East West 



Shotpoint Shotpoint 



Figure 17. Cross Section Determined by Nearby Drill-Hole Data Explains 

 Apparent Sediment Velocities. The wet sediment has a true 

 velocity of 5,000 feet per second. However, with a dry sand 

 overburden of variable thickness, apparent velocities are 

 indicated on the seismic record. A high velocity appears 

 when the shot is fired at the east end and the seismic wave 

 travels upward through a progressively thinner dry sand as 

 it propagates westward. However, a shot at the west end of 

 the profile, which places energy in the wet sediment, rec- 

 ords successively longer times on geophones toward the east 

 due to the thickening of the dry overburden. The two appar- 

 ent velocities and the respective direction of propagation 

 for the seismic energy are indicated on the cross section. 



27 



