[KING-SHAW] SOUND THROUGH EARTH AND ROCK 75 



That waves of disturbance should be capable of travelling long 

 distances with very little loss is borne out by the fact that the coefïïcient 

 of attenuation of heavy seismic earthquakewaves, now known with 

 fair accuracy, is remarkably small. ^ If Eq is the original energy, E, 

 the energy at distance Akm. from the epicentre measured along a great 

 circle, is given by the formula E =Eo e"'^-^ where K = . 00032 km."\ 

 In other words, such a wave is capable of travelling 2100 km. (1300 

 miles) before having its intensity diminished to half its original value. 

 As regards the medium through which seismic and microseismic 

 disturbances are propagated, geological evidence suggests- that at 

 depths of a few miles below the surface, pressures are so great that no 

 open cavities can exist and that the rock at this depth is an extremely 

 homogeneous medium free from flaws and fissures. In these cir- 

 cumstances, the possibility of microseismic disturbances taking their 

 origin from the impact of sea-waves on rocky coasts and travelling 

 hundreds of miles through the upper layers of the earth's crust need 

 occasion no surprise. 



The evidence just discussed has been brought forward to show 

 that the bed rock (at any rate a few miles from the surface) provides 

 a good medium for the transmission of elastic solid waves of the type 

 represented by microseisms (Rayleigh waves or surface tremors). 

 Whether this medium would be suitable for the transmission over long 

 distances of short period elastic waves having a frequency of several 

 hundred cycles per second, can only be settled by direct experiments: 

 the evidence which has just been brought forward would indicate that 

 experiments in this direction would give sufficient promise of success 

 to be worth attempting. 



! 



Section 2. — Notes on the Means Available for Generating 

 Elastic Waves in Rock Media. 



Until quite recently, easily controllable means of setting up rock 

 vibrations have not been available. The somewhat evident methods 

 of igniting explosive charges or striking blows on the bed rock by 

 means of hammers, etc., are neither efficient nor controllable to a 

 desirable extent. A very promising solution of this problem lies in 

 the adoption of the Fessenden Submarine Telegraph Oscillator for the 



^Report of the Chief Astronomer, Dominion Observatory, Ottawa, 1911. pp. 

 22-23. 



^Adams, F. D., "Experimental Contribution to the Question of the Depth of 

 the Zone of Flow in the Earth's Crust," Journal of Geology, Vol. XX., No. 2, Feb.- 

 Mar., 1912. See also King, I,.V. ihid. 



