Ocean bottom

A new method is proposed to infer the geotechnical properties of the sea floor from its response to the frequency-modulated pulses emitted by the subbottom profiler called Chirp Sonar. The environment is assumed to be a multilayered medium, composed of homogeneous layers, or an inhomogeneous half-space with depth-dependent properties. The acoustic response of the sediment is computed using the Biot-Stoll theory. The Levenberg-Marquardt method is applied to fit the synthetic response to the experimental response of an homogeneous layer overlying the sea floor. The porosity, the permeability, the mean grain diameter, the mass density, the bulk modulus and the shear modulus within this sediment layer can be estimated. A multilayered medium with depth-dependent properties could be applied to this inversion technique in the future.

The acoustic field in the ocean is difficult to model theoretically, due to the complexity of the environment. This is particularly true if the water depth is range dependent, such as in the coastal region, where a fully three dimensional description is required. Propagation effects result in horizontal refraction, shadow zones, and the existence of regions with strong interference patterns. As a result, all of the existing theoretical models are based on significant simplifying assumptions. One such assumption is to model a region of the ocean as a water column overlying a planar sloping bottom. To test the accuracy of these theories model scale measurements of the acoustic field under highly controlled conditions have been undertaken in this study. Two experiments were performed on models with a sloping bottom. The first model consisted of a fast fluid bottom, and the second model consisted of a thin epoxy layer, to model a sediment, overlying a concrete layer, which modelled the substrate rock. The measurements performed included pulse, CW traverse, and depth profile measurements in both the across slope and down slope directions, in order to demonstrate the three dimensional features of the field. The features of the results are discussed and where possible are compared with existing theories. The results indicated that the three dimensional propagation effects in a fluid bottom wedge are described accurately by a theoretical model which uses an effective depth correction. No three dimensional theory was available for the shear wave supporting bottom case but the fluid bottom theory was found to provide accurate predictions. Down slope propagation over a shear wave supporting bottom was also shown to be accurately predicted using a two dimensional finite element parabolic equation code.

A benthic environments classification system is devised from digital interpretations of multi-spectral IKONOS satellite imagery for 1,360 km2 of the carbonate platform and presented in a comprehensive digitized map. The classification scheme is designed as a 7th order hierarchical structure that integrates 5 Physiographic Realms, 24 Morphodynamic Zones, 11 Geoforms, 39 Landforms, 6 dominant surface sediment types, 9 dominant biological covers and 3 densities of biological covers for the description of benthic environments. Digital analysis of the high-resolution (4 m) IKONOS imagery employed ESRI's ArcMap to manually digitize 412 mapping units at a scale of 1:6,000 differentiated by spectral reflectance, color tones, and textures of seafloor topologies. The context of each morphodynamic zone is characterized by the content and areal distribution (in km2) of geomorphic forms and biological covers. Over 58% of the mapping area is occupied by sediment flats, and seagrasses are colonized in almost 80% of the topologies.