590 



SEISMIC METHODS 



[Chap. 9 



reduced.) Ground motion and instrument record are here about 180° out 

 of phase. Critical damping adds no particular advantage; it cuts ampli- 

 tudes and introduces an additional phase shift. (In station seismometers, 

 damping is usually less than critical.) In an undamped accelerometer 

 (n = ^) considerable interference is caused by natural oscillations. 

 When damped, the natural oscillations are eliminated. The instrument 

 record is nearly in phase with the ground motion and much reduced in 

 amplitude. 



In the remaining part of Fig. 9-101 the reaction of various seismographs 

 to a single impulse of one-half of a sine wave is indicated. At the begin- 

 ning both record and ground amplitude are assumed to be zero. At reso- 

 nance, the impulse starts a train of waves, the first recorded peak being 90° 



Tu/i/nf Facttr 



Undimpri 



n-0 



Cr/fictl/i/ Damped 

 1,./ 



n.) 

 Jit sunn e I 



iltfitn 

 Snsmtmtttr 



n^'i 



Fig. 9-101. Reactions of damped and undamped seismographs to onsets of periodic 

 and transient vibrations (adapted from Wilson). 



out of phase. With a critically damped seismometer, a double wave is 

 recorded with reduced amplitude, lagging 90° in phase behind the ground 

 impulse. An undamped station seismometer reacts rather unfavorably to a 

 single impulse. A train of low-frequencj' oscillations is started. The first 

 peak coincides with the ground motion peak but is 180° out of phase. 

 With a critically damped instrument, a double instead of a single impulse 

 is recorded, with less than 180° phase shift between peaks. An undamped 

 accelerometer records a single impulse essentially with its own natural 

 oscillation. When damped, it records three peaks instead of a single one, 

 the main peak being almost in phase with the ground motion. 



The behavior of various types of seismometers for an initial phase differ- 

 ence of 90° between ground motion and instrument position has been 

 calculated by H. Martin.^* Aside from minor details the response for 90° 



" Veroeff. Reichsanstalt, 26 (Jena, 1935). 



