774 



EXPLORATION GEOPHYSICS 



structure of the layers above the basement rock may be deduced. This 

 technique is useful in mapping the relative structure, but is not a method 

 for obtaining absolute structural measurements. 



Intersecting Spreads 



The methods to be described here are applicable when approximate 

 values of offset distances are known. These can be determined by computing 

 average delay times from preliminary and subsequent field data and using 

 the offset distance vs. delay time graph. 



Consider an arrangement (ring shooting) wherein two spreads 

 Oi^i and O2S2 intersect at a point M which is an approximate offset 

 position for the two shot-points Oi and O2. (Figure 469A.) The delay 

 times associated with the common offset position for the two spreads will 

 be approximately equal. 



The expressions for the intercept times are 



bi = tm' + tn^ ' 



&2 = ttn + ^ « ' 



Hence, 



h — i ' — f ' 



(122) 



Fig. 469. — A, element of ring shooting arrangement; B, complete ring shoot- 

 ing arrangement. (Gardner, Geophysics.) 



Evidently, Equation 122 can be used to calculate the difference in delay 

 times from the observed intersect times ho and bi. The corresponding 

 difference in depths may be obtained from the depth vs. delay time graph. 

 Also, a seismometer at ^'3 will have a common offset position at M 

 with the seismometer at ^2; hence, 



bs — b2 — t fi t „ ^ 



