Combined Seismic Reflection And Refraction Methods In Coastal Engineering Site Investigation

A combined seismic reflection and refraction approach to coastal site investigation is<br>described using case studies at two sites in South Wales, UK. The land-based surveys, using the<br>BISON 9024 seismograph, were carried out in the intertidal zone with a target depth of 4m to<br>40m below surface.<br>The reflection data show a dominant frequency of 550Hz which, coupled with seismic<br>velocities averaging 16OOm/s, produces a quater-wavelength vertical resolution of 0.7m. Results<br>have imaged local rockhead and lithological boundaries in the overlying unconsolidated<br>sediments. Comparisons of hammer, buffalo gun and detonator sources are shown and their<br>relative suitability for use in the intertidal zone is discussed.<br>In interpreting shallow reflection data the requirement of rockhead depth control from<br>seismic refraction surveying and/or boreholes is emphasised. This is of particular importance if<br>the bedrock is deeply weathered and has a lower acoustic impedance contrast with overlying<br>sediments than other reflecting interfaces. Combined refraction and reflection surveys can<br>provide detailed information on rockhead depth and overlying sediment geometries. This<br>information can be valuable to the coastal engineer in both the design and location of man-made<br>structures.