Shallow Seismic Data Acquisition, Processing, and Interpretation at Playa 3, Pantex Plant, Carson County, Texas

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Shallow seismic refraction and reflection data were collected in 1993 at Pantex Playa 3, a small (0.5-kilometer diameter), nearly circular ephemeral lake near the northern boundary of the Pantex Plant, as part of a hydrogeological study of the Pantex area playa and interplaya environments. These studies will be used to help understand the hydrogeological framework of the Pantex Plant and the paths of groundwater and potential contaminants in the subsurface.

Seismic refraction data collected along two reversed spreads show that near-surface compressional velocities increase from less than 400 meters per second at the surface to 700 to 1200 meters per second a few meters below the surface. Two shallow seismic reflection lines across Playa 3, each 1.8 kilometers long, reveal the presence of four major reflecting horizons beneath the playa basin. Horizon 0, the shallowest, is interpreted to be from the Ogallala caprock and appears to be absent directly beneath Playa 3. Horizon 1 is interpreted as a fine-grained zone within the upper Ogallala Formation that may perch groundwater above the main Ogallala aquifer. Horizon 2, the strongest reflector on the seismic sections, is a lower Ogallala reflector that may be either a stratigraphic unit or a horizon related to past Ogallala water levels. Horizon 3, the deepest major reflector recognized, is interpreted to be the top of Permian or Triassic bedrock.

Each horizon visible on the reflection lines mimics surface topography. Relief increases with depth: the playa floor is 8 meters below the upland, Horizon 0 (caprock) has 16 to 24 meters of relief, Horizon 1 (upper Ogallala fine-grained zone) has 30 meters of relief, Horizon 2 (lower Ogallala reflector) has 35 meters of relief, and Horizon 3 (bedrock) has 75 meters of relief. Increasing relief with age, coupled with the presence of internal bedrock reflectors that dip toward the basin center beneath the margins of Playa 3, indicate that subsidence has been important in the formation of the basin. Subsidence is probably caused by dissolution of underlying Permian evaporites.


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