30 ENVIROMENTAL MONITORING OF LAND SUBSIDENCE IN THE COASTAL AREA OF PADANG CITY USING SENTINEL 1 SAR DATASET * Fajrin 1 , Almegi 2 , Aljunaid Bakari 3 , Risky Ramadhan 4 , Yudi Antomi 5 *1 Department of Geodetic Engineering, Institute of Padang Technology (ITP) – Indonesia 2 Department of Geography Education, Universitas Islam Negeri Sultan Syarif Kasim – Indonesia 3 Institut Agama Islam Negeri Sultan Amai Gorontalo – Indonesia 4 Department of Geography Education, Universitas Negeri Padang – Indonesia 6 Technology of Remote Sensing, Universitas Negeri Padang – Indonesia Email: fajringeo@gmail.com *Corresponding Author, Received: March 8, 2021. Revised: March 21, 2021. Accepted: May 20, 2021 ABSTRACT: The land surface in the Padang City is thought to be experiencing a continuous relative subsidence due to natural processes and man-made activities. Factors that affect land subsidence include earthquakes, sea level rise, infrastructure development, sediment transport, and excessive use of groundwater sources. The purpose of this research is to map the rate of land subsidence which is processed from the Sentinel 1-A radar, satellite imagery using the Differential Synthetic Aperture Radar (DInSAR) method. The data used are two pairs of Sentinel-1A level 1 Single Looking Complex (SLC) imagery which were acquired in 2018 and 2019. Image processing is carried out by filtering and multilooking techniques on Synthetic Aperture Radar (SAR) images. The following process changes the phase unwrapping to the ground level phase using phase displacement. Land subsidence in 2018–2019 from DInSAR processing reached -10.5 cm / year. The largest land subsidence occurred in North Padang with an average of -7.64 cm/year. Land subsidence in the Padang City, which is located near the estuary, is due to the nature of the alluvial sediment material. The use of Sentinel 1 SAR remote sensing data can provide important information in the context of mitigating land subsidence in the Padang City. Therefore, we need the right policies to handle future land subsidence cases. Land subsidence mapping is one of the factors that determine the vulnerability of coastal areas to disasters. Keywords: Land Subsidence, SAR, DInSAR 1. INTRODUCTION Recent mapping has identified coastal plain areas with sediments accumulating in alluvial basins or coastal plains as the most vulnerable to Land subsidence. Coastal plains have grown rapidly in most regions of the world. The World Bank report in 2012 states that there has been urbanization in all regions of Indonesia, the report notes that there are 11 metropolitan cities and one of them is the Padang City which has experienced population growth and is predicted to continue to increase in the period 2010-2025. [1] Land subsidence is triggered by several things, one of which is the process of decreasing the amount of groundwater which causes the gradual subsidence of most of the land over months and years. This land subsidence has quite damaging effects on the affected area, including it will result in an increase in environmental disasters such as the risk of flooding, cracks, and damage to buildings and infrastructure [2,3,4]. Remote sensing technology that continues to develop can bring new alternatives in knowing land subsidence in a large area, one of which is by utilizing SAR data from remote sensing [5,6,7,8], besides that it is also time-saving and low-cost. Land subsidence in coastal areas can be influenced by several main factors, including the number of buildings that increase the surface load of the land, this will have a negative impact, to reduce the negative impacts that may arise, it is necessary to research this phenomenon as part of disaster mitigation efforts. To date, the use of Synthetic Aperture Radar (SAR) to monitor soil deformation associated with subsidence has been demonstrated in a number of studies [9,10,11,12]. Sentinel-1 is the first of a series of seven satellite missions launched as part of the Copernicus programme initiated by the European Sumatra Journal of Disaster, Geography and Geography Education, Month, Year Vol.5, No. 1, pp.30-34 Disaster, Geography, Geography Education http://sjdgge.ppj.unp.ac.id/index.php/Sjdgge ISSN : 2580 - 4030 ( Print ) 2580 - 1775 ( Online), Indonesia ISSN : 2580 - 4030 ( Print ) 2580 - 1775 ( Online), Indonesia