780 EXPLORATION GEOPHYSICS 



the most universally used in exploration geophysics. This classification 

 may be divided into : 



(a) Acceleromcters, a type of seismometer which is responsive par- 

 ticularly to the acceleration of the ground movement. These instruments 

 are generally characterized by high natural periods. Mechanically, they 

 are devices for measuring force or pressure acting on relatively fixed 

 inertia mass.f 



(b) Velocity-type seismometers, which measure, within certain definable 

 limits, the rate of movement of ground with reference to a relatively fixed 

 inertia mass. The natural periods of these instruments are intermediate in 

 magnitude. 



(c) Displacement-type seismometers, which react to the ground move- 

 ments in such a manner that their output is proportional to the displacement 

 or movement of the ground. Instruments of this type are generally of 

 extremely low frequency or long period, and are usually unstable and 

 difficult to design. 



The second classification deals with seismometers which may be tuned 

 or untuned. When the natural period of the seismometer lies within the 

 range of the frequencies to be recorded, the instrument is tuned; if the 

 natural frequency lies outside of this range, it is untuned. 



The mechanical make-up of a seismometer usually comprises a vibra- 

 tory mechanical system, of a desired frequency range or periodicity, which 

 works in conjunction with some form of transducer or energy transfer 

 device. It is common practice to dampen the oscillatory systems of seis- 

 mometers just short of the critical stage of damping. Usually the amount 

 of damping is referred to as a fraction of {h), such that when /i = there 

 is no damping, and when h = \ there is critical damping. The generally- 

 used value is h = 0.7. 



The essential parts of a seismometer are : a rugged waterproof case 

 which rests firmly on the earth, a heavy mass or weight which is attached 

 to the case by means of a spring or other elastic arrangement, and a 

 movable element which transforms the movement of the heavy mass to a 

 form of energy suitable for recording. 



The arrival of seismic waves at the seismometer produces two momen- 

 tary effects : ( 1 ) a displacement of the case which is in contact with the 

 earth and (2) a displacement of the inertia reactor. That is, the seismic 

 waves set both the case and the suspended inertia mass vibrating. The 

 frequency of vibration of the case is approximately the same as that 

 of the waves. The frequency of vibration of the inertia reactor depends 

 on the natural frequency of the reactor system (mass plus suspension) 

 as well as on the frequency of the seismic waves. In the absence of 



t L. D. Statham, "Accelerometer with Electric Strain Wires," U. S. Patent 2,453,548, Nov. 9, 

 1948. 



H. E. Webber, "Accelerometer." U. S. Patent 2,455,394, Dec. 7, 1948. 



C. R. Abraham, "Means and Method of Indicating Acceleration," U. S. Patent 2,457,620, 

 Dec. 28, 1948. 



C. M. Hathaway, "Accelerometer," U. S. Patent 2,440,605, April 27, 1948. 



