Towards Modeling the Seismic Velocity Structure of the Pohang Enhanced Geothermal System Site in Korea

The seismic velocities are mostly affected by the microstructure of rocks such as porosity, geometry of the pore space and bulk modulus of the pore fluid. Their effects on the seismic velocities can be modeled using the effective medium theory, which can clarify a link between P- and S-wave velocities (Vp and Vs) and lithology, fluid content and the shape of fluid-filled cracks. In this study, we conduct systematic modeling based on the differential effective medium (DEM) theory to understand seismic velocity structure beneath the Pohang enhanced geothermal system (EGS) site in Korea. The Mw 5.4 earthquake occurred on 15 November 2017 at the depth of 4.5 km beneath the EGS site, and fluid injection was suggested as one of its causes. The DEM theory models the effective medium of a two-phase composite that consists of background medium and inclusion phases. The modeling is performed by incrementally adding inclusions with a constant aspect ratio to the background medium and then re-calculating the new effective elastic tensor of the composite at each increment. For the background medium, we use sandstone and granodiorite, which are major geologic strata at depths of 0.4 - 1.3 km and 2.4 - 4.4 km, respectively, beneath the EGS site. At the initial step of calculation, crack-free sandstone is assumed to have Vp = 5.5 km and Vs = 3.6 km, and crack-free granodiorite rock to have Vp = 6.246 km/s and Vs = 3.669 km/s. Inclusions of water-filled or air-filled cracks are incrementally added and aligned parallel to one axis in the background medium. The modeling results show that both Vp and Vs depend on the volume fraction, fluid type and geometry of cracks. We expect that the velocity perturbations due to fluid, crack shape and its distribution can be at most 10% beneath the Pohang EGS site, considering previous seismic tomography results for the Geysers geothermal field at shallow depth. A decrease in Vp and Vs of crack-free rocks by 10% requires 14 - 16% volume fraction of water-filled cracks with spheroidal shape and 18 - 20 % volume fraction of spheroidal air-filled cracks. For water-filled cracks with ellipsoidal shape, the required volume fraction is only 2.2 - 3.7% for 10% reduction in both Vp and Vs. For ellipsoidal air-filled cracks, the required volume fractions are 1.7% and 3.9% for 10% reduction in Vp and Vs, respectively.