Abstract
We conducted MT measurements in the Bajawa geothermal field in central Flores Island, eastern Indonesia, under a joint research project among the Geological Survey of Japan, New Energy and Industrial Technology Development Organization, and Directorate of Mineral Resources Inventory, Indonesia. Then, we have applied two-dimensional (2-D) and three-dimensional (3-D) inversions to the MT data for the geological interpretation of geothermal reservoirs. The inversion algorithms used in this work are based on the linearized least-squares inversion with smoothness regularization. In addition to the subsurface resistivity structure, static shifts in apparent resistivity were also solved simultaneously in the 3-D inversion. The comparison of 2-D and 3-D models has revealed consistent results with each other and that the 3-D inversion technique has become practical. However, there still are some drawbacks in both 2-D and 3-D inversions. When applied to real 3-D environments, the 2-D inversion may create a distorted structure caused by a 3-D structure beneath or off the survey line. On the other hand, since the 3-D technique applied in this work assumes a homogeneous media when computing the Jacobian matrix (sensitivity matrix), it may also create ambiguous anomalies when applied to a structure of large resistivity contrast. The 3-D resistivity models of the major survey area, Mataloko, in Bajawa have the following features. The surface layer is mostly resistive, corresponding to less altered volcanic rocks, except the geothermal manifestation zone, where resistivity is low from the surface. Below this surface layer is a very conductive layer that spans almost the entire survey area. Beneath the manifestation zone, resistivity is approximately 1 ohm-m and its thickness is approximately 400 -500 m. This layer is interpreted as the clay-cap of the reservoir system. Pilot drilling data revealed the existence of montmorillonite in this zone. The resistive basement below the low resistivity layer is interpreted as the hot-water circulation zone. The basement is the shallowest at the manifestation, approximately at a depth of 500 m, while it becomes deeper rapidly in the western half of the surveyed area.
