The Application of Triangular Mesh for Gravity Inversion to Reconstruct Subsurface Structure in Hululais Geothermal Prospect, Bengkulu

Abstract

Inversion of gravity data need an appropriate initial model in order to guide the process obtaining reasonable subsurface model. Downhole data from well (such as lithology, density logs) are useful to construct the initial model in order to constraint the inversion results. Second Vertical Derivative (SVD) are also useful to give the additional information, especially in delineating the fault type and its orientation. Based on these information an inversion domain of triangular mesh are constructed. The triangle inside the domain are tailored to the fault orientation and its estimated location, so it will give the possibility to construct the inversion geometry result as appropriate as geometry of the fault. This inversion that use conjugate gradient algorithm has been tested to a synthetic model and show the improvement in reconstructing density structure distribution of the original model compare to rectangular mesh. The triangular mesh inversion was applied to gravity data in Tompaso Geothermal Prospect. The Bouger Anomaly ( =2.65 gr/cm3) of this field vary from 125 to 150 mGal and the prospect area is characterized by the existence of low gravity anomaly associated with existing manifestation that are mainly striking in the NE-SW direction. The density model of triangular mesh inversion in a line that perpendicular to this area prospect show that, there was subsurface density structure distribution with high-low-high density value associated with depression structure in Tondano Caldera. The plane of fault estimated has dip angle ~80o became tracks of the manifestation presence in the surface.