452 SEISMIC METHODS [Chap. 9 



for the first and second preliminary waves have been pubHshed in various 

 books on seismology. The time of arrival is not a linear function of the 

 distance traveled. The velocity is not constant but dependent on 

 distance and thus on depth of the strata traversed. 



4. Characteristics of waves observed in seismic exploration. In seismic 

 exploration one does not have to deal with as many types of waves as in 

 earthquake seismology. This fact is due, first, to the smaller distances 

 and shorter time intervals involved; and second, to the fact that not so 

 many components are recorded, making it impossible to identify trans- 

 verse waves with certainty. It is true that transverse impulses have been 

 recognized in records of quarry explosions, but in such cases greater dis- 

 tances were involved and horizontal components were also recorded. 



Three types of longitudinal waves are observed in seismic exploration: 

 (1) directly transmitted; (2) refracted; and (3) reflected. Longitudinal 

 waves transmitted at the immediate surface are rarely observed; vir- 

 tually all first breaks, even those recorded close to the shot point, are 

 refracted waves. Surface waves arriving at the end of a record are prob- 

 ably not simple longitudinal but Rayleigh waves. Their frequency is 

 generally low, from 10 to 15 cycles. The frequency of the refracted waves, 

 on the other hand, covers a wide band from around 15 to 60 or 80 cycles, 

 while the frequency of reflected waves is frequently near 50 and covers 

 the range from 30 to 70 cycles. 



C. Laboratory and Field Methods for the Determination of 

 Elastic Moduli and Wave Velocities 



It was shown in the preceding sections that elastic moduli are the 

 physical parameters relating stresses and deformations and that the elastic 

 wave velocities are functions of these moduli. In the study of the elastic 

 behavior of rocks and formations, it is thus possible to attack the problem 

 in two ways: (1) by measuring elastic moduli in the laboratory; (2) by 

 making velocity determinations directly in the field. Direct velocity deter- 

 minations have the advantage that the speed of seismic waves is deter- 

 mined in situ, that is, under the natural conditions of moisture, pressure, 

 weathering, and the like. Furthermore, unconsolidated formations cannot 

 be moved and must be tested in the field. 



Elastic constants may be expressed in various units. Pounds per square 

 inch is the unit most frequently used in the testing of construction ma- 

 terials in this country. Another technical system uses the atmosphere; 

 scientific publications employ dynes per cm^, megabars, and baryes. In 

 many articles there is confusion as to the correct usage of the terms bars 

 and baryes. In Europe the normal atmosphere and the technical at- 

 mosphere (kg per cm^), the dyne per cm^, and kg per mm^ are preferred. 



