SEISMIC METHODS 7A7 



ures, and draw a continuous contoured surface across the fault. This 

 would be an unpardonable error in the eyes of most geologists. 



A common method used for the adjustments of misclosures is the linear 

 correction which is based on a constant depth adjustment per unit length 

 of profile. Another method makes use of an adjustment proportional to 

 the magnitude of the computed dip appearing in the profile. It is common 

 practice to restudy a prospect when large misclosures are determined. In 

 this manner the misclosures are established either as anomalies or as errors 

 of accumulation. 



A completed contour map in some cases requires a correction for a 

 lateral velocity variation, because routine computations generally are based 

 on a velocity-depth function which varies with either the depth or the 

 reflection time. The customary correction for the lateral variation of 

 velocity is made by obtaining products of the depth Z below the low velocity 



layer and the ratio — , where Vt is the actual average velocity in the vertical 



1^ z 



direction and Vz is the originally-employed average velocity in the vertical 



2Z 

 direction for a time T equal to -7^ . The correction, although customary, 



1^ z 



is approximate, because the dip of the structure is not considered in the 

 correction of the data. A contoured interface which gives a constant time 

 for reflections is also helpful when studying the effect of non-vertical 

 velocity gradients. 



In areas where two components of dip are measured at each station for 

 the purpose of resolving the components into the resultant true dip of the 

 underground strata, horizontal plan maps are often prepared showing the 

 location of the shot-holes and the position of the vertically projected dips 

 into a horizontal plane. These maps are particularly valuable to geologists, 

 who are accustomed to handling much of their measured dip and strike 

 data in this form. An illustration of this type of map is given by the dip 

 strike plotting on Figure 433. 



One advantage of plotting of dip and strike in horizontal plan is that 

 the computed direction of the true dip from component measurements is 

 independent of any error in the assumed velocity depth function. Results 

 expressed in this form are usually more accurate in areas of moderately 

 steep dips than in areas of very flat-lying strata. 



Graphical Analogue Computer 



Field experience has shown that in many regions the empirical velocity 

 data obtained from deep well shooting can be very closely fitted to the 

 linear-increase, velocity-depth function already discussed in a former 

 section. The assumption involved in this velocity distribution led to the 

 development of a very simple graphical analogue computer, which is 

 completely rigorous and will enable one to plot any seismic reflection at 

 its proper position and attitude on a vertical section profile when the 

 arrival time T and the move-out time t are correctly set into the machine. 



