EWING: ACOUSTIC PROPERTIES OF THE SEA FLOOR 



we get from the sea bottom. That says at least two things to me: 

 That many sub-bottom interfaces have rather high reflectivity and, 

 probably to nobody's surprise, that the attenuation in these moder- 

 ately soft sediments is certainly not very high for frequencies in 

 the 20 Hz range. 



We see this low attenuation demonstrated in a slightly different 

 way as we travel along almost any ocean basin where the igneous rock 

 surface of the earth's crust is covered by a variable thickness of 

 sediment. As you cross such a bottom and make a low- frequency echo 

 sounding record, you can see little difference in the intensity of 

 the reflections from the basement surface whether it is covered by 

 a few tens of meters or a few hundreds of meters of sediment. 



We have recently made several airgun-sonobuoy measurements in 

 the Hatteras abyssal plain in connection with some joint work with 

 NUSC. I was particularly interested in the Hatteras abyssal plain 

 because I remember from some of the early work in bottom loss 

 measurements made at 3.5 kHz that the Hatteras abyssal plain was con- 

 sidered to be about as good a reflecting bottom as we knew. We knew 

 from piston coring and some of the Glomar Challenger work that this 

 abyssal plain had quite a lot of sand and silt in it, so it ought to 

 be a good reflecting bottom. We also knew there was another reflector 

 about 500 meters below bottom, one that we observed very broadly over 

 the North Atlantic Ocean, and we knew that it corresponded to some 

 closely spaced layers of chert (flint) in otherwise soft sediment. 

 I was quite interested to see how the reflectivity of the sea floor 

 in this nice, smooth abyssal plain would compare with the reflector 

 about 500 meters below the sea floor that I knew had some fairly 

 hard rock associated with it. 



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