Chap. 9] 



SEISMIC METHODS 



565 



These relations are identical with eq. (9-78a). They may be applied in 

 various ways. For a given set of conditions (in an area where the velocity 

 is known and the distance between center of spread and shot point are 

 kept constant), diagrams such as shown in Fig. 9-87 may be prepared, 

 showing, in vertical section, intersecting lines of equal gradient and equal 

 time to center of spread. After the time gradient, or step-out time, has 

 been measured, the point corresponding to these values is located in the 

 diagram, which gives the depth of the reflection point. By connecting 



Depth 



DeMors SMpoint 



Fig. 9-87. Graph for determining depth and dip from total time and step-out time 



(after Pirson). 



this point with the shot point and drawing a perpendicular, the dip is ob- 

 tained. A vertical change of velocity may be incorporated in the diagram. 

 Eqs. (9-79c) may also be utilized for direct dip-calculations. By sub- 

 tracting the two equations, 



v'(D.G..^d - U.G.-U - x^A- x^ 



42 



= sm (p. 



(9-79d) 



in which the last part of the numerator becomes when equal distances on 

 either side of the shot point are used. Eqs. (9-79a) and (9-796) are, in 



