SPATIAL PROBLEMS IN GEOMETRICAL SEISMICS 



73 



in determining the direction of the ray by means of the Wulff net and the 

 accuracy of the graphical constructions. The former depends primarily on 

 the size of the Wulff net. After some practice of accurate work with a WulfF 

 net 20 cm in diameter, an accuracy of 0.5° can be obtained in determining 

 the direction of the ray. If a higher degree of accuracy is required, the dia- 

 meter of the net can be increased and more divisions can be marked on it. 

 The accuracy of the graphical constructions depends on scale, and is limited 

 by an inherent margin of error which experiment has shown to be 0.2 — 0.3 mm. 

 A transverse scale must therefore be used in constructing the graphs. Scales 

 of 1:10,000 and 1:25,000 may be regarded as convenient. 



0° s s 



o — 180= 



Fig. 16. a — Chart of isochrones; b — vertical cross section of structure with generators 



of ray surfaces used in the construction. 1 — shot point; 2 — isochrones; 3— points 



of em^l^ence of rays from which the fields were calculated. 



