EWING: ACOUSTIC PROPERTIES OF THE SEA FLOOR 



Below the 500-meter interface is reasonably firm sediment down 

 to about 800 meters where calcium carbonate chalk turns to limestone, 

 really hard limestone. This transition also produces a very good 

 reflection and a very sharp drilling break. At 1,200 meters we hit 

 basalt under the limestone. 



We now have several such holes from the JOIDES program that are 

 well enough cored and geophysically examined so that we are starting 

 to know what causes a lot of our reflectors. I think you can under- 

 stand that it is not only interesting to us in a geological sense to 

 identify the reflectors, but that the identification also permits us 

 to use geological reasoning to interpolate between data points and 

 gradually to build up a more complete geoacoustic model. 



Figure 8, a section based on seismic data and drilling in the 

 Atlantic, is the southern part of the Hatteras abyssal plain. We 

 have identified some friendly Atlantic reflectors here. We have an 

 interface in the sediments, fairly shallow in some places, deeper in 

 others, labeled "A" which we now know is a series of chert beds, 

 nearly the same age as those in the Pacific. A thick layer of clay 

 is underneath, then again nice hard limestones (3) near the base of 

 the section, and then the basalt (B) , 



REFERENCES 

 Dix, C. H., Geophysics, 20:68-86 (1955) 



Bryan, G. M. , "Sonobuoy Measurements in Thin Layers," in Physics of 

 Sound in Marine Sediments , L. Hampton, ed. , Plenum Press (1974) 



265 



