Relationship between Time Lapse Microgravity Anomaly with Na and Cl Content for prediction of Sea Water Intrusion in Tourism Area Kota Lama Semarang Indonesia

Abstract

This research is motivated by changes in infrastructure that occur in the old city, the changes in question are the frequent occurrence of flooding in this region. The flooding that occurred was estimated to be caused by subsidence. In addition, subsidence that has other effects, for example seawater intrusion in the Old City, which is located close to the north coast of Java, is also relatively close to about 3 km. The purpose of this study was to find a relationship between seawater intrusion and micro-gravity anomalies between times. The methods carried out are as follows: (1) choose Gravimeter with micro precision. For this study using the Autograv CG5 type, (2) determining the location of the measurement point, which is not expected to experience physical damage due to natural or human behavior for a certain interval, (3) measuring gravity for two different periods in September 2017 and March 2018 at the same point, (4) processing data with initial corrections, namely tidal correction to reduce tidal effects, float correction, and correction of the dynamics of groundwater levels associated with rainfall, and (5) conduct chemical tests of water samples taken from residents’ wells at the Research site. The results showed that the negative time-weight gravity anomaly correlated with an average groundwater level decrease of 15,468 cm. This is in accordance with the fact that during this range rainfall trends tend to decline from September 2017 to March 2018. This decrease in ground water causes maximum subsidence and occurs in the south-southwest 2.3 cm. This zone experiences sea water intrusion which is characterized by the content of Na (60-100%) and Cl (100-160%).