54 



E. I. Gal'perin et al. 



the ray to the vertical and from the depths of its extreme points. We sum 

 the travel limes of the waves for the different sectors and record the points 

 at which each relevant ray emerges onto the observation surface. An isochro- 

 nous chart is constructed by interpolating the travel time values along each 

 ray. In addition to the time fields we can construct the fields of azimuthal 

 deviations and the angles of emergence (*>. 



Fig. 4. Construction for radial conic surface formed by rays emerging from source 

 at an angle of 20° with the vertical. 



a — plan; b — cross-section of structure in vertical plane from 270-90° azimuth. 

 1 — structure contours of interface (for cylindrical surface taken at every 20 m ; for 

 plane — at every 100 m) ; 2 — trace projection of intersection of radial conic surface 

 with interface ; 3 — geometric locus of points of emergence of rays of 20° conic surface 

 on to observation plane; 4 — ^points of emergence of individual rays; 5 — projection 

 of rays; 6 — azimuthal deviation suffered by the ray on reflection at point c; 7 — line 

 joining cyUndrical surface and plane. 



In order to solve linear problems, therefore, we must have a plan on the 

 appropriate scale with isohypses for all the interfaces. The plan must show 

 the position of the source of vibrations (shot point). 



Let us now examine the technique for making the constructions, taking 

 concrete examples first of two layered and then of three layered media. 



Structure contour [Editor's note]. 



