22 



METHODS OF GEOPHYSICAL EXPLORATION 



[Chap. 2 



points is homogeneous in horizontal and vertical direction, the arrival 

 times will be proportional to distance, and therefore the travel time curve 

 will be a straight line, its slope giving the velocity in the medium. If the 

 ground is horizontally stratified and if a high speed medium occurs beneath 

 a low speed medium, only the first part of the travel time curve will give 

 the speed in the upper medium. From a certain distance on, waves that 

 have taken a "detour" through the lower high-speed medium will overtake 

 and therefore arrive ahead of the wave through the upper medium (see 



Fig. 2-10. Wave path, schematic record, and travel-time curve in single-layer refrac- 

 tion problem. 



Fig. 2-10). The simultaneous arrival of the two waves will be indicated 

 by a break in the travel time curve; the slope of the second part of the 

 travel time curve will correspond to the velocity in the lower medium. 

 From these two velocities and the abscissa of the break in the travel time 

 curve, the depth of the interface may be calculated. 



If more than one interface exists, depths are calculated from the cor- 

 responding breaks and velocities. In the case of dipping beds the slopes 

 of the travel time curve no longer give a true but only an apparent velocity. 

 Compared with the horizontal bed, an up-dip profile shows a greater ap- 



