SEISMIC METHODS 



769 



which the total offset distance a corresponding to a total delay time f 

 can be obtained. 



The general types of depth vs. delay time and offset distance vs. 

 delay time graphs frequently obtained in practice are shown in Figure 465. 

 (The change in the slopes of these curves with increasing t' is due to 

 increase of velocity with depth.) 



A. B. 



Fig. 405. — A, general type of depth vs. delay 

 time graph; B, general type of offset distance vs. 

 delay time graph. (Gardner, Geophysics.) 



Correlation of Refractions 



The correlation refraction method of seismic surveying is more labor- 

 ious to prosecute than reflection methods. Consequently this type of pros- 

 pecting is usually confined to those areas in which the conventional reflection 

 methods fail to obtain satisfactory results. 



In the application of refraction methods it is known that for the case 

 of a multi-layered problem, there exists between the shot-point and the 

 recording station a distance where a number of refracted energy arrivals 

 may be observed corresponding to the seismic wave penetration of the 

 respective layers. The basis of the refraction method of shooting is that 

 the distance between shot-point and receiver is a measure of the wave 

 penetration for a given vertical velocity distribution. Conversely, there is 

 a reliable means for estimating the minimum distance required to obtain 

 the arrival of the so-called "grazing" wave. The changes in refracted 

 wave velocities will indicate the velocity discontinuities of the strata 

 penetrated by the wave. The sequence and the apparent surface velocity 

 of the refracted arrivals depend upon the depths, dips, velocities and record- 

 ing distances. Conversely, given the recorded sequence and surface velocity 

 of the refracted arrivals, and the velocities of the penetrated layers, deduc- 

 tion can be made of the dip and depth of the layers. f 



The recording distance used for correlation refraction shooting usually 

 is determined by the following factors : desired penetration to include 

 layers to be investigated, economical amount of explosive energy to secure 



t L. W. Gardner, "An Areal Plan of Mapping Subsurface Structure by Refraction," Geo- 

 physics, Vol. VII, No. 2, April, 1942. 



