1. INTRODUCTION
The presently active LUSI mud-volcano is the dominant, central, surface discharge feature of a geo-pressured, low temperature geothermal system. It started with a major eruption of hot fluids and mud on 29 May 2006, close to the 2.8 km deep Banjar Panji-1 (BJP-1) exploration well, drilled by PT Lapindo near Sidoarjo in E Java (for location see Figure 1).
Minor eruptions occurred a few days later along a c. 1 km long segment within a probably c. 250 m wide, NE trending fracture zone. Only the first eruption centre, c. 200 m SW of the BJP-1 well, has remained active. At the start of eruptions, the 2.8 km deep well had encountered a sequence of over-pressured and under-compacted Pleistocene sediments below 1.3 km depth.
The well was not cased below 1 km depth which facilitated rupturing within the fracture zone; an M 6.2 earthquake had occurred on 27 May 2006, about 200 km away in Central Java.
Figure 1: Location map of LUSI mud volcano near Sidoarjo (Java)
Gas (CH4, CO2, minor H2S), hot water, and hot mud at boiling point ascended to the surface, initially with a flow-rate of c. 0.5 m3/s. The discharge fluctuated and increased on average to > 1 m3/s after 2 months with fluctuations reaching an inferred peak discharge rate of c. 1.5 m3/s in December 2006.
The terrain around the discharge centre is rather flat (3 to 10 m a.s.l. within a radius of a few km). The size of the flooded areas increased rapidly from c. 1 km2in June to 6.5 km2in May 2007.
The flooded area has been encased and contained by an extensive dam building program. Monitoring and assessment of discharge rates, subsidence (using GPS), and fluid characteristics began in June 2006. Surveillance included micro gravity-, resistivity-and micro seismic surveys at irregular periods up to April 2007.
Some results of this first monitoring period have been published. The aim of the first publications (Mazzini et al., 2007, Davies et al., 2008.) was to prove or disprove the importance of a possible triggering effect of the drilling operation. Attempts to predict future subsidence and mass accumulation were based mainly on earlier monitoring data collected until May/June 2007 (Istadi, pers.com.).
Monitoring efforts declined after May 2007 but were taken up again in April/May 2008 with new micro-gravity- and micro-earthquake surveys, augmented by a few shallow ground temperature studies. The mud discharge from the central vent has continued unabated since June 2006.
Since most monitoring studies were made at irregular intervals, often by different groups from different organisations, it has been proposed to continue with a more coherent and regular monitoring programme of important parameters and features of the mud-volcano with the aim of detecting trends which might allow a prediction of shortand long term changes.
Such programme, if successful, could assist socio-economic planning aimed at mitigating the catastrophic effects of the LUSI eruption which has already caused the displacement of up to 40,000 people who lived in the now flooded areas.
Based on discussions with engineers from Badan Penanggulangan Lumpur Sidoarjo (BPLS), involved in the ongoing containment (management) of the mud discharge, and scientists who undertook some of the earlier monitoring, as well as using published and unpublished reports, papers, and satellite photos, an assessment of the monitoring efforts is given here.