PREFACE 



The absorption of sound in the bottom sediments is the primary cause for a loss of 

 energy when a sound wave interacts with the ocean bottom. This bottom interaction has 

 an important effect on passive and active ASW systems. Knowledge of the ocean bottom 

 can assist in understanding optimum array depths and degradation of signal coherence. 

 Systems that are located near the bottom are strongly affected by bottom interaction. In 

 addition, below 100 Hz the bottom begins to interact with long range sound propagation 

 and thus has a direct effect on noise background (ambient noise) in which the surveillance 

 systems must operate. 



Because this parameter is so important, the Naval Ocean Systems Center (NOSC) 

 (along with its predecessor the Naval Undersea Center) for several years has maintained a 

 coordinated research program on interaction of sound with the sea floor. This work falls 

 into four categories: 



1. Studies of the acoustic and related properties of the sea floor and production 

 of geoacoustic models 



2. Basic studies in sound propagation theory, especially as related to the sea floor, 

 and the development of bottom loss models 



3. Use of geoacoustic models of the sea floor and theoretical, mathematical 

 models of sound propagation and bottom loss to reconcile experiments at sea with 

 theory 



4. Prediction of both geoacoustic models and bottom loss versus grazing angle, 

 or reflection coefficients, for areas not experimentally occupied 



The objectives of the program for the past three years have been to investigate 

 properties of the sea floor of interest in underwater acoustics, including velocity gradients 

 in the sea floor, density, shear-wave velocities and other properties; conduct research on 

 acoustic propagation models especially at low frequencies (from about 2 to 200 Hz); de- 

 velop accurate and efficient methods for coupling geoacoustic models to standard propaga- 

 tion models such as ray theory, normal mode theory, and P. E. (a numerical method using 

 the Parabohc Equation approximation to the wave equation). 



We have published numerous reports that have been distributed to the acoustic 

 community, calculated various predictions for the surveillance community using our geo- 

 acoustic and acoustic models and provided support to others who are developing models. 

 We also have interfaced and supplied support on a continuous basis to surveillance pro- 

 grams including the Indian Ocean, MSS, SURTASS and other efforts. 



The summary report gives a brief, general review of our research work during the 

 three-year period, 1974-1977. This information, coupled with our referenced publications, 

 provides a basis for more detailed study. 



