CHAPTER VII 

 SEISMIC METHODS 



The constant search for knowledge by those investigating the natural 

 phenomenon of earthquakes led to the discovery of the fundamental 

 principles which have been applied to the seismic method of geophysical 

 exploration. Several large accidental explosions caused seismic waves to 

 be initiated within the earth's crust. These were recorded by stationary 

 seismographs set up for earthquake wave observations, and thus the first 

 effects of artificial earth shocks were recognized by seismologists. From 

 these early seismograms information was obtained on the velocity and the 

 frequencies of seismic waves. These waves were of relatively shallow 

 penetration into the earth's crust as compared with earthquake waves. 



Earthquake-recording seismographs have been in use for approximately 

 100 years recording the natural tremors of the earth. However, most of 

 the advancements in these earth studies have been made within the last 

 forty years. And it was not until 1924 that seismic methods of geophysical 

 prospecting became an acceptable tool of the exploration geologist. Im- 

 provements in both instrumentation and field technique followed rapidly 

 from that date, though the fundamental principles of earthquake seismology 

 remained the basis of the application of seismic methods to exploration. 

 Only the details of instrument construction and operational technique have 

 changed. Most of these changes were necessary because the frequencies of 

 artificially initiated elastic waves within the outermost crust of the earth 

 are many times greater than those of natural earthquake waves which 

 penetrate to much greater depths and travel long distances. 



The study of earthquake seismology has revealed much about the 

 physical interior of our planet. Analyses of travel-times of earthquake 

 waves have indicated that various concentric shells of materials of quite 

 different elastic properties make up the interior of the earth. In a similar 

 manner, analyses of the travel-times of artificially-produced shock waves 

 observed for local and relatively shallow penetration of the earth's crust 

 reveal the presence of materials of various elastic properties. This informa- 

 tion, when studied together with the local geology, often leads to informa- 

 tive conclusions as to the subterranean distribution of geological formations 

 characterized by the diflferent elastic properties. 



Relation Between Seismic Prospecting and Seismology 



Seismology is that science which deals with natural earthquakes and 

 related phenomena. This includes theories of causes of earthquakes, such 



