Microgravity survey in 2009-2010 around Bacman geothermal field, Philippines - gravity corrections and interpretations

Abstract

A microgravity survey was conducted in 2009-2010 around the Bacman geothermal field, Philippines. The observed gravity readings were subjected to the standard reduction process such as tidal, instrumental drift, latitude, and terrain corrections using Gravos, Gnet, Terra and Terrb reduction programs. Free-air and Bouguer corrections were calculated by using the Bouguer program. After these corrections, the optimum reduction density was determined analytically by using the Parasnis method. To enhance the local anomalies, the regional trend in the Bouguer maps was removed. The regional field was estimated to be planar. Positive anomalies in the central part of the field are within the Bacman Fault Zone. These are interpreted to be dense, intrusive rocks such as diorite which are present beneath this area based on core samples. In addition, these anomalies extend to the western and northern regions of the field near Mts. Tikolob and Kayabon and may be related to the resistivity anomaly seen in the western part of the field. Negative anomalies represent the sedimentary formations and collapse features in the southeastern and northern parts of the area, respectively. Using limiting depth analysis, the dense intrusive rock was approximated as a spherical body and its centre was found to be at a depth of around 900 m. In addition, an irregularly shaped body was assumed to be present beneath the western parts of the field and a depth to the top of this body was found to be about 1750 m.