A 3-D interpretation of magnetotelluric data at the Atadei geothermal field, Lembata Island, Indonesia including sea effects

Abstract

A three-dimensional (3-D) interpretation has been performed to magnetotelluric (MT) data obtained at the Atadei geothermal field, Lembata Island, Indonesia, for a detailed investigation of geothermal reservoirs. A preliminary interpretation has been made through inversion of MT data. The 3-D inversion scheme, which is based on the linearized iterative least-squares method with smoothness regularization and MT modeling algorithm using a 3D finite difference method, considers static shifts as unknown parameters in the inversion. The resulting resistivity model contains a low-resistivity in a shallow layer and a highly-resistive basement. However, for more precise interpretation of MT data at the Atadei field, we need to consider effects of surrounding sea and islands on the MT data, since the island is relatively small in a sense of MT survey. In this study, we first analyze effects of surrounding sea and islands on MT data in Lembata Island. In the analysis, we made MT simulation for several models; e.g., a model describing a large area including the Lembata Island and nearby islands and sea, a model of small area only describing part of Lembata Island and sea close to the Atadei field, and models with different areas with proper simplifications. Through the analysis, we selected a proper base model for inversion, which is as small as possible while explaining not only MT data in Lembata Island but also effects of surrounding sea and islands. Using the base model, we made 3D inversion of MT data in Lembata Island including sea effects.