SEISMIC METHODS 729 



medium included between the low velocity layer and the reflecting bed is 

 substantially constant. 



The time of travel for the first ray corrected to the base of the 

 low velocity layer is given on page 667; that is 



1 0-1 — tl— y 



where Ta-i is the time recorded at seismometer number 1 from shot-point 

 A and tx is a correction time factor (equal approximately to the vertical 

 travel-time through the low velocity layer below seismometer 1 less the 

 time of travel from the shot to the base of the low velocity zone) . Evidently 

 this equation may also be written in the form : 



V^= ,^ ^ , ., (84) 



If, therefore, one were to plot t the square of the quantity reflection 

 time Ta-k less the correction time factor t^ versus the square of the distance 

 Xjc from shot-point to seismometer, the resulting points should fall on a 

 straight line having a slope 1/V^ and an intercept 4h-/V^. (Here, as usual, 

 k applies to the kth seismometer.) 



This procedure is followed : the distance x is measured from that shot- 

 point for which the record was obtained, the data from both shot-points 

 being plotted on the same graph. The effect of low velocity travel-times, 

 which would be prominent if only one shot-point were used, is largely 

 cancelled when the double arrangement of shot-points is used. Similarly, 

 the effect of the dip of the stratum and of the base of the low velocity 

 layer in the direction of the spread is largely cancelled. Nevertheless, for 

 accuracy of final data it is important that, so far as is feasible, an area 

 be chosen for velocity shooting in which these strata, particularly the low 

 velocity stratum, are substantially horizontal. 



To obtain useful accuracy it is often necessary to employ such extremely 

 long spreads that they cannot be reached by the usual cables or covered 

 adequately by the usual number of seismometers. In this event, spreads 

 are shot in tandem, i.e., the length to be covered is traversed in steps, 

 wherein successive spreads are begun where previous ones stopped, and 

 one trace is left in common in order that accurate tie-in between records 

 be available. For such long spreads it is necessary first to determine 

 whether some of the reflecting horizons persist over the full length of 

 the spread. 



Refraction Method 



In the refraction method, refracted waves through the sediments are 

 recorded at various distances from the shot-point. Care must be taken in 

 locating the refraction shooting so that the line of profile is substantially 

 in the line of the strike of the sedimentary layers. This is important 



t See also C. H. Green, "Velocity Determinations by Means of Reflection Profiles," Geo- 

 physics, Vol. 3, No. 4, p. 295, October. 1938. 



