Jurnal 1

Time lapse vertical gradient microgravity measurement for subsurface mass change and vertical ground movement (subsidence) identification, case study: Semarang alluvial plain, Central Java, Indonesia.


Application of microgravity survey by measuring gravity change in time had been used extensively in many fields. The major causes of gravity change are mass change in reservoir include ground water level change (subsurface) and vertical ground movement (subsidence). While the observed gravity change, called as time-lapse microgravity anomaly, is as superposition of all the causes, hence how to identify each source is very important because some of causes could have similar response. As example is increase of subsurface density (mass) and subsidence.

In order to minimize this uncertainty, time-lapse vertical gradient microgravity associated with its time-lapse microgravity is analyzed. Theoretical background of this analysis is that vertical gradient microgravity value at the surface will be constant if there is only subsidence (no subsurface mass change). Therefore, response of its time-lapse vertical gradient microgravity for subsidence will be zero. In contrast, subsurface density change is indicated by anomaly values that direct proportional with its subsurface density contrast.

To demonstrate this technique, repeat microgravity and vertical gradient microgravity measurement were conducted in Semarang alluvial plain area that has 2 to 17 cm/year subsidence rate and 1 to 5 m/year ground water level change. Their time-lapse microgravity and vertical gradient anomalies indicates existence of ground water decrease, subsidence, and combination between subsidence and tidal flood. This result confirm with elevation change measurement and ground water level change from well data.

KEY WORDS: Time-lapse, vertical gradient, microgravity, subsurface mass change, subsidence

Oleh Wawan G.A. Kadir, Djoko Santoso and Muh Sarkowi

Geophysical Engineering Department, Bandung Institut of Technology – Indonesia (wawan@unix.lib.itb.at.id), Indonesia.