848 



EXPLORATION GEOPHYSICS 



Streams is sufficiently close to permit the field men to reach much of the 

 area in small power cruisers or boats. In this case the procedure is identical 

 with the procedure on land, with the exception that the equipment is 

 mounted in boats instead of in trucks. Special precautions must be exercised 

 in the design of equipment in order to prevent damage due to exposure ; for 

 example, the seismometers are placed in water-tight cases. 



Frequently, however, the swamp is cut up by streams which are too 

 small to permit the use of boats and too wide to be crossed by marsh 

 buggies. In order to avoid the necessity of transporting the amplifiers 

 and recorders across difficult marsh by foot, a method of "remote control" 

 is employed wherein the seismometers are located at a great distance from 



Fig. 531. — Laying cables in swamp areas. 



the amplifiers. Remote control work often has been done with the seismom- 

 eters as much as five miles from the amplifier ; in these cases, it is necessary 

 to separate the wires enough to prevent cross-feed from one line to the 

 next due to mutual inductance between lines. (In difficult swamps this 

 requires a considerable amount of labor.) 



Figure 531 illustrates working conditions often encountered in laying 

 cables in swamp areas. 



Recent applications of helicopter service to geophysical operations 

 have overcome many of the obstacles encountered in swamp lands, rough 

 terrain and in virgin country without trails or roads. Figure 532 illustrates 

 helicopter operations in swamp areas. 



In this type of operation, two helicopters are usually employed, one 



