SPATIAL PROBLEMS IN GEOMETRICAL SEISMICS 



61 



of observation. The azimuthal deviation for a ray with an azimuth in the 

 first medium of 70° is shown in the illustration by means of arrows and is 

 equal to 15°. 



We find the intersection trace of the ray surface with the plane of 

 observation as the geometrical position of the points of emergence of rays 

 belonging to one ray surface. These points are calculated for each of the 

 rays of the ray surface from the known depth of the point at which the 

 ray intersects the interface and the angle the ray makes with the vertical after 

 reflection. This angle is taken from the Wulflf net when the direction of the 



>96 1-00 



1-20 I-40 1-60 1-80 2-00 2-20 2-40 2-60 2 80 3-00 

 sec 

 Fig. 9. Chart of isochronous lines. 



1 — points of emergence of rays from which the field was 

 constructed. 



90° 



reflected ray is determined. In the general case we can find this point by the 

 same methods as we use for determining the projection trace of a ray inter- 

 section with an interface. In contrast to the case analysed above what we 

 are here examining is not "descent" of a ray but its "climb". The point 

 on the projection of the reflected ray which corresponds to zero depth will 

 also be the point of emergence of the corresponding ray (see line 4 in Fig. 4). 

 When we have joined all the ray emergence points by a smooth hne we 

 shall obtain the intersection trace of our ray surface with the observation 

 plane. 



For every point on line 2, then, there is a corresponding point on line 3 

 (see Fig. 4). 



Similar constructions can be made for all ray surfaces (Fig. 8), where (a) 

 the lines 4 correspond to radial surfaces of 10°, 13°, 15°, 17°, 20°, 22°, 



