74 



KWINti AND NAFK 



[('HAP. r> 



in an area of thick sedimentary cover. (For a general description of time- 

 distance graphs and of their relationship to various structures, the reader is 

 referred to Chaj)ter 1.) This is ty|3ical of the seismic data from which sedi- 

 mentary velocities and thicknesses are computed. The Ri, Rn and Rm curves 

 are determined bv reflected waves from the first, second and fourth sub- 

 surface interfaces respectively. The lines G3 and G4 are determined by refracted 

 arrivals associated with the tops of Layers 4 and 5. Note that no Gi line (corres- 

 ponding to refracted arrivals along the sea floor) is observed. This is tjrpical 

 of practically all, if not all, deep-ocean areas. The significance of the absence of 



15 



Velocity (km/sec) 



3 4 ! 



~i 1 r 



10 5 5 10 



DIRECT WATER WAVE TRAVEL TIME IN SECONDS 



15 



Loyer 5 



Fig. 1. Tiine-distance graph, sLructui'e section and I'ay diagram for end-to-end profiles 

 Sl-98 and 99. Velocity versus depth relationship shown at left of section. Raj' paths at 

 left of receiving position are for waves refracted at interfaces. At the right are ray 

 paths for reflected waves and waves refracted by the velocity gradient. 



such arrivals is that, in most places, the seismic wave velocity in the upper 

 part of the deep-ocean sediments must be equal to or less than that in the water 

 at the sea floor. The fact that reflected arrivals are received from this interface 

 indicates that the acoustic impedance, pC (density x velocity), is different 

 above and below the interface. The water and sediment mixture must be more 



