dip stepout. In Figure 26-5 the total stepout is due to spread, whereas in Figure 

 26-6 it is a combination of dip and spread effects. 



One of the purposes of seismic calculations is to derive from the total 

 stepout the specific stepouts required — for example, in dip computation, one 

 would be concerned chiefly with the dip stepout; whereas in some procedures 

 designed to find average velocity, the spread stepout becomes of primary im- 

 portance. 



Consider now the reflection travel-time curve for the simplified two layer 

 case with a dipping interface in which the seismometers are spread on lines 

 normal to the strike (fig. 26-6). Ray-path geometry readily shows the position 

 of the subsurface coverage for the spreads chosen. The coincident point is the 

 point on the reflecting horizon where the ray which has traveled down and back 

 on itself has been reflected. If the interface were horizontal and a detector were 

 positioned at the shotpoint, then the coincident point would be located vertically 

 below the shotpoint; likewise if the interface were dipping, the coincident point 

 would be offset or migrated from its zero dip position. Thus in dip work, it is 



datum 



AX 



sin <t> = XAT^ 



AX 



No. I sejsrrjometers^-ghQtppinj No. 1 1 



! /t 



I /// 



^/ / / 



V = average velocity 

 ATjj, = dip stepout time j / 



' $f I / Z ° = depth t0 



¥1 / 



coincident point 



subsurface coverage 



coincident 

 point 



x Q = offset or 



migration 

 distance 



<t> = dip of horizon 



RAY- PATH DIAGRAM OF DIPPING REFLECTING 

 HORIZON (STRAIGHT- RAY ASSUMPTION) 



Figure 26-7. Straight-ray ray-path diagram of a dipping reflecting horizon. If the seismo- 

 meters are deployed on a line normal to the strike of the reflecting horizon and if it is 

 assumed that the reflections return as a_j>lane wave, then the dip of the horizon may be 

 computed from the equation sin <j> = V&T<p/Ax> 



570 



