-SEISMIC METHODS 781 



damping, the vibration of the inertia reactor system would continue even 

 after the seismic waves ceased arriving. For this reason, the inertia 

 reactor of practically all seismometers is provided with some form of 

 external damping. The damping may be mechanical, electromagnetic, 

 or both.* 



General Conditions Affecting Design. — The amplitude of the elastic 

 wave created by the explosive or mechanical disturbance decreases with in- 

 creasing distance from the source. (Compare p. 651.) In particular, 

 the change of amplitude with depth is especially marked, and, due to 

 this factor, high sensitivity of seismometers and high amplification are 

 necessary. (In this connection, it should be noted that the seismometer, 

 amplifier and recording galvanometer all act together to produce the 

 final overall response.) 



The frequencies generally encountered in seismic exploration vary 

 with the area. In the immediate vicinity of the disturbance the waves 

 have a wide range of frequencies with the higher frequency components 

 constituting a major portion of the energy. Generally, the absorption is 

 greater for the higher frequencies ; hence, as the distance from the disturb- 

 ance is increased, the preponderance of wave energy shifts toward the 

 lower frequencies. 



Considering the detecting system (seismometer and recording appar- 

 atus), one may note that the equipment must be so designed that the 

 following three conditions are fulfilled: (1) The system must be 

 sufficiently sensitive to detect and amplify the useful seismic disturbance; 

 (2) the system must discriminate sufficiently against undesired energy 

 to produce usable records; and (3) the system must have sufficient 

 damping that successive seismic impulses can be recognized individually. 

 In the matter of sensitivity, the energy output of the seismometer is 

 of primary consideration. The seismometer must supply an impulse 

 to the first stage of the amplifier which is sufficiently above the noise 

 or microphone level of the first stage to allow effective amplification. 

 (Above this level, amplifier units can be made to produce any degree of 

 amplification desired.) The usual seismic amplifier has a gain of from 100 

 to 150 DB. Damping means must be present in both seismometer and 

 amplifier units to avoid a sustained oscillation in either. Where the visual 

 interpretation of records is employed, the frequency response of the overall 

 system must be sufficiently narrow that both the low frequencies character- 

 istic of ground roll and the high frequencies characteristic of wind and other 

 disturbances are greatly attenuated. The amplifier and seismometer should 

 be designed in such a way that their characteristics are complementary 

 in order to obtain the desired overall response. 



* For data on seismometer characteristics, particularly damping, see, for example, 

 H. R. Prescott and A. P. Lipski, "System Sensitive to Vibrations," U. S. Patent 

 2,058,106, issued Oct. 20. 1936, and H. R. Prescott and K. C. Woodyard, "Seismo- 

 phone," U. S. Patent 2,084,561, issued June 22, 1937. 



