SEISMIC METHODS 



787 



The natural frequency of the seismometer was 6.5 cycles per second, with 

 a constant of .727 volts /cm/ second at 250 ohms load. In conformity with 

 theory, the outputs are a maximum in the neighborhood of the natural 

 frequency when the seismometer is underdamped to the extent that the 

 value of h is somewhat less than 0.7. The smaller the damping coefficient, 

 the larger the peak output becomes. This usually corresponds to the larger 



FREOUENCY: CYCLES PER SECOND 



Fig. 477.- 



-Effect of various damping coefficients on_ frequency response of a 

 Type 301 seismometer. (Century Geophysical Company.) 



amplitudes of the inertia mass. Another important generalization to be 

 observed from Figure 477 is that the apparent maxima of the various curves 

 shift to the right of the natural frequency. This indicates that the frequency 

 of resonance increases with increasing values of the damping coefficient h. 

 This is an important factor for consideration when designing any oscillatory 

 system of a specified frequency response. Particular attention should be 

 given to the character of the curve for h = about 0.7, because this is the 

 amount of damping usually chosen for seismometers and galvanometers. 



Effects of Ground Motion 



The discussion just completed involving the solution of Equation 133 is perfectly 

 general and applies to pendulums of all types, as well as galvanometers and other oscil- 

 lating systems. In this theory neither friction nor mass of the spring was considered. 

 In all cases the restoring force was assumed to be proportional to the negative displace- 

 ment. The constant (n) is defined from Equation 133 as the square root of the restor- 

 ing acceleration per unit displacement. 



