Application of Tomography Inversion Methods to Determine the Seismic Wave Velocity Structure (Vp, Vs, Vp/vs) of the Meq Data on “ALPHA” Geothermal Field

Abstract

Micro earthquake occurred due to of hydraulic fracturing process in a geothermal field. Six array seismometers installed in “ALPHA” geothermal field and during the period of July 2012 to December 2012 recorded 133 micro earthquake event’s (MEQ). P and S wave travel time of MEQ data relocated earthquake simultaneously and calculated 3-D velocity model. P wave velocity (Vp) and S wave velocity (Vs) also has been compared to the velocities through the process of Tomography inversion. Tomography inversion output are cross section of the velocity model Vp, Vs and Vp/Vs, then it will be a basis of the analysis to identify the state of the subsurface in “ALPHA” geothermal field. Result of inversion model was indicated that “ALPHA” geothermal field has 10 until 15 layers of rock with different in Vp and Vs. The elevation range 580 m to 1100 m was interpreted as a zone of cap rock and an impermeable layer with P wave velocity (Vp) between 2,615 km/s until 3,051 km/s. Another result in order to value of the ratio Vp/Vs is relatively low between 1,7 until 1,8 at depth of 1 km until 2 km from velocity model. The ratio value was analyzed as a layer of low ranging from 1,6 to 1,807 on the tomogram velocity model, than it is analyzed as a coating zone associated with gas-saturated rock and “ALPHA” geothermal field indicated steam dominated of geothermal filed type.