Magnetotelluric (MT) Resistivity Surveys in Various Geothermal Systems in Central Philippines

Abstract

Since PNOC-EDC adapted magnetotelluric (MT) resistivty surveys in 1995, hundreds of MT data have been obtained in various Philippine geothermal areas. Interpretation of these data yielded a generally similar MT model for Philippine geothermal systems. The MT data provided a clearer and coherent resistivity structure and can better determine the actual size of the resource than the Schlumberger data.
The MT-derived resistivity model of Philippine geothermal systems is generally a three-layer structure. The surface cover is usually composed of approximately <100-300 m thick resistive layer of <100-500 ohm-m which coincides with the young, less altered volcanics. Immediately below the surface layer lies the highly conductive horizon with values of <10-15 ohm-m of varying thickness. This conductor represents the hydrothermal system's clay cap composed mainly of smectite. The base of this conductive second layer coincides with the transition from smectite to smectite-illite dominated argillic alteration. In the central part, the underlying third layer is a higher resistivity zone of <20-100 ohm-m. This relatively high resistive substratum is attributed to the decreasing amount of clays and to secondary assemblage dominated by higher temperature but less conductive minerals such as illite, chlorite, epidote, biotite and actinolite. This third layer corresponds with the hottest part of the geothermal system. It is then bounded by materials of varrying resistivity depending on the country rocks surrounding the area. The configuration and depth of the third layer point to the most probable location of the center of the resource.