced Land Subsidence Detected by Sentinel-1 SAR Images: An Example from the Historical Tadeusz Kościuszko Salt Mine at Wapno, Greater Poland Voivodeship, Poland

There are many mines in Poland that have been in operation for over 100 years, with theTadeusz Kościuszko mine being a large salt mine in Wapno, northern Poland. The mine was closed in1977 due to the greatest catastrophe in the history of Polish mining, but in the first days of 2021, a verylarge hole has been created in this area due to land subsidence. This article uses InSAR technology withSentinel-1 images to determine settlement and ongoing deformation in this mine. The study results areuseful for policymakers, managers, and authorities because land subsidence has caused serious anddangerous effects on people living in the area. The results processed by the Persistent Scatterer InSAR(PSInSAR) method with the Sentinel Application Platform and the Stanford Method for PersistentScatterers software packages show that deformation in the Wapno village area has been detected in bothresidential and non-residential areas, with maximum subsidence of up to −19 mm/yr. The subsidence inthe mine reaches −12 mm/yr, and that at surrounding area range from 0 to −18.8 mm/yr.

Persistent Scatterer Interferometry for Pettimudi (India) Landslide Monitoring using Sentinel-1A Images

The continuous monitoring of land surface movement over time is of paramount importance for assessing landslide triggering factors and mitigating landslide hazards. This research focuses on measuring horizontal and vertical surface displacement due to a devastating landslide event in the west-facing slope of the Rajamala Hills, induced by intense rainfall. The landslide occurred in Pettimudi, a tea-plantation village of the Idukki district in Kerala, India, on August 6–7, 2020. The persistent-scatterer synthetic aperture radar interferometry (PSInSAR ) technique, along with the Stanford Method for Persistent Scatterers (StaMPS), was applied to investigate the land surface movement over time. A stack of 20 Sentinel-1A single-look complex images (19 interferograms) acquired in descending passes was used for PSInSAR processing. The line-of-sight (LOS ) displacement in long time series, and hence the average LOS velocity, was measured at each measurement-point location. The mean LOS velocity was decomposed into horizontal east–west (EW ) and vertical up–down velocity components. The results show that the mean LOS, EW, and up–down velocities in the study area, respectively, range from –18.76 to +11.88, –10.95 to +6.93, and –15.05 to +9.53 mm/y, and the LOS displacement ranges from –19.60 to +19.59 mm. The displacement values clearly indicate the instability of the terrain. The time-series LOS displacement trends derived from the applied PSInSAR technique are very useful for providing valuable inputs for disaster management and the development of disaster early-warning systems for the benefit of local residents.

Seasonal Surface Fluctuation of a Slow-Moving Landslide Detected by Multitemporal Interferometry (MTI) on the Huafan University Campus, Northern Taiwan

A slow-moving landslide on the Huafan University campus, which is located on a dip slope in northern Taiwan, has been observed since 1990. However, reliable monitoring data are difficult to acquire after 2018 due to the lack of continuous maintenance of the field measurement equipment. In this study, the multitemporal interferometry (MTI) technique is applied with Sentinel-1 SAR images to monitor the slow-moving landslide from 2014–2019. The slow-moving areas detected by persistent scatterer (PS) pixels are consistent with the range of previous studies, which are based on in situ monitoring data and field surveys. According to the time series of the PS pixels, a long period gravity-induced deformation of the slow-moving landslide can be clearly observed. Moreover, a short period seasonal surface fluctuation of the slow-moving landslide, which has seldom been discussed before, can also be detected in this study. The seasonal surface fluctuation is in-phase with precipitation, which is inferred to be related to the geological and hydrological conditions of the study area. The MTI technique can compensate for the lack of surface displacement data, in this case, the Huafan University campus, and provide information for evaluating and monitoring slow-moving landslides for possible landslide early warning in the future.

ESTIMATION OF LAND SUBSIDENCE IN THE MEKONG DELTA AND ITS SURROUNDING AREAS

Land subsidence becomes a trouble for the different regions of the Mekong delta, where many places will be severely affected by sea-level rise in the context of climate change. Land subsidence could amplify this situation by inducing interactive hazards such as submerged land and saline intrusion, etc. Mapping the spatial distribution of land subsidence become a crucial task, and the Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) approach was applied to 120 Sentinel-1A images within three scenes, captured from October 2016 to October 2020 with the interval of 36 days between two consecutive images. This approach allows mapping ground displacement by continuously extracting deformation signals and estimating the position of targets that persistently scatter radar beams. The average velocity map shows that eight main subsidence areas in the Mekong delta have been affected in recent years with the maximum velocity of -39.61 mm year-1 and the cumulative displacement ranging from 60 to 100 mm in the Line Of Sight (LOS) direction over four years. The validation using 40 Sentinel-1B images, captured in identical periods indicates a consistent result in comparison with the one issued from Sentinel-1A. These pieces of knowledge are essential for improving both citizen’s life and reducing the impact of land subsidence on the natural environment.