The Importance of Coupling Passive and Active Seismic Methods in Geothermal Fields - A Case Study at Paralana, Australia

The Paralana Enhanced Geothermal System is situated in South Australia and is a promising project aiming to exploit natural heating from the radiogenic basement. A 5-day stimulation of the well, in July 2011, induced more than 7000 microearthquakes. Seismic event waveforms are complex, exhibiting a high amplitude seismic phase arrival between direct-P and direct-S waves, a phase which complicates event analyses and needs to be explained. In such projects, site characterization should use all available data, a key issue being a proper knowledge of both P- and S-velocity models. In the present case, a combination of active and passive seismic data, acquired before and/or during hydraulic fracturing, was paramount to the definition of a proper model. Using existing 2D seismic profile interpretations and a seismic log, and applying ray tracing to constrain interval velocities, a 3D velocity model was built in order to improve the event locations. Furthermore, to define the origin of the unknown phase, 3D ray-tracing modeling was systematically applied to test different scenarios. Actual results show that in term of arrival time, these phases most likely correspond to converted waves on interfaces between 1.6- and 2.5-km depth rather than on nearby steep faults.