per Wilson's tables, in predicting in-situ sound velocity in the pore water. The 

 double layer water phenomenon may be more influential in acoustic energy absorp- 

 tion than we have imagined. 



MATHEMATICAL MODEL DISCUSSION 



The data obtained from the laboratory and in-situ measurements are being sub- 

 mitted to the multi-layered mathematical model for determination of reflection 

 coefficients. These results will then be compared with the field acoustic measure- 

 ments over the same bottom. 



The primary data inputs to the model are density, velocity, and layer thickness, 

 where the layer thickness refers to that of the acoustic impedance layer. It should 

 be noted here that the lithologic strata in the sediment column do not necessarily 

 correspond to the acoustic impedance strata. In order to determine the impedance 

 values we must know the values of sound velocity. This has not been a readily 

 measurable quantity and, for the most part, has been inferred from the engineering 

 properties of the sediment. The value of the absorption coefficient is required and 

 has been estimated using Shumway's data (1960) as a guide. Use of the first-power 

 frequency dependence, as recommended by B. F, Cole (1964), has compared favor- 

 ably with the field acoustic data. By utilizing these data we then compute the reflec- 

 tion coefficients. 



In the Bermuda area study the sound velocity values were obtained from the 

 table of estimated values and were used to compute the impedances. The model 

 derived amplitude reflection coefficients were then compared with those from the 

 corresponding field acoustic test. At the large incident angles, where the depth of 

 acoustic penetration is approximately comparable to the core lengths, the curve fit 

 was fairly close, Menotti, etal. , (1965). However, it is felt that there is insufficient 

 geological data to effectively describe the insonified area of the bottom. 



SUMMARY 



The motivating factor in undertaking this investigation is to obtain significant 

 behavioral reltionships between geological and acoustical properties of the soil mass 

 which will lead to development of a predictive capability of acoustic energy loss in 

 bottom bounce sonar operations, based upon ocean bottom environmental data. To 

 this end our approach is to thoroughly examine small, smooth areas of the bottom, 

 thereby eliminating extensive horizontal variations in lithology and reverberation 

 effects caused by a rough surface. The data gathered will be used as inputs to the 

 multi-layered mathematical model which assumes a smooth bottom interface. 



It appears that the state of the art of soil science in the marine sediments pre- 

 vents proclamation of precision analysis and, thus, prediction of acoustic energy 



399 



