Yayan Sofyan

Application of A10 Absolute Gravimeter for Monitoring Land Subsidence in Jakarta, Indonesia

To investigate the cause of land subsidence occurring in Jakarta, Indonesia, we have conducted absolute gravity measurements near or at the existing GPS points in 2008, 2009, 2010, 2011 and 2013, by using a portable absolute gravimeter Micro-G LaCoste A10 serial number 017 (MGL A10-017). The A10 measurements, particularly those conducted before 2010, were adversely affected by the influence of the high-temperature and high-humidity climate, which caused problems with the ion vacuum pump, laser controls and unstable behavior of the data processing personal computer. Moreover, some of the gravity points were lost mainly due to construction work. For these reasons, the number of reliable gravity data was restricted. Nevertheless, the observed gravity changes along the northern coastal area of Jakarta showed an increase in gravity, which was consistent with the GPS results. The analysis of the height changes versus the gravity changes suggested that the subsidence was caused by soil compaction and/or groundwater level changes, which are likely connected with excessive groundwater extraction.

Monitoring Hot Spring Aquifer Using Repeat Hybrid Micro-gravity Measurements in Beppu Geothermal Field, Japan

Repeat hybrid micro-gravity measurements were conducted to detect the gravity change caused by hot spring water production around Beppu in eastern Kyushu, Japan. An A10 #017 absolute gravimeter (Micro-g LaCoste) and a CG-5 #549 gravimeter (Scintrex) were used for this study in intervals of three to four months at eight gravity stations. According to the results obtained with the absolute gravimetry, a gravity change of up to 33 μgal was detected at the Beppu Geothermal Research Laboratory (BGRL) reference station. The observed absolute gravity was compared with the groundwater level, and there was a good correlation between the gravity changes and the groundwater level changes. Based on the precipitation, groundwater level, and soil character, the effect of the water content changes in the unsaturated zone was estimated precisely by using a Gwater-1D. This calculation can explain that the gravity seasonal changes were caused by the groundwater level changes. After removal of noise effects (e.g., tidal movement, precipitation, and shallow groundwater level changes), the residual gravity changes, which were measured by the relative gravimeter, were subdivided into two types of responses. Gravity changes up to 90 μgal were observed from April 2014 to July 2015. After that, gravity became stable, except for small seasonal changes.