Land Subsidence Monitoring by SBAS-InSAR Technique with the Conversion to Horizontal-Vertical Displacements in Semarang

The main factors of land subsidence are excessive pumping of groundwater for industry, agriculture, snow melting, cooling, pumping of natural gas, and mines’ tunnel excavation. The city of Semarang, the capital of Central Java province, is located on the northern coast of Java island in Indonesia. Land subsidence in this city has enormous potentials for many hazards threatening people and urban infrastructures, such as buildings, roads, or whatever necessary for livelihoods. To accurately measure the land subsidence in Semarang, time series analysis of Small Baseline Subset (SBAS) or Permanent/Persistent Scatterers (PS) and Distributed Scatterers (DS) InSAR analysis is essential. This study shows the recent land subsidence volume in Semarang city with the SBAS technique and compares it to historical InSAR analysis. We also conducted the 2.5-dimensional analysis to convert the satellite line-of-site (LOS) displacement into the horizontal and vertical direction to see if there are differences between ascending and descending data. The SBAS analysis result showed the displacement volume in a range of - 18.2cm subsided and +6.8cm uplifted. The displacement rate was estimated to be substantially constant over time without any apparent seasonal effect. There is no significant difference in the results between ascending and descending datasets. The 2.5-dimensional analysis identified the horizontal and vertical displacements that we corrected any noise and inclined directional values using incidence angle.