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.

New laboratory measurements of sediment properties in cores from the Bering Sea, North Sea, Mediterranean Sea, equatorial Pacific, and other areas, have been combined with older measurements and the results, with statistical analyses, are presented (for various sediment types in three general environments) in tables, diagrams, and regression equations. The measured properties are sound velocity, density, porosity, grain density, and grain size; computed properties are velocity ratios (sediment velocity/water velocity) and impedance. Mineral-grain microstructures of sediments are critical in determining density, porosity, and sound velocity; compressibility of pore water is the critical factor in determining sound velocity. New regression equations are provided for important empirical relationships between properties. Corrections of laboratory values to sea-floor values are discussed. It is concluded that sound velocity and density are about the same for a given sediment type in the same environment in any ocean if porosity is about the same. Given the mean size of mineral grains, or average porosity, of a sediment, the average sound velocity can be predicted within 1% or 2% in most environments. Comparisons with recent in situ measurements validate the laboratory measurements.

In‐situ measurements of compressional (sound) velocity and attenuation were made in the sea floor off San Diego in water depths between 4 and 1100 m; frequencies were between 3.5 and 100 khz. Sediment types ranged from coarse sand to clayey silt. These measurements, and others from the literature, allowed analyses of the relationships between attenuation and frequency and other physical properties. This permitted the study of appropriate viscoelastic models which can be applied to saturated sediments. Some conclusions are: (1) attenuation in db/unit length is approximately dependent on the first power of frequency, (2) velocity dispersion is negligible, or absent, in water‐saturated sediments, (3) intergrain friction appears to be, by far, the dominant cause of wave‐energy damping in marine sediments; viscous losses due to relative movement of pore water and mineral structure are probably negligible, (4) a particular viscoelastic model (and concomitant equations) is recommended; the model appears to apply to both water‐saturated rocks and sediments, and (5) a method is derived which allows prediction of compressional‐wave attenuation, given sediment‐mean‐grain size or porosity.

In studies in underwater acoustics, geophysics, and geology, the relations between sound velocity and density allow assignment of approximate values of density to sediment and rock layers of the earth's crust and mantle, given a seismic measurement of velocity. In the past, single curves of velocity versus density represented all sediment and rock types. A large amount of recent data from the Deep Sea Drilling Project (DSDP), and reflection and refraction measurements of sound velocity, allow construction of separate velocity-density curves for the principal marine sediment and rock types. The paper uses carefully selected data from laboratory and in situ measurements to present empirical sound velocity-density relations (in the form of regression curves and equations) in terrigenous silt clays, turbidites, and shale, in calcareous materials (sediments, chalk, and limestone), and in siliceous materials (sediments, porcelanite, and theft); a published curve for DSDP basalts is included. Speculative curves are presented for composite sections of basalt and sediments. These velocity-density relations, with seismic measurements of velocity, should be useful in assigning approximate densities to sea-floor sediment and rock layers for studies in marine geophysics, and in forming geoacoustic models of the sea floor for underwater acoustic studies.

The ratio of compressional wave velocity Vp to shear wave velocity Vs, and Poisson’s ratio in marine sediments and rocks are important in modeling the sea floor for underwater acoustics, geophysics, and foundation engineering. Vp and Vs versus depth information was linked at common depths in terrigenous sediments (to 1000 m) and in sands (to 20 m) to yield data on Vp vs Vs, and Vp/Vs and Poisson’s ratios versus depth. Soft, terrigenous sediments usually grade with depth into mudstones and shales; Vp/Vs ratios vary from about 13 or more at the sea floor to about 2.6 at 1000 m. Poisson’s ratios vary from above 0.49 at the sea floor to about 0.41 at 1000 m. In sands, Vp, Vs, and Vp/Vs have very high gradients in the first few meters; below about 5 m, Vp/Vs ratios decrease from about 9 to about 6 at 20 m; Poisson’s ratios vary from above 0.49 at the surface to above 0.48 at 20 m. The mean value of Vp/Vs in 30 laboratory samples of chalk and limestone is 1.90 (standard error: 0.03); mean Poisson’s ratio is 0.31. Literature data on basalts from the sea floor are reviewed. Equations relating Vp to Vs are given for terrigenous sediments, sands, and basalts.