Gravity Modelling And Second Vertical Derivative Calculation To Analysis Subsurface Structure At Wayratai Geothermal Prospect Area, Lampung

Abstract

Wayratai Geothermal Prospect Area, Lampung is near a cluster of Mount Ratai with geological condition in the form of young volcanic sediment such as lava, breccias, and tuff obtained from the eruption of Mount Ratai. To determine geothermal potential in Wayratai area, we needed some preliminary surveys such as geological, geochemistry, and geophysical survey. Based on geological mapping, we obtained several point of manifestations such as 5 hotspring points (with a temperature 86-90°C), mudpool, steaming ground, and alterated rock. Afterthat, performed with a geophysical survey using the gravity method to determine subsurface structure and describe the initial conditions of the geothermal system. Gravity method is used because this method is sensitive to analyze the vertical changes, and can describe the subsurface conditions based on density variation of rock. Based on data acquisition and literature studies performed on Wayratai geothermal prospects area, obtained 2D and 3D subsurface model based on density variations. From the model, obtained area with a density value 2.3-2.6 gr/cm3 and depth from 400 to 2000 m were assumed to be a reservoir zone. Then, the area with a density value 2.8-3.2 gr/cm3 and depth from 2000-2500 m assumed to be a heat source. With these density, we can assume that the rock in reservoir area is calcareous-breccia and in the heat source is an andesitic-basaltic rocks. Based on the analysis of second vertical derivative calculation, obtained value of SVD maximum is greater than SVD minimum which indicated that the area is a basin structure or normal fault, and its strenghtening of the assumption that the modelling of the geothermal reservoir is graben form.