Interwell seismic imaging at the Savannah River Site, South Carolina

Crosswell and continuity logging seismic measurements were made beneath a l arge tank (27 m diameter) used for processing radioactive waste at the Depa rtment of Energy (DOE) Savannah River Site in the Atlantic Coastal Plain of South Carolina. We used the data to delineate a low-velocity zone (soft ma terials) and image the connectivity of a clay unit between wells. The low-v elocity zone depicted on the crosswell seismic tomogram integrated with dat a from cores and well logs revealed soft materials in the region between 15 0 and 180 ft (46-55 m). The bottom boundary of this low-velocity zone corre lates with a reflection observed in the crosswell seismic image at a depth of 180 ft (55 m). This reflection corresponds to the impedance contrast bet ween the soft materials and the more rigid Tinker Formation. The low-veloci ty zone of soft materials indicates a dissolution margin of a carbonate uni t (which is part of the Utley limestone) and the presence of loose sands of the Griffins Landing Member. Ray tracing and common source seismograms sho w that the rigid part of the Utley limestone extends horizontally about 12. 5 ft (4 m) west of the receiver well. The continuity logging data showed le aky and normal modes in the region between 140 and 150 ft (43-46 m). The co mputed group velocity contours of leaky and normal modes are consistent wit h waveguide models based on well logs and crosswell seismic data. This indi cates that the low-velocity tan clay (confirming unit) within the Griffins Landing Member is connected between wells.