CHAPTER I 

 INTRODUCTION 



Volume Reverberation 



When sound is propagated in the ocean any inhomogeneities in the medium will scatter a portion of the 

 sound incident upon them. This scattered sound is termed volume reverberation. If volume reverberation levels 

 are high, the operation of an active sonar can be adversely affected. Specifically, high reverberation levels can 

 mask an echo reflected from a particular target of interest. 



During World War II, researchers studying sonar echo ranging at the University of California Division of War 

 Research (UCDWR) discovered that volume reverberation levels were usually vertically stratified. Layers of 

 high reverberation, on the order of 1 00 m thick, were found, usually within the upper 1 ,000 m of the water column, 

 extending over large geographic areas of the Pacific Ocean [1 ,2]. Other researchers subsequently found such 

 layers in other oceans [3,4]. These layers came to be known as deep scattering layers (DSL). In addition, it was 

 found that these layers frequently rose to shallower depths around sunset and descended around sunrise. This 

 gave rise to the hypothesis that deep scattering layers were caused by biological organisms, which were known 

 to undergo diurnal vertical migrations [5]. 



The great difference between the acoustic impedances of air and sea water causes an air bubble to be a 

 much more effective scatterer than other objects of comparable size [6]. This fact led Marshall to examine the 

 possibility that small mid-water fish which contained air-filled swimbladders were the cause of DSL [7]. His study 

 strongly implicated such fish as major components of DSL. The primary scattering mechanism was considered 

 to be swimbladders which could resonate in the fundamental, or volume pulsation, mode when insonified at the 

 proper frequency. Subsequent research by Hersey and co-workers displayed the frequency dependence of 

 reverberation levels and provided further qualitative proof that resonant scattering by swimbladder-bearing fish 

 was the major cause of volume reverberation in the ocean [8-11]. The study of volume reverberation in the world 

 ocean has continued (for example, see reference 1 2) and it is now generally accepted that swimbladders of fish 

 are the predominant scattering mechanism in most geographic areas. However, it has not been until very 

 recently that quantitative comparisons between fish distribution data and acoustic volume reverberation data 

 have been made. 

 The fundamental volume reverberation parameter is the back-scattering coefficient of a unit volume of 



ocean, M, which is the ratio of the scattered intensity at a unit distance from the unit volume, l s , to the 



incident intensity, lj, [13], 



M = l s /li . (1-1) 



If it is assumed that the scattered signals add incoherently [13], then M can be defined in terms of the scatterers 

 as n 



M = 4WI°.' (l " 2) 



where n is the number of scatterers in volume V and a, is the ratio of scattered power to incident intensity of the 

 jth scatterer. a is called the acoustic cross section of the scatterer. 



Although M is the fundamental parameter, the quantity which is most often utilized in any discussion of 

 volume reverberation is the scattering strength per unit volume, S v , which is M expressed in decibels. 



Sv = 10 log M . (I-3) 



Sv is often simply called scattering strength [14]. 



In any acoustic measurement of volume reverberation, Sv is obtained basically from the equation 



S v = 10log(l s /I i ). 0-4) 



The determination of I, and l s is relatively straightforward and acoustic measurements of S v are fairly routine. 

 However, if a determination of S v is to be made from the distribution of scatterers, the equation employed is 



n 



S * = 10log hnV£ ai ]- °" 5) 



L i=i 



