Gallagher 



SUMMARY AND CONCLUSIONS 



The purpose of this single experiment was to test the mu I ti layered, 

 absorbing, mathematical model for sensitivity to changes in sound velocity. Two 

 closely spaced cores from each of two dissimilar depositional environments were 

 investigated. The slow depositional environment is represented by the Hatteras 

 Abyssal Plain cores, and the rapid depositional area, by the Tongue of the Ocean 

 cores. The sound velocity data used were values determined from direct meas- 

 urements on the core samples and computed values obtained with Sutton's 

 regression equation for the same cores. 



Based on this investigation. It is concluded that for given density values 

 higher sound velocity values will produce greater values of Impedance, but they 

 do not necessarily produce lower bottom losses over all incident angles. In addi- 

 tion, for given density values, the magnitude of the differences In reflection 

 coefficients derived from two sources over various Incident angles does not 

 necessarily reflect the magnitude of the differences between the sound velocity 

 values of these sources. The impedance values of the layers, the thicknesses of 

 the layers, and the angles and speeds at which the sound rays are traveling 

 through the various layers In a nonuniform bottom apparently combine to set 

 up constructive and destructive interferences, thereby regulating the amount of 

 acoustic energy returning to the Interface. 



This model Is currently being employed as a research tool. In Its present 

 capacity, bottom loss differences greater than 3 db between observed and pre- 

 dicted values will continue to be considered significant. Differences in bottom 

 losses of 3 db or greater over short lateral distances ore evidenced by single 

 source data obtained from two closely spaced core samples. If indeed this close 

 range variability is widespread within a physiographic province, and there is 

 every reason to believe that It Is for some provinces, then the accurate predic- 

 tion of changes In bottom loss of several db over extended distances, based on a 

 few sample points, will be difficult. Results of acoustic measurements made at 

 these core locations substantiate that a complex nonuniform acoustic impedance 

 layering structure exists over small lateral distances. 



The effectiveness of this model in accurately predicting In situ acoustic 

 bottom losses over core sample lengths appears limited by the sensitivity to 

 changes In sound velocity. The effects of small changes on layer thickness and 

 sound velocity will continue to be sought. The problem has already been mani- 

 fested by the fact that in some cores the sensitivity of the model to changes In 

 sound velocity produced more impedance layers within a core length than the 

 model program was capable of accommodating. Therefore, laboratory experi- 

 mental model studies are being planned to simulate the mathematical model to 

 compare measured and computed reflection coefficients for given data. Attempts 

 will be made to alter the number of original layers, while the original mass 



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