EWING: ACOUSTIC PROPERTIES OF THE SEA FLOOR 



have been looking at bottom effects through the use of expendable 

 sonobuoys and air gun sound sources and have recorded a large number 

 of variable-angle reflection profiles. Since our ships travel around 

 the earth widely, this has given us a chance to look at the bottom of 

 the ocean in many places and to compare the behavior in one place 

 with that in another. 



Although not completely satisfied with the quality of each of 

 our measurements, I do think that our total data bank is starting 

 to give meaningful information about what is going on, particularly 

 at low frequencies, when sound encounters the bottom. 



Figure 1 is an example of an airgun-sonobuoy reflection pro- 

 file, the ordinate representing reflection time and the abscissa 

 representing distance. At these low frequencies (about 20 to 60 Hz), 

 several reflectors appear quite clearly as a distinct reflection 

 hyperbolic curve. The sea floor reflection intercepts the ordinate 

 at about 6.2 seconds and prominent reflections from within the 

 sedimentary section have intercepts of about 7.2 and 8.2 seconds. 

 The intercept at about 9 seconds corresponds to igneous basement. 

 It is quite clear at ranges between about 12 and 16 km (corresponding 

 to grazing angles on the bottom of 30 degrees or so) that a lot of 

 these reflection curves are starting to run together. When they do, 

 we get some interference patterns showing up and the signal levels 

 observed in that part of the profile vary extremely widely over 

 (I would guess) something like 20 dB. 



Even before we get to these moderately small grazing angles, 

 at frequencies in the vicinity of 20 Hz, we are already getting a 

 lot of energy from the sub-bottom interfaces. By the time the grazing 

 angle reaches 45 degrees, in many places we get at least as much low- 

 frequency energy from reflectors at depths of 500 meters or more as 



250 



