Coastal Zone Geophysics: A Seismic Refraction Profile In North Norfolk, U.K.

摘要

The north Norfolk coast is an environmentally sensitive area with potential problems related to sea-level rise. Seismic refraction profiles have been used to supplement the borehole information in an investigation of the Holocene evolution and long-term coastal stability of this area. This paper describes the 1500 m long trial profile at Holkham, and covers reclaimed marsh, dunes and a broad beach. Modelling was based on the Generalished Reciprocal Method and despite some poor quality field data a dense shot point coverage allowed the variable velocity structure to be unravelled. The preconceived model of the Chalk basement increasing in depth seawards was found to be incorrect with the overlying Quaternary found in a shallow trough. Velocity variation in the Chalk is attributed to the contrast between weathered and fresh chalk. Low velocities of Quaternary sediments under the marsh (and apparently below the water table) may be due to gas bubbles and/or problems related to the modelling procedure. The seismic interpretation did not allow the muddy facies of the Holocene sediments to be distinguished from the underlying glacial sands. Overall, the geological interpretation of the velocities requires an understanding of the local processes in addition to borehole control.