SEISMIC METHODS 717 



Therefore an acceptable method of correcting for the delay of the 

 seismic waves in passing through the low velocity layer should give proper 

 results under all the above-mentioned conditions. 



Errors in the application of the so-called "weathering correction" 

 affect the seismic results in two general ways. The calculated depths to 

 the reflection surfaces are only as accurate as the correction used, other 

 factors being equal. In reflection correlation work, errors in this correction 

 may lead to erroneous correlations of reflected impulses recorded from 

 adjacent shot-points. Another problem arises when the proper impulses 

 are correlated and the depth computations are affected by unknown thick- 

 ening or thinning of the surface or low velocity layer. Another way in 

 which errors in the correction may influence the seismic results is found 

 in the dip computations. In areas of extremely low relief structures, 

 errors in the application of the low velocity data may completely mask the 

 changes due to structure. In areas where insufficient low velocity layer 

 data are obtained, the errors may be of the same order of magnitude as 

 the measured step-out time caused by the inclinations or attitudes of the 

 reflecting interfaces. It therefore follows that, when using the reflection 

 method to investigate areas in which the attitudes of the geological beds 

 are of the order of zero to three degrees from the horizontal, extreme care 

 must be used in obtaining measured data on the surface low velocity layer. 



Time Corrections for Low Velocity Layer. — For the sake of sim- 

 plicity the following discussion will refer all computations of depth and 

 reflection time to the base of the low velocity layer. Data reduced in this 

 manner may be referred to any other datum merely by adding algebraically 

 the interval between the bottom of the low velocity layer and the elevation 

 of the desired reference datum. 



For a shot at depth H, the time to be deducted from the observed 

 reflection time to eliminate the effect of the low velocity layer and to 

 refer the time to the base of the low velocity layer is : 



tu = —7} + -77 — for H <Uo 



(71) 



,_Us_ H-Uo 



C=iP-- ,. " forH>[/o 



1^ UyS y B 



where H is the depth of the shot 



Uq is the thickness of the low velocity layer at the shot-point 

 Ug is the thickness of the low velocity layer at the seismometer 

 Vu,o is the velocity of the low velocity layer at the shot-point 

 Vu,s is the velocity of the low velocity layer at the seismometer 

 Vc is the velocity of the consolidated section immediately below the 

 low velocity layer 



