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1 USING SENTINEL-1 TOPS SAR AND SBAS FOR LAND SUBSIDENCE MONITORING IN HANOI, VIETNAM Minh Nguyen, Chung-Pai Chang, Kuo-Hsin Tseng Center for Space and Remote Sensing Research, National Central University, Taiwan ABSTRACT: The rapid expansion of urban area has resulted in surface deformation in many countries. In the case of Hanoi, Vietnam, ground subsidence is noted as one of major influences on urban growth and has endangered the infrastructures such as historical places, buildings, and residential regions. The significant impacts of land subsidence were recorded recently at a rate of few millimeters to centimeters per year, while the rate keeps increasing since past few years as observed from remote sensing technologies. The Synthetic Aperture Radar (SAR) as one of the typical methods for land subsidence measurement with high precision and large spatial coverage, is applied to continuously observe such phenomenon in Hanoi. Even so, the limitations of data as well as high cost hamper the deployment and application of SAR technique in earlier years. To catch up the new trend of Sentinel series for open data with high precision and temporal resolution and overcome these limitations, the study intends to utilize Sentinel-1A TOPS SAR data to investigate the deformation in Hanoi where suffers severe subsidence since last few decades. Also, Hanoi experienced an expansion of 3.6 times in 2008 along with deployment of first-10 years new urban planning. In our preliminary result, the Small Baseline (SBAS) Interferometry SAR (InSAR) represents a large area subsiding with a rate up to 40 mm yr -1 in the period from April 2016 to September Along with the extension of the city centre, subsided area also expanded in the scale and new subsidence region were recorded with even higher rates. The valuable information of subsidence measurement combining with geology analysis as well as other data could be used for next period of Hanoi urban planning. In addition, the good agreement of SBAS result with recently published studies in Hanoi proves the capability of Sentinel-1 for ground displacement observations and further applications in Vietnam. KEYWORDS: Sentinel-1; SBAS; Land Subsidence; Hanoi 1. INTRODUCTION The work of estimating land subsidence in Hanoi, Vietnam had started approximately 20 years ago. Many studies have utilized the Synthetic Aperture Radar (SAR) imagery since the city has rapidly developed along with the expansion of centre area, which erased the boundary between urban and rural regions. The complexity of geological condition and a high demand of groundwater for the growth of population have also resulted in surface deformation in newly extended areas. These hazards were recorded and motivated administrative and researchers to conduct studies of ground displacement that supports urban planning in the future. The differential SAR Interferometry (DInSAR) was used at the beginning and the technique is updated through the time with Persistent Scatters InSAR (PSInSAR) or Small Baseline Subset (SBAS) InSAR. New techniques can improve the limitations of DInSAR and provide time series observation. However, the satellite data in the past was quite limited, and thus the information extracted for Hanoi subsidence was intermittent since the first attempt of using InSAR technology. Thanks to the open data policy trending in many space agencies in recent years, the new series of Sentinel satellites launched by European Space Agency (ESA) in 2014, addressing such issue and moreover, they made a breakthrough in providing open remotely sensed data with high precision and temporal resolution. This study intends to provide a timely land subsidence observation in Hanoi using Sentinel-1A TOPS SAR data with SBAS technique, that allows us to observe the deformation of the entire city and scan any unexpected displacement along with its rate in millimeters to centimeters level. The additional geology data could illuminate partially the potential reasons causing this phenomenon and furthermore, the study shares a favorable information to construct the new expansion area in future not only for Hanoi but also other larger region. 2. METHOD AND DATA COLLECTION Hanoi located in the northeastern of Vietnam (Fig. 1) is a 70-years-old capital with an important role in the country s economy. It is the second largest city following Ho Chi Minh City with it special foreign influence urban type. The latitude and longitude boundary of Hanoi are and , respectively, with an area of 3,358.9 km 2 that hosts approximately 8 million people. The city situated in the centre of Red river plain and has over 1000 years of history with a mixing of historical heritage, modern buildings, and residential areas. In 2018, there are undergoing the first 10-year of urban planning since the last expansion in 2008 with 3 times expanded in the scale. At the present, there is no

2 tectonic or fault in the study area, hence the deformation mostly depends on human activity. Hanoi has distinct four seasons annually and has complicated atmospheric condition along with the geology. According to surface geology, the area dominated by clay accounting for a high rate in study region which means that the subsidence of ground is likely to occur over these locations. 3. RESULTS & DISCUSSION Figure 3 denotes the Line of Sight (LOS) displacement velocity of SBAS result derived from Sentinel-1A imagery in the period from 3 April 2016 to 13 September In the central area, it can be seen that there are 2 subsidence hotspots located at Hadong and Hoangmai district. In fact, these two districts are recorded as deteriorating displacement zones since 2007 [2]. With a similar trend of surface deformation, this study continuously observers these subsided regions where the rate of Hadong is nearly 27 mm yr -1 in LOS displacement and a less severe displacement of 18 mm yr -1 to Hoangmai district. The good agreement of SBAS Sentinel-1A with published studies [1,2] proves that our study result is reliable and able to extend time series surrounding city centre. Figure 1. Location of study area: surface geology map of extended urban area and old central area derived from [1]. In Hanoi region, Sentinel-1A satellite provides regular swath of C-band radar imageries in the Interferometric Wide (IW) mode. The Terrain Observation with Progressive Scans SAR (TOPSAR) technique is used to provide vertical-vertical (VV) polarization with a spatial resolution at approximately 5 20 m. This data was processed by one of the advanced techniques namely the SBAS InSAR, which takes advantage of the small distance between satellite observation times to extract the displacement (i.e., Fig. 2). There are 42 scenes used to generate 107 interferograms and process with Generic Map Tool (GMT) GMTSAR, which is one of the newest package able to deal with Sentinel-1 directly. Figure 2. Baseline network of interferometric pairs. Lines represents 107 interferograms and black dots represent the timing and relative baseline between acquisitions. Figure 3. The LOS displacement velocity of Hanoi derived by SBAS InSAR Sentinel-1A from April 2016 to September Red boxes display 3 subsided areas: Hoaiduc, Hadong, Hoangmai (from left to right). The most interesting finding in our results is an unreported subsiding area located near the northwestern boundary of city center. This suburban area named Hoaiduc experiences the highest rate at more than 40 mm yr -1. This deformation had not been well-known in the past and the position is in one of the new residential areas adjacent to the main road, which connects the city to the western supported areas. It is clear that all of the subsidence bowls represent agricultural zone in the past and mostly dominated by clay, which potentially causing a compaction process. In addition, the rapid growth of population also induces the sudden increase of groundwater demanding since it has undergone the worst situation in recent years. Over 1.5 year of observation, the cumulative displacement as a result of continuous subsiding, the highest displacement cumulatively reaches approximately 90 mm. In case of no improvement, the worse scenario could be endangering most the facility in these new residential.

3 4. CONCLUSIONS In this study, we present the LOS displacement of SBAS InSAR from Sentinel-1A imageries to observe the surface deformation in Hanoi, Vietnam. There are 2 subsidence bowls continuously measuring the rate of displacement at 18 to 27 mm yr -1. The displacement velocity of SBAS found a new declining region whose rate is estimated to more than 40 mm yr -1. The geology factor accompany with excessive groundwater extraction could be the reason inducing this phenomenon. The good agreement of Sentinel-1A data processed by SBAS with published studies performs the successful of study in observing the land subsidence in Hanoi and high capability of further applications in Vietnam and provide valuable information for urban planning. Based on current results, there is a numerous analysis could be provided to extract further information and it is worth to keep monitoring this region with current workflow. REFERENCES [1] Le, T. S., Chang, C. P., Nguyen, X. T., & Yhokha, A. (2016). TerraSAR-X Data for High-Precision Land Subsidence Monitoring: A Case Study in the Historical Centre of Hanoi, Vietnam. Remote Sensing, 8(4), 338. [2] Dang, V. K., Doubre, C., Weber, C., Gourmelen, N., & Masson, F. (2014). Recent land subsidence caused by the rapid urban development in the Hanoi region (Vietnam) using ALOS InSAR data. Natural Hazards and Earth System Sciences, 14(3), 657. [3] Phi, T. H., & Strokova, L. A. (2015). Prediction maps of land subsidence caused by groundwater exploitation in Hanoi, Vietnam. Resource-Efficient Technologies, 1(2), [4] Senturk, S., Cakir, Z., & Ustundag, B. B. (2016, July). The potential of Sentinel-IA interferometric SAR data in monitoring of surface subsidence caused by overdrafting groundwater in agricultural areas. In Agro- Geoinformatics (Agro-Geoinformatics), 2016 Fifth International Conference on (pp. 1-4). IEEE. [5] Chen, M., Tomás, R., Li, Z., Motagh, M., Li, T., Hu, L.,... & Gong, X. (2016). Imaging land subsidence induced by groundwater extraction in Beijing (China) using satellite radar interferometry. Remote Sensing, 8(6), 468.