SEISMIC METHODS 



721 



same order of magnitude as the variations in the quantity ty, corresponding 

 to the various seismometers of the same spread. The difference in travel- 

 times through the low velocity zone to any two seismometers (Af^) is 

 obtained by subtracting two terms of the form given by Equation 75. 

 To avoid introducing the velocity Vc in the evaluation of A/„, the dis- 

 tances from the two seismometers to the shot-point are made equal ; also, 



43S' 



gza' 



Fig. 441. — Low velocity layer refraction record. (Courtesy of Western Geophysical Co.) 



the radical term can easily be reduced to unity within the general degree 



of accuracy.* Hence, the differential low velocity layer correction is 



where tk and t-u are the first arrival-times at the seismometer stations 

 "k" and "-k" respectively. The first arrival-times of a record are thus 

 used directly to obtain the correction which must be appHed to the dip 

 time difference AT to compensate for variations in the low velocity 

 layer. 



* Setting the radical term equal to unity is equivalent to assuming that the critical 

 angle is 90° or that the waves travel through the \ovi velocity zone in a vertical 

 direction. 



