Geoacoustic modeling of the sea floor

Geoacoustic models of the sea floor are basic to underwater acoustics and to marine geological and geophysical studies of the earth’s crust, including stratigraphy, sedimentology, geomorphology, structural and gravity studies, geologic history, and many others. A ’’geoacoustic model’’ is defined as a model of the real sea floor with emphasis on measured, extrapolated, and predicted values of those properties important in underwater acoustics and those aspects of geophysics involving sound transmission. In general, a geoacoustic model details the true thicknesses and properties of sediment and rock layers in the sea floor. A complete model includes water-mass data, a detailed bathymetric chart, and profiles of the sea floor (to obtain relief and slopes). At higher sound frequencies, the investigator may be interested in only the first few meters or tens of meters of sediments. At lower frequencies information must be provided on the whole sediment column and on properties of the underlying rocks. Complete geoacoustic models are especially important to the acoustician studying sound interactions with the sea floor in several critical aspects: they guide theoretical studies, help reconcile experiments at sea with theory, and aid in predicting the effects of the sea floor on sound propagation. The information required for a complete geoacoustic model should include the following for each sediment and rock layer. In some cases, the state-of-the-art allows only rough estimates, in others information may be nonexistent. (1) Identification of sediment and rock types at the sea floor and in the underlying layers. (2) True thicknesses and shapes of layers, and locations of significant reflectors (which may vary with sound frequencies). For the following properties, information is required in the surface of the sea floor, in the surface of the acoustic basement, and values of the property as a function of depth in the sea floor. (3) Compressional wave (sound) velocity. (4) Shear wave velocity. (5) Attenuation of compressional waves. (6) Attenuation of shear waves. (7) Density. (8) Additional elastic properties (e.g., dynamic rigidity and Lamé’s constant); given compressional and shear wave velocities and density, these and other elastic properties can be computed. There is an almost infinite variety of geoacoustic models; consequently, the floor of the world’s ocean cannot be defined by any single model or even a small number of models; therefore, it is important that acoustic and geophysical experiments at sea be supported by a particular model, or models, of the area. However, it is possible to use geological and geophysical judgement to extrapolate models over wider areas within geomorphic provinces. To extrapolate models requires water-mass data (such as from Nansen casts and velocimeter lowerings), good bathymetric charts, sediment and rock information from charts, cores, and the Deep Sea Drilling Project, echo-sounder profiles, reflection and refraction records (which show detailed and general layering and the location of the acoustic basement), sound velocities in the layers, and geological and geophysical judgement. Recent studies have provided much new information which, with older data, yield general values and restrictive parameters for many properties of marine sediments and rocks. These general values and parameters, and methods for their derivation, are the main subjects of this paper.