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EXPLORATION GEOPHYSICS 



Some computing methods treat the low velocity correction or the delay 

 by obtaining an average of the differences of the symmetrical traces, using 

 the above method and thus finding an average correction for the instrument 

 spread. Then an average is obtained for the uncorrected step-outs and the 

 average correction applied to this figure. 



Some argument has been presented suggesting that these methods of 

 averaging by least squares are too mechanical in that they allow equal 

 consideration to all traces, while field experience indicates that often some 

 seismogram traces give anomalous results because of peculiar soil condi- 

 tions, local noise, high background, etc., and therefore should not be equally 

 considered. It is sometimes argued that by a glance at a reflection seismo- 

 gram, giving consideration to the recording quality of each trace, an exper- 

 ienced interpreter can make a mental estimate of the effective average 

 step-out time which may be more real than the laboriously calculated value. 



Velocity Shooting 



All methods of computation of reflection data which present the results 

 in the form of depth or dip require the knowledge of the seismic wave 

 velocity through the underground strata. The sources of this velocity infor- 

 mation are well velocity surveys, reflection computations and refraction work. 

 These sources are named in order of importance and reliability. 



Reflection Method 



It was observed in the treatment of reflected ray paths through a 

 section consisting of two layers (p. 666) that velocity could be determined 

 directly from reflection times recorded at the end seismometers of a 

 spread. However, this method requires a large spread length. A step 



in refinement is to utilize reflection 



--*- H times recorded on each trace, to 



plot the times against distance be- 

 tween shot-point and seismometer, 

 and to obtain an average value of 

 velocity from the slope of the best 

 fitting curve through the data. 



Consider Figure 445, a simpli- 

 fied section showing reflection 

 ray paths from shot-points A and 

 B to seismometers 1 and 10, the 

 spread lying between these two 

 seismometers. Separate records are 

 of course obtained for each shot- 

 point. It is assumed that the re- 

 flecting bed and the base of the 

 low velocity layer are horizontal, 



-Determining average velocity by reflec- ^^^ ^^^^ ^^^ vclocity V of the 



Fig. 445.- 



