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Several geothermal prospect areas are situated near coastal regions. One of them is Jailolo geothermal prospect area located on the island of Halmahera. Magnetotelluric (MT) data obtained in the vicinity of the coast may be suffered from coast effect. Boundaries between ocean and land may induce severe distortion of electrical fields due to its extremely high conductivity contrast. The effect mostly distorted MT data in low-frequency and may produce some artifacts in deep parts of resistivity inversion model. In order to get a reliable subsurface resistivity model in Jailolo, a 3-D inversion of acquired MT data was carried out by including an oceanic model, which was set as a prior model, to overcome coast effect distortion. Before that, to examine coast effect influence on acquired MT data, 3-D forward modeling of a simple synthetic model was performed using a similar 3-D inversion mesh grid. Conductive seawater around survey area was also built in the mesh grid and adjusted to bathymetry data. Furthermore, synthetic MT data were then inverted in two schemes with and without oceanic model. Based on inversion results, use of oceanic model can significantly improve inversion result and give a more comparable inversion model with a synthetic model. Meanwhile, 3-D inversion of real MT data, which was carried out with a similar approach, successfully produces a representative subsurface resistivity model to describe geothermal system in study area.

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