The advent of suitable equipment for the magnetic recording and play-back 

 of seismic data introduced a ready versatility of section display. Full record 

 detail may be carried to the section. Individual traces or records may be 

 shifted with respect to each other in play-back process. Thus corrections can 

 be applied, and traces throughout the record section may be referred to any 

 desired common datum in time or depth. Even arbitrary velocity functions 

 may be applied to reduce time to depth. The datum may be sea level, or it may 

 be a particular time surface, for example, an unconformity. 



Desired mixing, compositing, and filtering may be accomplished at any. 

 stage in the play-back process, perhaps after all corrections have been introduced 

 instead of being restricted to the original recording as is required by non- 

 magnetic systems. 



The resulting record section may be presented by conventional traces as 

 illustrated in Figures 26-12 and 26-13, by the variable density tracks of Figure 

 26-14, by variable area tracks, or by clipped traces. 



Modern record sections are most illuminating and particularly helpful in 

 relating seismic data to the known subsurface geology. They are impressive, 

 perhaps too impressive, and may leave the unwary viewer with a sense of com- 

 placency. He should remember that they are unmigrated representations and 

 therefore in regions of high dip cannot represent the true subsurface. 



The second general form of data presentation is the subsurface map. It 

 may be a dip-strike map made from scattered reflection data, e.g. Salvatori 

 (1945), but, more probably, it will be a contour map based on a reasonably 

 continuous reflecting horizon. Contours may depict equal reflection times or 

 depths and may be based on migrated or unmigrated points. They may purport 

 to represent the top or bottom of a particular geologic stratum, but as pointed 

 out previously, dependence on this relationship should be tempered by judgment. 



Seismic contour maps are constructed sometimes directly from record data 

 but more often subsequent to cross sections from which the mapper should derive 

 great help, particularly in complicated areas. 



An intriguing method of constructing a migrated map has been proposed 

 by Hagedoorn (1954) . Through the use of templates derived from the applicable 

 velocity function, contours drawn from vertically plotted depths are translated 

 to their migrated positions. 



Figure 26-13. This seismic record section is similar to the section of Figure 26-12 except 

 that its vertical scale is in time instead of depth, with the unconformity instead of sea 

 level as the reference plane (Courtesy Geophysical Service, Incorporated). 



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