440 SEISMIC METHODS [Chap. 9 



consolidation, dynamo-metamorphism, and greater geologic age generally 

 exhibit a greater seismic wave speed. 



In regard to physical laws involved, seismic phenomena are comparable 

 to optical phenomena since they deal with a type of energy propagated 

 in the form of waves. Wave propagation may be said to be characterized 

 by velocity, frequency, intensity, direction, and certain associated or de- 

 rived characteristics and phenomena, such as travel time, wave length, 

 absorption, refraction, reflection, and the like. In optics, only quasi- 

 stationary phenomena are investigated, owing to the rapid rate of propa- 

 gation of light, whereas in seismic work, virtually all interpretation is 

 based on travel time. Comparatively little quantitative use is made of 

 direction, frequency, and intensity. The chief objective is to determine 

 the distance between the earth's surface and one or more refracting and 

 reflecting surfaces below. Sensitive detection devices are used to record 

 the arrival times of first (refraction) or later (refraction and reflection) 

 impulses. These detectors embody an inert mass which remains (at first) 

 stationary in respect to the ground, and whose movement may be magni- 

 fied mechanically or electrically. In addition, the instant of the explosion 

 is transferred to the receiving station; accurate time marks are provided 

 on the record so that the time elapsed between the shot and the arrival 

 of the elastic impulses may be found. Together with a determination of 

 distance between shot point and record, this permits of measuring the true 

 and apparent velocities of elastic waves. 



The simplest method of seismic prospecting is the fan-shooting method. 

 This consists of comparing the travel times from a single shot point to a 

 number of pickups arranged approximately in a circle around it. The ex- 

 istence and sometimes the nature of an intervening medium may be deduced 

 from these data. More detailed information is derived from the refrac- 

 tion method, which consists of shooting traverses with one (or two) shot 

 points and a number of seismographs set up in line, and determining the 

 travel time as a function of distance. The time-distance curves give in- 

 formation on the paths of seismic rays below, their refraction, and depths 

 to refracting surfaces. In the reflection method, the travel times of re- 

 flected waves allow depths to reflecting surfaces to be calculated. 



Observation methods and instruments in seismic prospecting and earth- 

 quake seismology are very similar in respect to time marking, time trans- 

 mission, and recording procedures. The inductive electromagnetic seismo- 

 graph, which is now widely used in geophysical exploration, was developed 

 for earthquake seismology by Galitzin more than twenty-five years ago. 

 On the other hand, important differences in instruments were brought 

 about by the needs of adaptation to geological exploration. This resulted 

 in a decrease of size, increase in portability, increase in natural frequency 



