Chap. 9] 



SEISMIC METHODS 



463 



servations. The oscillations of the speci- 

 men are picked up by a piezo-electric crys- 

 tal cemented to the specimen (or by a sepa- 

 rate microphone), are amplified and read on 

 an output meter. 



Longitudinal "bar" vibrations do not de- 

 pend on Poisson's ratio; their velocity is 

 given by 



vn= /l/f. (9-29a) 



Af 



W 



S 



Since, as before, for a bar of the length Z, 

 I = ix and V = X/o , the longitudinal bar 



velocity 



Vh = 2lfo 



Fig. 9-14. Apparatus for 

 electromagnetic determination 

 of natural bar frequency (after 

 Weatherby and Fai|pt). (B, 

 bar; M, magnet; C, coil; O, os- 

 cillator; S, supporting knife 

 edge; A, amplifier; m, meter; 

 r, rectifier.) 



(9-296) 



Methods for determining damping from resonance curves are discussed 

 on page 481. 



Rebound observations: Elastic properties of rocks may be determined by 

 observations of physical parameters related to the rebounding of a steel 

 ball dropped from a given height. Two methods have been used: (1) 

 measuring the size of the imprint, and (2) measuring the height of 

 rebound.^" 



If a steel ball of radius pt , mass nib , Young's modulus Eb (= 22-10^^), 

 dropped from a height H on the rock whose surface is blackened, leaves an 

 imprint of the radius r, then Young's modulus of the rock is given by 



1 

 £ 



16r 



lS.5mt2gHpl 



1^ 

 Eft' 



(9-30) 



Cr 



i 



CZ3=0^ 



o 



If a steel ball is dropped from a 

 height H on the rock surface and re- 

 bounds 'to the height h (see Fig. 9- 

 16), the so-called "restoration" coeffi- 

 cient is 



^/- kv 



Q = 



(9-31) 



J 



Fid. 9^5. Apparatus for electrostatic 

 detei/mination of natural bar frequency 

 (after Idc). (O, oscillator; C, con- 

 denser; D, dielectHc; F, foil; B, base 

 plate; V, polarizing voltage; S, speci- 

 men ; Cr, crystal ; A , amplifier ; M, out- 

 put meter.) 



The exact relation between the resto- 

 ration coefficient and Young's modu- 

 lus has not been definitely deter- 



10 J. Roess, Union Geod. and Geophya. 

 Internat. Assoc. Seism. Ser. A, Trav. Sci., 

 Fasc, IS, 3-69 (1935). 



