SEISMIC METHODS 



837 



progress is usually better with the more elaborate and more sturdy truck- 

 mounted instruments. The lighter- weight, portable type of equipment may 

 also be transported by helicopter for work in swamps and other difficult 

 terrain. Helicopter operations improve the time factor until approximately 

 the same production may be obtained as with truck-carried apparatus over 

 average terrain. 



Portable seismograph equipment, like its heavier counterpart for truck 

 mounting, must be designed to withstand fungus growths, corrosion from 

 intense humidity, and a certain amount of field hazard in the form of 



accidents resulting from rough usage. The lighter weight in the portable 

 equipment is obtained at the expense of a certain amount of ruggedness 

 and operational completeness. 



Recording Trucks and Reels. — -The recording trucks shown by Fig- 

 ures 523 and 524 illustrate common practice as regards the instrument 

 compartment and the arrangement of reels. In Figure 524 the reels are 

 kept under cover when not in use, whereas in Figure 523 an arrangement 

 is shown where the cable and reels are open to the weather. In both cases 

 the reels are power driven. Both trucks are of the four-wheel drive type, 

 making possible the negotiation of very difficult terrain. 



