Groundwater Change Detection by Gravity Measurement on Northern Coast of Java: A Case Study in Semarang City of Central Java of Indonesia

Abstract

Northern part of Semarang city is affected by tidal flood due to land subsidence. Land subsidence is triggered by soil compaction and massive groundwater extraction. One of famous methods in analysing groundwater is gravity method. This research aim to present groundwater change in Semarang city based on gravity measurement and to analyze problems related to equipment and groundwater behaviour. Gravimeter Scintrex CG-5 was applied for gravity measurement. Geographic coordinates were measured using GNSS receiver namely TopCon HiperII. This research occupied SRTM90 Plus and regional geology map at 1:100.000 scales. Groundwater change was detected by differentiating simple Bouguer anomaly of 2014 and 2016. Data in gravity unit were then converted by coefficient of 0.012 mgal per meter. Simple Bouguer anomaly in 2014 ranged from -3.423 mgal to 19.874 mgal, while 2016 data ranged from -3.427 mgal to 19.835 mgal. Discrepancy of gravity anomaly ranged from -0.326 mgal to 0.209 mgal. The data confirmed that groundwater change ranged from -27.160 m to 17.408 m. Groundwater depletion occurred commonly on alluvial plain in northern area, while recharging could be found in some locations in southern part. This research concluded that gravity measurement had potential to be applied for groundwater detection in coastal areas.