INVESTIGATIONS OF SEDIMENT PROPERTIES IN SONAR 

 BOTTOM REFLECTIVITY STUDIES 



by 



James J. Gallagher 

 U. S. Navy Underwater Sound Laboratory, New London, Conn, 



and 

 Vito A. Nacci 

 Civil Engineering Department, University of Rhode Island, Kingston, R. I. 



INTRODUCTION 



The objective of the investigation of sediment properties in acoustic bottom loss 

 studies is to develop a capability to predict acoustic energy loss in various types of 

 ocean depositional areas. This capability is being developed primarily by laboratory- 

 experiments on sediment samples and by model studies, utilizing the laboratory- 

 derived sediment properties values. 



This problem has been under study in this country for about ten years. The 

 general approach has been to establish the relationships between the structural and 

 acoustical properties of the sediments. Various mathematical expressions have been 

 derived relating principally density, median grain size, and porosity to sound velocity. 

 However, these expressions appear to be restricted to localized physiographic 

 provinces, and perhaps to geographic sectors or soil types. The problem of the 

 acoustic energy absorbing properties of the sediments is more severe, and the solu- 

 tion is less developed. Many investigators of acoustic energy loss utilize core 

 samples, selected at relatively large distance intervals, as being representative of 

 the bottom. However, variability in acoustic energy loss occurs over rather small 

 distances along the bottom, well within the limits of a physiographic province. 



Rather than seek relationships of wide-spread application, the Acoustics Research 

 Division of the Underwater Sound Laboratory is formulating a program to explain 

 this variability over specified areas of the ocean bottom. We have chosen our test 

 sites as those with little bottom slope and almost no bottom roughness features. 

 Sedimentation and soil mechanics relationships will also be studied. The properties 

 of those sediments that would most likely be found in a flat-bottom depositional area 

 are being intensively examined for energy absorbing qualities, for their effect on a 

 passing acoustic wave, and for the effect of a passing acoustic wave on the properties 

 of the sediment aggregate. In adopting the approach it is felt that the effects of 



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