576 SEISMIC METHODS [Chap. 9 



promise between accuracy and time available to obtain the correction data 

 in the field. The accuracy need be no greater than that for depth deter- 

 mination. The time required to obtain weathered-layer data in the field 

 depends on the lateral consistencj'' of the layer. Four possibilities exist: 

 (1) conditions may be the same in the entire area; (2) conditions may be 

 the same at the shot and receiving points but vary throughout the area; 

 (3) conditions may differ on shot and receiving points but be the same at 

 the individual receiving locations; (4) conditions may be different at the 

 shot point and differ at the receiving locations. 



The first condition prevails in some limited areas so that the correction 

 may be determined once and for all by vertical-time or refraction methods. 

 Assumption of the second condition furnishes sufficient accuracy in many 

 cases. The weathered-layer delay is then most conveniently determined 

 by measuring the vertical-time interval at the shot point with a shot-point 

 detector and correcting to shot level. In the third case (different condi- 

 tions at shot point and receiving points) the vertical time is measured at 

 the shot point, and a refraction profile is shot at the receptor spread. 

 Finally, when conditions are different at the individual receiving stations, 

 the vertical time is again determined at the shot point and the time delay 

 for each receiver is established by the method of differences (ABC system). 

 This is the procedure in most prevalent use. Further details are given in 

 the calculation records (Table 58). The vertical-time phone is usually 

 connected by a separate line to the truck and recorded on the fifth or elev- 

 enth trace. A correction to regional datum, from which the average veloc- 

 ity is reckoned, is generally added to the weatheiing correction. 



6. Calculation, interpretation. Reflections are not very difl&cult to spot 

 even in a fair seismogram. They appear with a marked change in ampli- 

 tude, consist of one to three or four waves (generally with declining ampli- 

 tude), and reveal themselves primarily by their almost simultaneous ap- 

 pearance on all traces (except when strong dips are present) (see Fig. 9-91). 

 A reflection may be recognized by comparing its time-distance gradient 

 with that of the first impulses. A graphical analysis of records taken in 

 an entirely new area by travel-time curves will identify reflections with 

 certainty, besides offering the advantage of a determination of their average 

 velocity. After reflections have been identified, they are timed, using 

 the trough of the first wave. The mean time, referred to the center of the 

 spread, is entered in the calculation form. In dip shooting, times are 

 read for the closest and the farthest detector. The recorded times are 

 then corrected for weathering to datum and for spread, as described below. 



Some reflections may not show very distinct first troughs. In case of 

 doubt, two and sometimes three troughs or "phases" are timed (see Fig. 

 9-92). Depth errors may readily be committed unless the correct phases 



