Utilization of Double-Difference Tomography for Geothermal Exploration: 3D Velocity Structure Interpretation and Fluid Determination

Abstract

Geothermal surface exploration entails a multi-geoscientific process, which is aimed to define the geometry and characteristics of the geothermal reservoir prior to drilling. Lately, micro-seismic event monitoring is becoming a standard procedure in inferring the structure of the potential geothermal reservoir. However, a good coverage of seismic station and abundant seismic events must be fulfilled in order to map the subsurface structure. Taking advantage of the well-designed seismic station deployed at the “ARD” geothermal field prior to its first drilling, a study of micro-earthquake tomography for 3D reservoir structure is performed in this field. A seismic network of 26 stations was set up for more than eight months from August 2011 within 20 km radius from the centre of the expected reservoir. There were 637 micro-seismic events had been detected and located, which is a very high number of seismicity for a region that is not yet under geothermal development. The purpose of this study is to construct a 3D seismic velocity structure using double-difference tomography and to infer fluid properties, i.e. steam and brine, from the ratio of the P- and S-wave velocity. Double-difference tomography is used to its ability to reduce uncertainties of the model associated with picking and velocity structure. A zone with low P and S velocity anomaly as well as a low ratio of Vp/Vs, which is interpreted as the steam dominated reservoir, is observed. Below this reservoir, there is a high P and S velocity anomaly and a high ratio of Vp/Vs which may be correlate to the non-permeable rock.