EWING: ACOUSTIC PROPERTIES OF THE SEA FLOOR 



Sure enough, in all of the profiles there, by the time we were 

 out to grazing angles of 30 degrees, we consistently had signal 

 levels coming from this subsurface reflector anywhere from 6 to 10 dB 

 higher than signal levels from the sea bottom. In some places, 

 usually at grazing angles of 30 to 45 degrees, it was common for the 

 largest signal received in any part of the signal train to be coming 

 from even deeper than the 500-meter level, sometimes coming from the 

 top of the igneous rock itself, 1,000 meters or more below bottom. 



One thing more. Notice in Figure 1 that at these farther ranges 

 some signals are arriving appreciably ahead of the reflected signals. 

 These are head waves coming from some of the deeper, high-velocity 

 layers. Although they are interesting and important to us in geo- 

 physics, they do not carry much energy. They may appear to be rather 

 energetic in the figure, but that is because this particular buoy is 

 an SSQ41 buoy with AGC. 



To summarize this part of my talk — there are large areas of 

 the sea floor where, at frequencies below 100 Hz, appreciably more 

 energy is returned to the surface by reflection from interfaces well 

 below the bottom (hundreds of meters) than is returned from the sea 

 floor itself. 



We also get velocity information from the airgun-sonobuoy pro- 

 files. The technique that we have been using is rather standard, 



developed for geophysicists by Dix many years ago. It is known as the 



2 2 

 X - T method and is a purely geometrical treatment of the problem 



that depends on the fact that the shot point and receiving point 



separate during the experiment. 



These measurements are easy to make. From them we can calculate 

 interval velocities for each layer that is bounded by distinct 



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