514 



SEISMIC METHODS 



[Chap. 9 



and (9-46a). Next follows the ratio a:/v, and in column xii is given the 

 time spent in all layers above the n*^. For successive layers column xii 

 is obtained by multiplying the 2/i/v values in column xv by the cosine 

 values in column ix. Column xiii is the sum of xi and xii, and xiii in 

 turn is deducted from ii, giving 2/i/v cos i in column xiv, which is divided 

 by cos in of column vi. Thus, the thickness (xvi) and the total depths 

 (xvii) are obtained. 



/qoao ZO.OOO 3nooo 



Fig. 9-51. Multiple layer travel-time curve. 



8. Vertical steps and domes. Travel-time curves obtained on faults, 

 terraces, buried escarpments, salt domes, and igneous intrusions often 

 approach the simple cases discussed in the follomng section. Contrary 

 to conditions treated previously, formations are not continuous in hori- 

 zontal direction or are not parallel with the surface for the entire length 

 of the profile. Hence, additional unknowns enter. These can be deter- 

 mined by taking two profiles in opposite directions or by shooting at two 

 shot points at different distances from the detector spread. In the fol- 

 lowing discussion the direction of shooting is assumed to be at right angles 

 to the strike; however, the same problems may also be handled by shooting 

 parallel with the strike. 



