ACOUSTIC PROPERTIES OF THE SEA FLOOR 



John Ewing 



Lamont-Doherty Geological Observatory 

 Columbia University 

 Palisades, New York 



Studies of the reflection and refraction of sound by the 

 ocean bottom and sub-bottom have provided the basis for 

 characterizing geographical regions (provinces) in terms 

 of sound velocity versus depth functions. Velocity gradi- 

 ents vary appreciably from province to province in response 

 to variations in sediment type and in mode and rate of 

 deposition. When the gradient is expressed as V = V + KT 

 (where T is one-way travel time) the value of K generally 

 lies between 0.5 and 1 sec" . These values represent 

 average gradients in 1 km or more of sedimentary section. 



Recent analysis of two data sets from the Hatteras abyssal 

 plain has provided an opportunity to examine local varia- 

 tions in the velocity versus depth function and to investi- 

 gate energy distribution among the various reflected and 

 refracted paths. The region can be characterized reason- 

 ably well by two linear velocity versus depth functions: 

 V = 1.5 + 2T for the upper 400-500 meters and V = 1.9 + T 

 for the lower part of the sedimentary section. Standard 

 deviations of sound velocity in the two data sets are be- 

 tween 50 and 100 meters per second. 



For frequencies in the range of 60 Hz and lower, the signal 

 amplitudes associated with rays penetrating to a reflector 

 500 meters deep in the sediments are, in a substantial range 

 of grazing angles, 6 to 10 decibels higher than amplitudes 

 associated with rays reflecting from the sea floor and 

 shallow interfaces. Comparably high signal amplitudes are 

 received at discreet ranges from still deeper levels, down 

 to the top of the igneous basement. Some variations in 

 signal level can be related to multipath interference and 

 geometrical focusing effects, as well as to change in co- 

 efficient of reflectivity associated with incident angle. 



The following is a brief summary of work we have been doing in 

 the marine seismology group at Lamont on ocean bottom acoustics. We 



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