wave, e.g., direct, refracted, or reflected longitudinal; he gets information can- 

 cerning velocities; he recognizes the preferred computation procedure (curved- 

 ray techniques may be required in the reduction to datum) and he is aided in the 

 selection of spread distances suitable to accomplish particular objectives. 



A close relationship exists among the travel-time curves, the ray-path dia- 

 gram, and the seismic record. Data for travel-time curves are obtained from 

 properly labeled records, and these curves in turn help to determine the nature 

 of the ray-path diagrams. The equations required to resolve the problems pre- 

 sented are then selected, record data are imposed, and computation proceeds. 



In the selection of spread distances to be used in prospecting, the travel-time 

 curves can be of much value. Referring to Figure 26-5, we see that if a re- 

 fraction technique is to be used to determine the depth, then seismometers must 

 be deployed beyond the critical distance so that undisturbed arrival times (first 

 breaks) will be from a refracted ray; naturally, refraction equations cannot 

 apply except to refraction data. If a reflection method is employed, then the 

 seismometers would be located with the x ir distance so that refractions would not 

 mask the wanted reflection arrivals. Although the direct wave arrival is present, 

 it may attenuate before the reflections are recorded. 



As the shotpoint-seismometer separation increases, the reflection time in- 

 creases even though the depth to the interface is constant. Time differences on 

 records observed between two traces or channels are termed either stepouts or 

 moveouts. If the stepout is due to spread, it would then be termed spread 

 stepout. In the no-dip case, spread stepout is often called normal moveout. The 

 difference between reflection arrival times which exists solely because of the effect 

 of dip is called the dip stepout time. Irregularities in the immediate sub- 

 surface can also cause time differences (to be discussed later), and these 

 time differences are known as datum stepouts. Generally speaking, in a constant 

 velocity section, the total time difference between any two traces (called the 

 total stepout time) equals the summation of spread stepout, datum stepout, and 



Figure 26-6. Relationship among the travel-time curves, the ray-path diagram, and the 

 associated idealized seismogram for the two layer case with a dipping interface between 

 the lower and higher velocity layers. The idealized reflection depicted at a later time 

 on the record illustrates the return from a deeper dipping reflecting horizon such as 

 shown in Figure 26-7. The seismometers are deployed on a line which parallels the 

 dip of the horizons. 



If the interface is not dipping, symmetry of travel-time curves exists about the 

 arrival-time axis (see fig. 26-5), but in the dipping case only the direct wave travel- 

 time curves are symmetrical to this axis. The least reflection time would be observed 

 if a seismometer were positioned at 2x , and the slopes of the refraction travel-time 

 curves are different on each side of the shotpoint. The inverse slope of the refraction 

 arrival time vs. distance curve would be equal to the velocity of the higher velocity 

 layer only if the spreads were parallel to the strike of the dipping interface; with 

 other orientations the reciprocal slope leads to an apparent velocity. 



569 



