Resistivity Structure of the Mt. Labo Geothermal Project Based on Magnetotelluric (MT) Sounding Measurements, Southern Luzon, Philippines

Abstract

Results from inversion modeling derived from MT sounding data collected in 1995 and the more recent 2012-2013 surveys in the Mt. Labo Geothermal Project (MLGP) generally show a thin high resistivity layer ( more than 30 ohm-m) overlying most of MLGP. Beneath this thin high resistivity layer is a conductive layer associated with the smectite and illite-smectite clays of the altered andesite and dacite rocks from the Mt. Labo central cone. Beneath the base of the conductive layer is a dome-like resistive feature, postulated to be the top of the reservoir. This resistive feature is bounded by thick conductive layers on both sides. An isoresistivity map at 1500 m below sea level shows a ~15-30 ohm-m resistivity anomaly and is postulated to be part of the geothermal reservoir. Isotherms from drilled wells indicate higher temperatures towards this intermediate resistivity anomaly and lower temperatures towards the conductive zones located in the margins. The postulated upflow in Mt. Labo is beneath the resistive dome-like feature where the highest temperatures were encountered by the drilled wells. Outflow of hot fluids is probably towards the southwest feeding the thermal springs of Kilbay and Alawihaw.