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Geological Society, London, Engineering Geology Special Publications; 1997; v. 12; p. 211-221;
DOI: 10.1144/GSL.ENG.1997.012.01.19
© 1997 Geological Society of London

Section 3: Site Investigation

The potential of on-shore multichannel high-resolution shallow seismic techniques when applied to coastal site investigation

P. J. Brabham1 & R. J. McDonald2

1 Department of Earth Sciences, University of Wales Cardiff, PO Box 914, Cardiff, CF1 3YE, UK
2 Terradat (UK) Ltd, Engineering & Environmental Geophysics, PO Box 319, Cardiff, South Glamorgan, CF1 3UJ, UK

A combined seismic P-wave refraction and shallow high-resolution multichannel common midpoint (CMP) reflection approach to onshore coastal site investigations in the UK is described, using case studies from two sites on the South Wales coast. The land-based surveys are performed within the inter-tidal zone, using a 24-channel seismic acquisition system, targeting to a depth of interest of 50 m.

Reflection events from rockhead and lithological boundaries within the overlying unconsolidated sediments are clearly imaged using CMP stacked data. The reflection data show a dominant frequency around 550 Hz, which combined with seismic velocities averaging 1600 m/s, produces a quarter-wavelength vertical resolution of 0.7 m. Comparisons of hammer, buffalo gun and detonator sources are made, and their relative suitability for high-resolution shallow reflection surveys within the inter-tidal zone is discussed.

For successful interpretation of shallow reflection data, the necessity of additional rockhead depth control from seismic refraction surveying and/or borehole data is emphasized. Additional depth control is of particular importance if the acoustic contrast between deeply weathered rockhead and overlying sediments is low. Combined refraction and reflection surveys are a relatively inexpensive way of providing information on rockhead topography, rock structure and sediment stratigraphy. Such information is invaluable to the coastal engineer in both the design and location of engineering structures.