812 



EXPLORATION GEOPHYSICS 



Table 30 gives the desired operating characteristics for seismic 

 amplifiers. 



Amplifiers are of two general types of construction. Figure 498 shows 

 a conventional type which allows rapid interchange of amplifier units, 

 should operating difficulties develop. In another design, weight and space 

 are conserved by mounting all amplifiers on a single chassis. An illustration 



TABLE 30 



GENERAL OPERATING CHARACTERISTICS OF 

 SEISMIC AMPLIFIERS 



Voltage Gain : 



Frequency 

 Response : 



Adjustable 

 Filters : 



Automatic Gain 

 Control : 



De-Coupled : 



Tubes : 



Power 

 Requirements ; 



Mounting : 



Components : 



100-150 D.B. (Usually measured from 200 ohm input to plate 

 of last tube.) 



2 to 200 cycles per second. 



Consisting of 1 or 2 high pass sections, and 1 low pass section 

 controlled by selector switches on the front panel. Provision 

 is made for choosing either 1 or 2 sections, depending upon 

 the type of filtering desired. 



Operating time should be from .075 to .15 second for large 

 ratio of signal change (such as "first breaks") and with a 

 slower rate for small ratios which remain substantially con- 

 stant regardless of the signal level or intensity, so that the 

 reflections have time to be reproduced with their normal con- 

 trast without interference of a "fast" automatic gain control. 



The amplifier should be sufficiently decoupled to permit use 

 of any group from a common power source such as a gene- 

 motor or vibrator power supply, without "cross-talk," feed 

 back, or coupling. 



Should be standard radio or television tubes, non-microphonic 

 mounted. 



1.5 to 6.2) volt filaments; 6 to 12-volt motor-generator; and 

 not to exceed 250* volts for plate supply. 



Front panel controls, with rack mounting to allow easy inter- 

 change of amplifiers for replacement or testing. Multi-circuit 

 connectors should be utilized for plug-in connections. 



Held to rigid specifications. Transformers and inductors 

 hermetically sealed, with multiple high-permeable and high- 

 conductive shields to permit operation of equipment in rela- 

 tively high magnetic fields such as may be encountered in 

 vicinity of power lines. All units should be interchangeable 

 both electrically and mechanically. 



*Some designers prefer to add an extra stage of amplification, and operate at a lower plate 

 voltage to minimize tube noises and leakage problems. 



