SEISMIC METHODS 



813 



of twelve separate channel amplifiers mounted on one stainless steel chassis 

 is given in Figure 499. All units are hermetically sealed against humidity 

 and made to plug into a standard vacuum tube socket. When provided with 

 standard tube prong base, a transformer or a coupling unit between ampli- 

 fier stages may be pulled out and replaced in a few seconds, exactly as in 

 changing a vacuum tube. All compo- 

 nent parts are mounted above the 

 chassis ; only the wiring is below. Test 

 equipment and charts are designed so 

 that each component may be instantly 

 checked for defects, allowing rapid 

 trouble-shooting in the field. The de- 

 sign of this type of amplifier is such 

 as to facilitate repair and upkeep 

 rather than to withstand all field haz- 

 ards. These twelve amplifiers with 

 expander type volume control, exclu- 

 sive of dry cell batteries, weigh only 

 thirty-seven pounds. This type of 

 portable equipment has proven satis- 

 factory for difficult terrain conditions. 



Fig. 499. — Twelve separate channel am- 

 plifiers, mounted on a single stainless steel 

 chassis using plug-in elements. (A) input 

 transformer; (B) output transformer; (C) 

 interstage coupling; (D) battery plug; (E) 

 input plug; (F) output plug. (Courtesy of 

 Geophysical Engineering Corporation.) 



Shielding. — All parts of the elec- 

 trical system (seismometers, connect- 

 ing cables, amplifiers, recording 

 camera, and controls) are preferably 

 shielded against disturbing magnetic 

 and electrostatic fields. This is 



particularly advantageous when working in the vicinity of high ten- 

 sion power lines, street car systems, etc.* The circuits carrying the 

 minute currents generated by the seismometers must be well insulated 

 from the case of the seismometer and from the ground. Variable earth 

 potentials are frequently present in the ground between the locations 

 of the various seismometers and the recording truck. These earth 

 potentials due to their varying magnitude may cause disturbances if picked 

 up by the input circuit. For use in the tropics, the equipment should be 

 protected against fungus. Metal parts should be cadmium or chrome plated. 



Instrument cases, as well as the cases of the seismometers, are usually 

 of iron or aluminum and serve both as electric shields and as mechanical 

 protection for the instruments. 



Photographic Recording. — The photographic method of recording, 

 because of its sensitivity, reliability, and accuracy, has superseded practically 



* A method of eliminating the effect of extraneous electrical disturbances, with 

 particular reference to power line interferences, ground motions, etc., is described 

 by K. C. Woodyard, C. A. Putnam and H. R. Prescott, "Means and Method of Making 

 Geophysical Explorations," U. S. Patent 2,164,196, issued June 27, 1939. 



