SEPARATION OF GRAVITY ANOMALY CAUSED BY SUBSIDENCE AND GROUND WATER LEVEL LOWERING OF TIME LAPSE MICROGRAVITY DATA USING MODEL BASED FILTER (CASE STUDY SEMARANG ALUVIAL PLAIN)

• Main Author: (NIM 32303002), SUPRIYADI
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/11463
• Institution: Institut Teknologi Bandung

The time lapse of microgravity method is a development of gravity method which is conducted by repeating measurements of the gravity within a certain time interval. Since the microgravity changes due to fluid movement in the subsurface, subsidence and topography are relatively small in number (< 100 uGal), therefore special equipment, planning, strategy and surveys are required to overcome the problems.<p>The objectives of this research are to make a Model Based Filter (MBF) and to examine the filter on the subsidence model and ground water level lowering synthetic data, and to analyze the filter ability based on the output gained. This is compared with its input and the application of MBF to the time lapse microgravity anomaly in Semarang.<p>The time lapse microgravity research was held in Semarang. The research started by preparing a synthetic data model that is used in order to find out the response of the time lapse microgravity caused by subsidence and ground water level change. The result of the synthetic data model showed that 1 meter ground water level change with porosity of 30 % creates 12.576 uGal. Negative anomaly showed lowering ground water level, and positive anomaly showed the increase of ground water level. Synthetic data simulation for 10 cm subsidence produced an anomaly response of 30 uGal.<p>MBF synthetic model was made based on 2D and 3D models. Anomaly object model can be seen on the subsidence and ground water level lowering. Based on 1D FFT and 2D FFT of gravity anomaly showed that the frequency of subsidence was higher than ground water level lowering. This occurred if subsidence dimension was smaller than ground water level lowering. At this condition the frequencies of both anomalies can be separated from mix frequencies using MBF. The MBF input was gravity anomaly caused by subsidence and ground water level lowering, while the output was the gravity anomaly due to subsidence or ground water level lowering. The result of the model showed that the MBF made for the model could separate the gravity of the anomaly well and it created no mistake.<p>The research of time lapse microgravity had been done 7 times every 6 months during 2002 to 2005 in Semarang. The equipment used was the Gravimeter L&R type G1158 that equipped by alliod system which has the accuracy of 5 uGal, to measure the gravity in every station. The Gravimeter L&R type G508 equipped by the electronic feedback was used to observe the tide of the gravity at the base. The Sipat Datar method was used to measure the elevation point. Time lapse microgravity anomaly in Semarang was showing the different anomalies values in every period of experiment. Maximum and minimum anomalies 26 and -8 uGal were occurred from September to June 2003. From September 2002 to November 2005 the maximum and minimum anomalies 75 and -30 uGal. The negative anomaly occurred from September 2002 to June 2003 was about 30 %. In fact, 60 % of the research area had negative anomaly from September 2002 to November 2005. It indicated that the lowering ground water levels occurred in this area.<p>The usage of MBF for time lapse microgravity anomaly data during September 2002 to November 2005 showed that a maximum of 48 cm subsidence occurred in the north Semarang while 0 – 4 cm in the south of Semarang. The measuring results by using the time lapse microgravity and Sipat Datar methods were showing the similar results. <br /> <br />