Chap. 9] 



SEISMIC METHODS 



589 



erally reached after the third or fourth peak of a sinusoidal impulse has 

 been recorded. Before that, the first term in eq. (9-89rf) plays an im- 

 portant part. How the seismograph records th6 "onset" of an impulse 

 depends on its damping, frequency characteristics, and initial conditions, 

 that is, the phase relation of ground and instrument motion. The simplest 

 assumption in regard to starting conditions is that at the time t = 0, the 

 amplitudes and velocities of both ground and instrument motion are zero, 

 and that the ground motion is a train of sinusoidal waves. The corre- 

 sponding seismograph motion has been calculated by H. A. Wilson. 



Fig. 9-100. Dynamic response of mechanical seismograph, as a function of tuning 

 factor and relative damping (after Meisser). 



Some of his results are reproduced in Fig. 9-101 for undamped and criti- 

 cally damped seismometers and for various tuning factors. 



At resonance the onset of a wave train is recorded by an undamped 

 seismometer with rapidly increasing ampUtudes and a phase shift of 90°. 

 For critical damping, the amplitude is considerably reduced, but the 90° 

 phase shift in maximum amplitudes remains. In a station seismometer 

 {n = 3) the reproduction of the ground motion is more faithful. (For a 

 tuning factor of 10, phase shift and amplitude would be still further 



"Physics, 2(3), 186-199 (March, 1932). 



