SEISMIC METHODS 651 



tion parallel to the line of propagation, and (b) transverse or shear waves, 

 i.e., waves in which the motion of the particles is at right angles to the 

 direction of propagation. When the disturbance is an explosion — the usual 

 case in seismic prospecting — the shear produced in the elastic medium is 

 small compared to the change in volume ; hence, in this case, most of the 

 wave energy will be propagated in the form of longitudinal or com- 

 pressional waves. 



If the elastic solid is bounded, as is true of the earth, a third type 

 of wave is produced. These waves travel along the surface and are 

 called surface waves. Such waves are generated either by shear or change 

 in volume close to the surface — this is the more important mode of 

 origin — or when elastic waves reach the surface. 



The sudden stress generally utilized in seismic prospecting is the 

 explosion of a charge of dynamite. The explosion of a charge buried in 

 the ground produces a strain in the walls of the cavity or hole in which 

 the charge is placed due to the enormous pressure of the expanding gas. 

 This strain is transmitted to the surrounding layers and propagated out- 

 ward through the earth as elastic waves, chiefly longitudinal and surface 

 waves. The longitudinal waves are classified conveniently into two 

 types: (a) direct waves which travel in approximately straight lines 

 from the shot-hole to the various seismometer stations and (b) waves 

 which are reflected and refracted at various subsurface boundaries before 

 reaching the seismometer stations. The paths traversed by the second type 

 of longitudinal waves are determined by the wave velocities in the media 

 constituting the subsurface and by the shapes of the boundaries between 

 the media. 



Distribution of Energy in Reflected and Refracted Waves 



In an unbounded isotropic homogeneous m.edium, the energy radiates 

 uniformly in all directions from the source, and the decrease in energy 

 due to absorption depends only on the distance.* In the case of seismic 

 waves traveling through the earth, however, the longitudinal waves spread- 

 ing out from the shot-hole encounter boundaries separating media of 

 different elastic constants or density, and the energy of the impinging 

 waves is distributed among the several waves produced at these boundaries. 



When a longitudinal wave is incident on a boundary separating media 

 of different elastic constants or density, the energy of this wave is gen- 

 erally distributed among four new waves, a longitudinal and a trans- 

 verse wave in each of the media. 



Formulas for computing the relative amounts of energy transferred 

 to these four waves have been given by Knott, t and formulas for com- 

 puting relative amplitudes have been given by Zoeppritz.J 



* The absorption of energy is due to internal friction. 



t C. G. Knott, "Reflexion and Refraction of Elastic Waves, with Seismological Applications," 

 Philosophical Magazine, Vol. 48, July, 1899, p. 64. 



% K. Zoeppritz, "Ueber Erdbebenwellen Vllb," Nachrichten der Koniglichen Gesellschaft der 

 Wissenschaften zu Gottingen, mathematische-physikalische Klasse, 1919, p. 57. 



