ANISOTROPIC SEISMIC TOMOGRAPHY OF A POTENTIAL HOT DRY ROCK RESERVOIR BEFORE AND DURING INDUCED PRESSURIZATION

To assess the effects of changing fluid pressure in a potential hot dr y rock reservoir, two cross-hole seismic surveys were conducted: one a t ambient pressures and one with the investigated volume hydraulically overpressured by similar to 3.3 MPa. Sources and receivers were locat ed in two straight subvertical boreholes similar to 100 m apart. Trave ltime data were inverted for velocity information in two phases. Durin g the first phase the seismic velocities were constrained to be isotro pic, and during the second they were allowed to be anisotropic. Accoun ting for anisotropy enhanced greatly the interpretability of the tomog rams. Before pressurization, a low-velocity anomaly at 455-475 m depth correlated with a subhorizontal zone of hydraulically permeable fract ures connecting the two boreholes. Although the absolute magnitude of anisotropy is not well constrained, the dominant orientation of the TI -axes is consistent with the local stress field. Hydraulic over-pressu rization resulted in a pronounced extension of the low-velocity zone, but had no significant effects on the velocity structure elsewhere. Th e results of this study suggest that anisotropic seismic tomography ma y be a key method for characterizing hot dry rock reservoirs.