Subsurface Methods as Applied in Geophysics 1087 



where rj = index of refraction 

 i = angle of incidence 

 r = angle of refraction 

 Vi = velocity in upper medium 

 V2 = velocity in lower medium. 



Of primary importance in refraction-seismograph methods is the 

 critical angle of incidence where the angle of refraction becomes 90°. 

 A ray striking the interface at the critical angle travels along the interface 

 and returns to the detectors at the surface at the same angle. Rays striking 

 the interface at angles less than the critical may be both refracted and 

 reflected. Those striking at angles greater than the critical are reflected 

 only. The angle of incidence of a reflected ray is equal to the angle of 



Figure 576. Three of the many wave paths that rays may travel after 

 a seismic disturbance at or near the surface. 



reflection, as in optics. In order that the seismic methods may be used as 

 a prospecting tool, variations in the physical properties of the rocks 

 must occur with depth. 



In all modern seismic methods there are a shot point and a number 

 of receiving points, the disposition of the receptors depending upon the 

 method used and local conditions. Energy is introduced into the ground 

 by the explosion of dynamite, which may be placed from a few feet to 

 hundreds of feet below the surface in drilled shot holes, or from the 

 explosive force of dynamite suspended above the ground. The latter 

 method, called the "Poulter method," is at present in the development 

 stage. ^^ As shot-hole drilling represents one of the largest expenditures 

 on a seismograph crew, shooting above the ground should prove attractive 



^ Kastrop, J. E., The Poulter Method of Geophysical Seismic Exploration: World Oil, vol. 128, 

 no. 9, pp. 53-60, Jan. 1949. 



