SEISMIC METHODS 7ZZ 



The presence of low velocity strata is confirmed by the two wells and a 

 correlation of these strata is noted. The interval velocities were determined 

 from the difference of time over a depth difference of 500 feet. A 

 considerably more accurate determination of interval velocity would be 

 possible if the two times at the ends of the interval could be determined 

 simultaneously. Multiple seismometers separated by a constant interval of 

 cable are designed to realize this accuracy.! (When no special seismometers 

 are available for well shooting but an abandoned well can be located in the 

 area under investigation, it is possible to obtain information about the 

 increase of the velocity with depth by shooting small charges — for shallow 

 depths a cap is sufficient — at diff'erent depths and using a seismometer very 

 close to the well as a detector.) 



It is sometimes desired to determine the velocity V at any depth by the 

 use of a curve such as Figure 459 which shows the average velocity V 

 as a function of depth. The velocity V is assumed to be a continuous 

 function of the depth. By the definition of average velocity (the average 

 with respect to time is used throughout this chapter), one has 



"■ " (87) 





— <!!-— V (88) 



On differentiation, one obtains 



dh ~ h \ V 



or 



dh 



The velocity at any depth is thus obtained from a knowledge of the 

 average velocity to that depth and the rate of change of the average 

 velocity at that depth. 



In an interpretation of well shot data, Dix$ corrects for lateral varia- 

 tions in velocity which are usually found in shooting a number of wells. 

 The variations have been accounted for and the wells tied together by 

 tilting the linear distribution of velocity with depth in the proper way. 



Curvature of the Reflecting Interface 



Ray path and dip equations have been treated thus far on the basis 

 of a plane reflecting horizon. The effect of curvature in the reflecting 

 horizon, however, can be significant.* 



t H. Salvatori. U.S. Patent 2.1.37.985. Issued July 9. 1937 



t C. H. Dix, "The Interpretation of Well Shot Data," Geophysics, Vol. XI, No. 4, October, 

 1946, pp. 457-461. 



* For example, the reflecting bed could be so severely distorted that for a given 

 shot-point the spread would be actuated by several reflecting portions scattered over 

 the bed, a condition of multiple reflections. 



