The Geophysical Model of the Mahanagdong Sector, Leyte Geothermal Production Field, Central Philippines Based on Magnetotelluric Data

Abstract

The resistivity structure of Mahanagdong consists of 1) a highly resistive cap (>50 .-m) which can be correlated to the relatively fresh to less altered volcanic deposits blanketing the survey area, 2) a very conductive middle layer (<10 .-m) which is associated with the Smectite and Smectite-Illite clay alteration, and 3) a moderately resistive third layer (>10-50 .-m) which corresponds to higher temperature but less conductive minerals such as illite, biotite and chlorite.

The postulated upflow region coincides with the dome-shaped elevation contour interface between the second and third layers and the 25-40 .-m Occam anomaly located beneath the Mahanagdong collapse where the highest subsurface temperatures were measured. The permeable zones are provided by the faults coincident with the steep resistivity gradients as indicated in the 1-D Occam isoresistivity map at -700 m elevation.

Furthermore, the magnetotelluric data delineated a possible extension of the geothermal resource still untapped by drilling. It is about 6 km2 farther east of the Mahanagdong collapse and is bounded by the Upper Mahanagdong Fault in the north, the Maagas Fault in the east and the Macape Splay Fault in the south-southeast. Drilling a well to intersect the Magaas Fault will help determine the extent of the resource to the east.