Acoustic observations in seabed materials

Abstract

It is current practice to investigate surficial seafloor sediments by the acoustic technique of vertical-incidence reflexion profiling whether the investigation is done for the purposes of foundation engineering, of underwater sound propagation or pure sedimentological research. Such a technique gives no information about the variation with depth of such properties of the seabed materials as the acoustic velocity and attenuation. Gross changes in these properties can be expected if sediments are gasified or ice-bonded. Comprehensive reviews of what is known about these phenomena are given. Briefer summaries are included of the significance to the fields of engineering and of underwater acoustics, of the acoustic properties of seabed materials. The long-established seismic techniques for measuring velocities on the basis of travel time measurements can be applied to seabed sediments, but require adaptation, particularly in deeper water. The necessary amendments to field equipment and data-reduction technique are described, with results from areas of Maritime Canada and from Lake Erie. A description is given of equipment built to gather 12-channel acoustic data on a scale suitable for the investigation of surficial sediments and results are presented from the Beaufort Sea (Arctic Canada). Application of synthetic seismogramme modelling to these records shows that the acoustic technique when analysed in this way is capable of discovering properties of seabed sediments which are not revealed by either the vertical incidence profile or the travel-time methods of velocity analysis. In particular, a substantially different velocity profile results from this analysis. Suggestions are made for optimising the future gathering of field data for treatment in this way.