Monitoring of LUSI Mud-Volcano - a Geo-Pressured System, Java, Indonesia

Abstract

The LUSI mud-volcano is the dominant surface manifestation of a geo-pressured, low temperature geothermal system in Java. It started with an eruption of hot fluids and mud at the end of May 2006, close to an uncompleted, 2.8 km deep oil exploration well, two days after a major earthquake. The mud discharges from a concealed crater have been irregular with inferred long-term volume rates between 1 and 1.5 m3/s and have continued ever since. The flooded area increased from c. 1 km2 in June 2006 to c. 6 km2 in May 2007; it displaced c. 40,000 people. The area has been enclosed and divided by a network of huge dams. The discharged hot liquid mud came initially from depths around 1.7 km where formation pressures of c. 30 MPa and temperatures of 100 deg C (gradient of c. 40 deg C/km) prevail. Pore pressure decrease in the upper sediments caused the initial subsidence over an area of the order of c. 7 km2 after one year. The resulting subsidence bowl has accommodated only part of the extruded mud whose spreading is constrained by the huge dams.
Monitoring of subsidence and mud discharge began in June 2006 but ceased in April 2007 when sites were flooded. Satellite records (INSAR analysis of radar data) have been used to monitor subsidence of the not flooded region during that period. The rather short (< 2 km) wave length of subsidence during the first year points to shallow (< 0.5 km) source depths of de-compressed sediments. Interpretation of IKONOS satellite photos allows an assessment of heat discharged by the central steam plume and the upwelling mud column. The heat loss by steam discharge over the crater has fluctuated between c. 3 and 150 MW, losses due to hot mud discharges were always > 200 MW. Discharge trends during 2008 pointed to an overall irregular decrease of steam and mud losses which, however, was followed by a renewal of discharge activity in February 2009. The rate of gas discharges (mainly CO2 and CH4) from the central crater and gas discharges over the flooded area have not been measured yet. Micro-earthquake and micro gravity surveys were started in 2006 and repeated in May 2008 together with a ground temperature survey.