Gravity Anomaly Evaluation of the Geothermal Field Distribution Around the Seismo-Volcanic Area of Toba Lake

Toba Lake is one of the largest caldera lakes in the world, formed by an enormous eruption about 74,000 years ago. The emergence of hot spring manifestations around Lake Toba is closely related to volcanic activity, which is also controlled by regional geological structures. There are two areas in Lake Toba: Simbolon and Pusuk Buhit, with several geothermal manifestations, such as fumaroles, hot springs, and steaming ground. With the large caldera area, there should be other locations with geothermal potential. Therefore, we analyzed the regional gravity anomaly to find specific patterns that might indicate a possible geothermal field. First, we obtained the data of Free Air Anomaly (FAA) with a resolution of 1.85 km from altimetry satellites and acceleration of gravity anomaly data of 0.1 mGal at 1 minute per grid with an altitude accuracy of 1 meter. In order to acquire the Simple Bouguer Anomaly, the data were corrected. Then, we analyzed the results based on derivative analysis. The result shows that the location has simple Bouguer anomaly values between - 26.2 mGal-36.0 mGal with an initial density of 1.74 gr/cc. Furthermore, we speculated that indicated regional patterns of structures and sub-surface properties might be associated with that prospective geothermal.