Integrated Geophysical Exploration in Mataloko Geothermal Area, Central Flores-East Nusa Tenggara,

Abstract

Integrated geophysical explorations including DC-resistivity, Magnetotelluric, gravity, magnetic and self-potential methods have been conducted to provide an integrated information on electrical resistivity distribution, to assess a likely structure and hydrology of geothermal reservoir in conjunction with the existing geological and geochemical data of the Mataloko geothermal prospect. The resistivity models of the MT in Mataloko area is very consistent with the 2-D resistivity models of the Schlumberger data. There is generally a thin high-resistive surface layer except the manifestation zones. Below it, the Mataloko area is entirely underlain by a low resistive layer in the shallow zones. Resistivity of this layer is very low (1 .m), near the manifestation zone. This is interpreted as a clayrich zone which corresponds to a cap layer of the geothermal reservoir system. A large high-resistive body is interpreted below this cap layer in the Mataloko surface manifestation zone. Residual gravity anomalies show a broad negative anomaly of -10 mgals which extends from Mataloko to the eastern part of the manifestation area. This anomaly may indicate a concealed graben structure. The negative anomaly could be caused by negative density contrast between the graben structure infill and the more dense surrounding rocks. Residual magnetic anomaly occurs over the central part of the geothermal prospect, which is slightly narrow negative anomaly almost superimposes the low resistivity anomaly. It is considered that a hydrothermal activity in this area demagnetized the volcanic rocks. Self-potential data shows the highest positive anomaly in the main geothermal manifestation area along the stream Wae-Luja. A 2-D hydrological and electrical structure model was numerically constructed and the result suggests the existence of high permeability zone around the main manifestation area. A local dipolar SP anomaly found in the southern area is corresponding to a fault zone suggested by gravity survey. This fault probably act as a conduit for ascending geothermal fluid in this study area.