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Felixberto C. Maneja, Carlos F. Los Baños,
Domingo B. Layugan, Nilo A. Apuada,
and David M. Rigor, Jr.
PNOC Energy Development Cnrp., Philippines






Interpretation of magnetotelluric (MT) soundings collected in 92 stations in 1995 and 2000 provide a better and deeper picture of the resistivity structure of the Northern Negros geothermal field (NNGF) than previously obtained from DC electrical resistivity surveys. The resulting three-layer resistivity structure interpreted from MT data is consistent with drillhole information as well as data from previous geophysical surveys. A thin, ~ 300-m thick, highly resistive (30 to >100 ohm-m) layer blanketing the area corresponds to young and fresh extrusives from Mt. Canlaon. Immediately underlying this stratum is a 0.5- to 1.0-km thick highly conductive (1-10 ohm-m) second layer that extends from Mambucal in the northwest to Sumaguan in the southeast. This conductor represents the hydrothermal system’s clay cap and may also partly coincide with the shallow and structurally-confined outflow in the northwest. Over the productive region of NNGF, the base of this conductive second layer coincides with the transition from smectite- and illite-smectite-dominated argillic alteration to a secondary assemblage dominated by higher temperature but less conductive minerals like biotite, epidote, and illite. The resistivity of the third layer varies from place to place. In Pataan, the bottom layer is a 2-km thick moderately resistive (20-30 ohm-m) stratum that drillhole data confirm to be part of the geothermal reservoir. This block is juxtaposed to the north by the Mambucal B Fault against highly resistive (30 to >100 ohm-m) third layer that underlies the Catugasan and Mambucal and which corresponds to cold geothermal aquitard. In Sumaguan, southeast of Pataan, the third layer becomes slightly more conductive (10-20 ohm-m) implying the presence of saline and hotter fluids in this sector; the identification of Sumaguan as the prime resource block in NNGF should be tested by deep exploratory drilling. South of Pataan, a narrow resistive block bounded by the Pataan C and Maao East faults separates the high-temperature geothermal reource in Pataan-Sumaguan from that in Hagdan. In addition to being geophysically less coherent, the Hagdan resource is believed to be of lower grade and merits lower priority for deep delineation drilling.




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