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Environmental Geosciences, V.
Two-dimensional seismic refraction tomography of a buried bedrock valley at Hallsands beach, Devon, United Kingdom
1Department of Geography–Geology, Campus Box 4400, Illinois State University, Normal, Illinois 61790-4400; present address: 25519 Owl Landing Lane, Katy, Texas 77494; [email protected]
2Department of Geography–Geology, Campus Box 4400, Illinois State University, Normal, Illinois 61790-4400; [email protected]
3Department of Geography–Geology, Campus Box 4400, Illinois State University, Normal, Illinois 61790-4400; [email protected]
4Department of Geology, Appalachian State University, 033 Rankin Science West, 572 Rivers Street, Boone, North Carolina 28608; [email protected]
5Department of Environment, York University, Wentworth Way, University of York, Heslington, York YO10 5NG, United Kingdom; [email protected]
Two-dimensional seismic refraction tomography was used to map the bedrock topography beneath Hallsands beach in southwest Devon, United Kingdom. Seismic refraction data were acquired from 11 spreads, 4 parallel to the beach and 7 normal to the beach, with either 12 or 24 geophones at 5-m (16-ft) spacing. Eight sediment cores were used to calibrate the velocity model. The bedrock consists of metasedimentary rocks that have a seismic velocity of 2100–2500 m/s (6900–8200 ft/s) and is overlain by variable amounts of gravel, peat, and muddy peat. Wood peat and peaty mud are differentiated within the peat as 700-m/s (2300-ft/s) velocity for wood peat and 1200-m/s (4000-ft/s) velocity for peaty mud. These refraction data were collected and processed in two dimensions, then imported into Petrel, a three-dimensional (3-D) geological modeling software package. The 3-D geologic model was built using the velocity attribute of the seismic refraction data. These selected data points were used to create 3-D horizons, surfaces, and contacts constraining the target bedrock surface from the overlying unconsolidated deposits.
The bedrock surface beneath Hallsands beach is marked by two paleochannels. One paleochannel occurs in the north end of the beach beneath the axis of the modern valley. A second paleochannel occurs in the southern section of Hallsands beach centered along the axis of a tributary valley. Bedrock occurs at a depth of approximately −10 m (−33 ft) in the southern and northern sections of the main valley. Bedrock occurs at a depth of approximately −2 m (−6 ft) along the valley wall at the southern end of the beach east of the parking lot. Shore-perpendicular refraction lines differentiate layers within the peat, whereas shore-parallel lines delineate wood-peat, peaty-mud, and bedrock topography.
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