VARIABILITY IN DERIVED SEDIMENT SOUND VELOCITY AS A FUNCTION 

 OF CORE ANALYSIS AND ITS EFFECT IN DETERMINING THEORETICAL 



BOTTOM REFLECTIVITY 



James J. Gallagher 

 U. S. Navy Underwater Sound Laborafory 

 Fort Trumbull, New London, Connecticut 



INTRODUCTION 



Acoustic field tests have been conducted over limited portions of flat-bottomed 

 ocean areas. Core samples were collected from these insonified bottom areas, 

 where each area was usually about two square miles. A multilayered, absorbing, 

 mathematical model is being used to transform the geological core data into an 

 acoustic description of the unconsolidated ocean bottom .^"^ 



This description is supposed to provide an understanding of the influences 

 that a nonuniform bottom exerts on an acoustic signal as it travels through that 

 bottom. The results of the model investigations are being compared with corre- 

 sponding 3.7 kHz field data to determine whether the mechanisms affecting 

 acoustic behavior are, in truth, being explained by the model . Relationships 

 between geologic and acoustic properties of marine sediments have been studied 

 by other investigations. Inasmuch as the results of these studies will not be 



reviewed here, it should be noted that the products of these investigations have 

 direct application to the mathematical model studies. 



The salient features of the multilayered model as currently programmed are 

 (1) it treats up to 10 liquid layers overlying a solid layer; (2) it considers acous- 

 tic absorption; and (3) it requires a smooth, nonsloping surface. 



This model is being employed as a research tool. In its present capacity 

 disagreement between observed and predicted reflection coefficients of about 

 0.05, which may correspond to a change in bottom loss of several db, is con- 

 sidered significant. The sensitivity of this model to changes in sound velocity 

 and layering thickness is of concern and is being explored at the present time. 

 This report discusses the results of a preliminary investigation that was designed 



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