Interwell seismic imaging at the Savannah River Site, South Carolina

Crosswell and continuity logging seismic measurements were made beneath a large tank (27 m diameter) used for processing radioactive waste at the Department 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 materials) and image the connectivity of a clay unit between wells. The low-velocity zone depicted on the crosswell seismic tomogram integrated with data from cores and well logs revealed soft materials in the region between 150 and 180 ft (46-55 m). The bottom boundary of this low-velocity zone correlates with a reflection observed in the crosswell seismic image at a depth of 180 ft (55 m). This reflection corresponds to the impedance contrast between the soft materials and the more rigid Tinker Formation. The low-velocity zone of soft materials indicates a dissolution margin of a carbonate unit (which is part of the Utley limestone) and the presence of loose sands of the Griffins Landing Member. Ray tracing and common source seismograms show 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 leaky and normal modes in the region between 140 and 150 ft (43-46 m). The computed group velocity contours of leaky and normal modes are consistent with waveguide models based on well logs and crosswell seismic data. This indicates that the low-velocity tan clay (confirming unit) within the Griffins Landing Member is connected between wells.