Interpretation of Gravity Surveys at Tongonan Geothermal Field

Abstract

Residual gravity anomalies of the Tongonan Geothermal Field are interpreted to be the result of a positive density contrast between an intrusive diorite body and the surrounding volcanics. A possit i v e anomaly in the Mahiao, Sambaloran and Malitbog parts of the field, is associated with the occurrence of the Mahiao Plutonic Complex, as encountered in most deep wells. However, known plutonic bodies in the Mamban and Mahanagdong areas do not produce high gravity values because of overlying low density rocks. Another positive anomaly occurs in the nearby Bao valley. Interpretation of this anomaly suggests that it could be modelled by another plutonic body, which may originally have been related to the Nahiao Pluton, but has since been displaced by left lateral movement ong the Philippine fault.