His results suggested that backscattering of sound from a 

 natural bottom is due more to its roughness than to the 

 sedimentary particles that make it up, and that at normal 

 incidence the bottom is sufficiently rough so that little 

 sound is returned by specular reflection. The increase in 

 scattering for sand-rock bottom seemed to be due to the 

 greater roughness. Again., in I960, Urick 6 utilized two 

 transducers to study side scattering over a uniform bottom. 

 The results indicated that isotropic bottom scattering may 

 be safely assumed in applications utilizing separated trans- 

 mitting and receiving transducers at kilocycle frequencies. 

 As examples of sea scatterers of sound, Urick listed marine 

 life, the air bubbles existing near the sea surface in bad 

 weather, and the roughnesses of the sea floor. The latter 

 are known to be the dominant sound scatterers in shallow 

 water when the bottom is hard and when downward refrac- 

 tion exists. 



Mackenzie made bottom reverberation measurements 

 with 530 and 1030 c/s omnidirectional sound sources in 

 2100-fathomwater near San Diego. An analysis of nonspecular 

 reflections to obtain a scattering constant for the bottom 

 revealed that, for clays, muds, or fine-grained sands, 

 there appears to be no significant frequency dependence 

 over a range of seven octaves. Mackenzie 8 stated that, for 

 frequencies from 200 to 3000 c/s in long-range shallow- 

 water propagation, bottom reverberation is probably the 

 only important type of reverberation. He pointed out that 

 accurate close-range reverberation studies over different 

 bottoms and especially at lower frequencies are needed 

 for grazing angles from 1° to 10°, and he noted that rever- 

 beration strength is more significant than reverberation 

 level. 



In discussing the theory of sound reflection from 

 flat and uniform fluid bottoms, Mackenzie 9 stated that 

 more measurements should be made of the bottom reflection 

 properties, especially for grazing angles of incidence from 

 zero to 30° where the results are more critically dependent 

 on the bottom properties. 



Lieberman measured the reflection coefficient of 

 the ocean bottom for supersonic sound (24 kc/s) at grazing 



