464 



SEISMIC METHODS 



[Chap. 9 



mined. Roess established an empirical relation (Fig. 9-17) by determin- 

 ing both q and E for the same rocks by the two methods described above. 

 Rebound observations are applicable only to fine-grained materials. If 

 the ball hits a large mineral grain, the elastic coefficient of the grain and 



not that of the aggregate is obtained. 

 Another difficulty arises from the fact that 

 the surface constitution of the rock must 

 not deviate from the composition of its in- 

 terior; otherwise the restitution coefficient 

 would express surface conditions only. As 

 a matter of fact, rebound observations are 

 used in the metallurgy of steel for hard- 

 ness determination. 



2. Field methods {velocity determinations). 

 Field determinations of horizontal velocities 

 are made by measuring the time interval 

 which elapses between the firing of a shot 

 and the arrival of the longitudinal or 

 transverse energy at the location of a de- 

 tector set up at a measured distance. One 

 shot point may be used with a number of 



Fig. 9-16. Record of steel ball 

 rebounding from surface of mar- 

 ble slab (after Roess). 



f . (Youfff's 



(Rest oration 

 ^ cotffidenfj 



Fig. 9-17. Young's modulus E as a function of restoration coefficient (g) for some 

 homogeneous rocks. (Numbers refer to tabulation of rock specimens, pp. 11 and 

 12, Roess' article.) (After Roess.) 



detectors, and the latter may be moved when one spread is not sufficient 

 to cover all desired distances. Another method is to leave the detectors 

 at their places and to move the shot point. In practice, the application 

 of this method is not quite so easy as it may appear. Errors of instru- 

 mental and geologic nature occur. The former arise from (1) inaccuracies 

 in the determination of the exact instant of the shot, (2) errors in the tim- 



