6. THE CRUSTAL ROCKS 



R. W. Raitt 



1. Introduction 



Knowledge of the depth, velocity and thickness of the rock layers beneath 

 the unconsolidated sediment has been gained mostly from observations of the 

 first arrival times of waves refracted through these layers. Later arrivals some- 

 times supply additional useful information, but on the whole our present ideas 

 regarding oceanic crustal structure depend almost entirely on the interpretation 

 of the travel times of first arrivals. 



The minimum distance at which a bottom-refracted wave is observed to 

 arrive ahead of the bottom reflection is about 6 or 8 km in the typical deep-sea 

 location. At this distance its intensity is strong, even for a small explosive 

 charge of only a few pounds weight. There is usually no difficulty in measuring 

 its time of arrival to a precision of about 0.01 sec. 



Layer 4 

 8.09 km /sec 



Layer 3 

 6.71 km /sec 



Layer 2 

 5 km /sec 



10 



20 30 



Distance (km) 



40 



50 



Fig. 1. Average travel-time-distance plot of an oceanic refraction profile. 



Fig. 1 is a travel-time-distance plot which illustrates the main features of 

 oceanic refraction profiles. It represents the calculated travel times for a model 

 of the average oceanic structure as summarized by Hill (1957), consisting of 

 the following layers : 



Layer 



Thickness, km 



Velocity, km/sec 



The refracted arrivals have three segments representing the three layers 2, 3 

 and 4 respectively. The distance intervals within which a given layer is observed 



[MS received July, 1960] 85 



