INTRODUCTION 



The objectives of the geology and geophysics part of the task are to: 



1. Determine, study and predict those characteristics (or properties) of the sea 

 floor affecting sound propagation and prediction of sonar and surveillance system 

 performance 



2. Place these properties in a form usable by underwater acousticians and 

 engineers 



3. Produce geoacoustic models of the sea floor for specific areas as required for 

 experimental, predictive, or theoretical work. 



The work thus involves marine geological and geophysical studies in direct support 

 of underwater acoustics. 



When sound interacts with the sea floor, the acoustician concerned with sound 

 propagation, reflection coefficients, or bottom losses must have a full range of information 

 about the sea floor. This information includes the basic physics of sound propagation in 

 marine sediments and rocks, measured acoustic and related properties, properties determined 

 by empirical relationships and outright estimates or extrapolations based on geological and 

 geophysical probabilities. 



At higher sound frequencies, the acoustician may be interested in only the first meters 

 or tens of meters of sediments. At lower frequencies (and higher grazing angles) information 

 must be provided on the whole sediment column and on properties of the underlying rock. 

 This information should be provided in the form of geoacoustic models of the sea floor. 



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 geo- 

 acoustic model details the true thicknesses and properties of the sediment and rock layers in 

 the sea floor. 



Geoacoustic models are 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 fol- 

 lowing for each layer. In some cases, the state of the art allows only rough estimates, in 

 others information may be non-existent. 



• Properties of the overlying water mass from Nansen casts and velocimeter 

 lowerings 



• Sediment information (from cores, drilling, or geologic extrapolation): sedi- 

 ment types, grain-size distributions, densities, porosities, compressional and shear wave 

 attenuations and velocities, and other elastic properties. Gradients of these properties 

 with depth, for example, velocity gradients and interval velocities from sonobuoy 

 measurements 



• Thicknesses of sediment layers (in time) determined at various frequencies 

 by continuous reflection profiling 



