• Locations, thicknesses, and properties of reflectors within the sediment body as 

 seen at various frequencies 



• Properties of rock layers; those at or near the sea floor are of special impor- 

 tance to the underwater acoustician 



• Details of bottom topography, roughness, relief, and slope as seen by under- 

 water cameras, sea-surface echo sounders and deep-towed equipment 



It has been shown by acousticians that the above types of information are essential to 

 an understanding of sound interactions with the sea floor. Among the above properties and 

 information, the following is the basic, minimum information on properties of the sediments 

 and rocks required for most current work in sound propagation. 



1 . Thicknesses of layers 



2. Compressional wave (sound) velocity profile and gradient through the layers 



3. Sound attenuation in each layer 



4. Density in each layer 



Newer and more sophisticated mathematical models involving sound interaction with 

 the sea floor, especially at lower frequencies, require (in addition to the above): 



5. The profile and gradient of sound attenuation through the layers 



6. The density profile and gradient through the layers 



7. Shear wave velocity and attenuation profiles and gradients through the layers 



8. Additional elastic properties (e.g., dynamic rigidity and Lame's constant); 

 given compressional and shear wave velocities and density, these and other elastic 

 properties can be computed. 



Examples of newer mathematical models involving sound interactions with the sea 

 floor are given by Bucker (Part II of this report) and Bucker and Morris (1975). Additional 

 examples are those models used at the Applied Research Laboratories of the University of 

 Texas to study the effects of various sediment properties on bottom losses (Hawker and 

 Foreman, 1976; Hawker et al, 1976, 1977). 



Where sound penetrates the whole sediment layer (and sedimentary rock layers if 

 they are present) and reflects from and refracts in the surface of the acoustic basement, it is 

 necessary to know the properties of the basement surface (i.e., compressional and shear 

 wave velocities and attenuations, and density). An example of this is in the Northcentral 

 Pacific where 50 to 100 meters of pelagic clay overlies basalt. 



A continuing project in the geology-geophysics group is improvement of geo- 

 acoustic modeling and acquisition and refinement of properties in coordination with acous- 

 ticians to supply required information and to anticipate future needs. Except where specific 

 geoacoustic models are required for experimental work, our emphasis is on the general case 

 so that reasonable predictions can be made in the absence of specific measurements. 



At the start of the three-year project in 1974, considerable work had been done by 

 our laboratory (then NUC) in the acoustic and related properties of marine sediments. These 

 studies were based on in situ measurements by divers and from submersibles and from 

 measurements in cored sediments in the laboratory. Much of this work, with appropriate 



