A vertical line from the bottom-water velocity (1517 

 m/sec, fig. 4) intersects the probable sound-velocity 

 gradients between 20 and 45 meters, thus predicting the 

 thickness of the layer in which the velocity of sound is less 

 in the sediment than in the water just above the bottom. 

 These thicknesses are in accordance with those noted in 

 the Atlantic. 46 ' 47 



The sediment surface at the Mohole Site is red clay; 

 at the first cored depth in the hole (EM 8-1) at 29 meters, 

 it is distinctly different: a clayey silt. The thickness of 

 the red clay is indeterminate; gravity cores in the area 

 were all red clay between 1 and 2 meters in length. 

 Rittenberg et al . note that red clay cores to the north and 

 south rarely contain more than 1 meter of red clay, and 

 that, allowing for compaction in the gravity cores, the red 

 clay might be no more than 2.5 meters thick. Krause 

 notes that on the echo-sounder record there is a strong 

 reflector (or "double bottom") at a depth of 5 to 7 meters in 

 the area. 27 It is a distinct possibility that the "double 

 bottom" marks the lower limit of the red clay but other 

 possibilities, such as ash layers, cannot be excluded. 



SUMMARY 



For either theoretical or experimental studies of 

 sound in relation to the sea floor, it is necessary to choose 

 a model, or models, of the upper few tens of feet in the 

 sediment and rocks of the sea floor. In the past, a wide 

 variety of these models have been postulated; some are 

 valid and some are not worthy of consideration. 



In the first part of this report the information neces- 

 sary to form a valid geoacoustic model of the sea floor 

 was listed. These items will not be repeated here, but, in 

 general, the following are essential: (1) a detailed topographic 

 chart based on electronically determined ship's positions, 

 (2) sub-bottom layering within the sediment body in sufficient 



34 



