of the formation interval adjacent to the tool is inversely proportional to the 

 time of travel corrected for the time consumed in the well bore. The purpose 

 of the acoustic insulator is to eliminate pulse travel through the tool itself. 



The rather complicated surface equipment of a velocity or acoustic logger 

 produces a continuous depth record of formation velocity over the transmitter- 

 receiver separation interval (say 5 feet) and also a continuous integrated time 

 curve representing travel time to the surface. This second curve is comparable 

 to the time-depth plot derived from a conventional well velocity survey. 



PRESENTATION Having assembled all available seismic in- 



OF RESULTS formation, it behooves the interpreter to dis- 



play his results in lucid fashion. Two general 

 forms of presentation are prevalent: the cross section and the subsurface map. 

 The cross section may have as its vertical dimension either time or depth; 

 for depth sections a suitable velocity function must be imposed to convert time 

 to distance. Significant events are transferred from the records to the section 

 where they are represented by appropriate symbols. If the symbols are plotted 

 vertically below the seismometer spread, an unmigrated section such as shown 

 in Figure 26-10A results. Often in areas of steeper dip, a migrated section 

 (idealized in fig. 26-10B) forms a more probable representation. Construction 

 of a migrated section is facilitated by use of wave-front charts (fig. 26-11) or 

 dip plotters. If the symbols appear to form a more or less continuous line as 

 illustrated in Figure 26-10, they may be presumed to represent a continuous 

 reflecting horizon. A break in the continuity of symbol alignment may indicate 

 zones of structural displacement or stratigraphic variation, although it should 

 be borne in mind that a loss of reflections does not always imply a loss of bed- 

 ding. Only the recorded events which appear significant to the interpreter reach 

 the manually-plotted section and much detail is necessarily omitted. 



Figure 26-10. Comparison of a highly idealized unmigrated time section (fig. 26-10,4) and 

 a migrated depth section (fig. 26-105) as constructed from selected seismic events. It 

 has been assumed that the line of shotpoints parallels the general dip in the area. 

 The value of the migrated section is well illustrated by events C and D as recorded at 

 shotpoints 15 and 16. Reflections from both the upthrown and downthrown segments 

 of the C and D horizons appear on the same records, thus when these data are plotted 

 on the unmigrated time section a geologically improbable condition apparently obtains 

 — such is not the case when these same data are properly migrated. Horizon D as 

 depicted under shotpoints 3 and 4 on the unmigrated section appears to be faulted, 

 but on the migrated section the position of these horizon segments is such as might be 

 anticipated with tighter folding. Fragments of horizon C (as recorded from shotpoints 

 3 and 4) when properly migrated aid in the better location of the unconformity. Ill 

 areas of relatively small relief (as indicated by horizons A, B, and E), the unmigrated 

 section is sufficient to indicate regions of interest. 



577 



