Magnetic Anomalies Over the Kamojang Geothermal Field (West-Java, Indonesia)

Abstract

A high level (2500 m) airborne magnetic survey made in 1986 has shown that a broad, negative total force anomaly occurs over the centre of the Kamojang field. The anomaly can be interpreted in terms of demagnetised, andesitic reservoir rocks extending from about 300 m depth to at least 900 m depth. Measurements on cores also indicate that rocks from productive wells are demagnetized where T>150?C (commonly below 300 m depth). Modelling of topographic effects of andesitic flows exposed outside the prospect gives an average magnetization of 2.5 A/m for these unaltered rocks. The magnetic model covers an area of about 10 km2; all productive wells lie within this area. The study shows that demagnetisation of volcanic reservoir rocks can also take place in a vapour-dominated system.