Joint Geophysical Imaging Using Microearthquakes and Magnetotelluric in Delineating Geothermal Resource

By: David M. Rigor, Jr., Carlos F. Los Banos and Randy J. Tugawin

Geosciences Department

Abstract

Energy Development Corporation, embarking on the application of better practices and new technologies, is currently conducting a new geophysical method, Joint Geophysical Imaging, to be conducted in the Mahanagdong sector of the Leyte Geothermal Project and plans to do the same in Mt Apo Geothermal Project. The Joint Geophysical Imaging studies, a combination of two known geophysical methods, microearthquake (MEQ) and magnetotellurics (MT) is now being widely used for geothermal characterization both during exploration and exploitation. This method will enhance EDC’s capabilities in delineating permeable faults and structures and complement our existing tools in identifying potential drilling targets.

The Mahanagdong JGI aims to find new injection sinks in two areas; Cambantog, south of the current Mahanagdong-A RI sector, and the area west of Mahanagdong-A production field. Another objective of the study is to find drilling targets for the expanded production area at Matikug, northeast of the current Mahanagdong-B production field.

The MEQ technique utilizes both natural and induced microseismicity as seismic sources to be recorded by an array of surface seismometers that will be operated continuously until enough earthquake events are accurately located. Hutchings (2013) and Julian and Foulger (2012) have enumerated diverse information that can be obtained from analysis of MEQ data. These are:

Plot of hypocenters with high resolution that could accurately identify fractures and track where fluids flowed and where permeability was increased
Fault plane solutions, that can explain nature of fracturing
Seismic tomography that can identify reservoir fractures and permeability and improve resolution for drilling targets

Seismic tomography uses mathematical modeling of the compressional and shear wave travel times to map velocity perturbation. Passive method uses seismic waves generated by microearthquakes as the source of energy which pass through in all directions and are recorded by data loggers. Generally, seismic wave travels faster in areas of low temperatures, high pressures and of solid phases. Conversely, it has slower velocity in areas of higher temperature, lower pressure and in liquid phases.

MT method on the other hand, measures naturally occurring time-varying electromagnetic waves at the earth surface. Each MT station, usually a kilometer apart, will record the fluctuations of the magnetic and electric field for a continuous period of at least 40 hours. The electrical properties or resistivity of the underlying ground is determined by the relationship between the components of the electric and magnetic field transfer functions. Generally, the resistivity structure of a high temperature geothermal system is an increasing resistivity layer beneath a very conductive surficial layer, reflecting an increasing temperature with depth.

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