SEISMIC METHODS 833 



The frequency analysis of a single trace of one such record is shown 

 in Figure 516. The top trace shows the complex wave as generated by the 

 actual ground motion. Many of the reflections are masked by the varia- 

 tions in phase, amplitude, and frequency of the different reflections. By 

 playing back the record through a narrow band-pass filter (usually 2 to 5 

 cycles wide) and scanning the record from the low to the higher frequency 



Fig. 515. — Mirragraph reproducing and analyzing equipment. A, reproducer unit; 

 B, control unit; C, frequency analyzer; D, exciter lamp power supply; E, filament power 

 supply; F, graphic recorder; G, control panel; H and I, plate current power supply. 

 (Courtesy of Western Electric Company.) 



limits (usually 2 to 300 cycles), the complex seismic wave may be broken 

 down into its components. The other traces in that figure show seven of 

 the different frequency components, their arrival times, and their relative 

 amplitude and persistence. The higher frequency response of this particular 

 recording is low due to the characteristics of the seismometer employed, 

 which was peaked at about 25 cps. Later equipment employs high fidelity 

 seismometers and amplifiers. 



Detailed studies of conventional oscillograph records from many areas 

 show changes in frequency and wave characteristics diagnostic of sub- 

 surface conditions. Although there is a general change in frequency versus 

 depth, the frequency is influenced by factors other than mere depth below 

 the surface. The frequency of the reflected waves seems to be governed 

 by the characteristics of the "reflecting" bed and the character of the inter- 



