Chap. 9] 



SEISMIC METHODS 



451 



waves. The preliminaiy waves, or forerunners, 

 come to a seismic station through the interior of 

 the earth, while the surface waves, as their name 

 indicates, propagate along the earth's surface. 

 The forenmners in turn are divided into the lon- 

 gitudinal and transverse preUminary waves, while 

 the surface waves are divided in the same manner 

 into the Love waves and the Rayleigh waves, named 

 after the investigators who first described and 

 analyzed them. 



Usually there are three phases in a long-distance 

 seismogram (see Fig. 9-6): (1) the primae, or 

 normal longitudinal preliminaries, P; (2)" the 

 secundae, or transverse preliminaries, S; and (3) 

 the surface waves, L (= longae). Abrupt arrivals 

 are designated by the subscript i (impetus); a 

 gradual appearance is designated by e (emersio). 

 The P waves, as well as the S waves, are divided 

 into the (1) normal, (2) reflected,^ and (3) 

 refracted waves. Alternating waves, mostly of 

 the reflected type, are designated by the letters 

 PS or SP, depending upon whether they were 

 running first as longitudinal or transverse waves. 

 The subscript n denotes the normal preliminaries; 

 the subscript c designates waves which have 

 passed the core of the earth. Bars above the 

 sjonbols indicate refractions ; double letters, reflec- 

 tions. The mam part of a seismogram is generally 

 divided into (1) the arrival of the surface waves, 

 L; (2) the maximum, M; and (3) the coda, C. 



An analysis of travel-time curves makes it 

 possible to determine the depth of penetration 

 of seismic waves in the earth's interior, their 

 path, and their velocities along various portions 

 of this path. Obviously, only the preliminary 

 waves can be used for investigations of this 

 character. As the longitudinal waves have the 

 greatest velocity and arrive first, they can be more 

 accurately identified and timed than can later 

 impulses. Travel- time curves for the directly 

 transmitted, for the once and twice reflected,* and 



' Reflected at the surface. 



* It has been suspected that some unreasonably deep 

 refliBctions recorded in seismic exploration are of a similar 

 nature, that is, reflected once at the surface and twice 

 on the reflecting bed. 



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