SEISMIC METHODS 



849 



for transporting personnel and supplies, and the other for moving the 

 recording equipment, geophones and cables. When moving a short distance 

 to another line of profile, both helicopters are used for transporting the 

 geophone cable, one on each end. Both helicopters are employed when 

 moving the drilling crew and equipment. To see this operation would 

 amaze and delight some of the old-timers who remember swamp operations 

 in the late twenties and thirties. It is a much faster operation than by 

 marsh buggy, and production records approach those of normal automobile- 

 equipped crews operating over poor roads. Generally the equipment is of 

 the portable type, with a reduced crew personnel. The usual heavy-duty 

 drill is dispensed with, and in its place is provided a light-weight drill or 

 one of the small hydraulic units for digging shallow shot holes. 



Fig. 532. — Helicopter service applied to swamp areas in seismic prospecting. (Courtesy of 

 Seismograph Service Corporation.) 



(a) Helicopter with basket underneath for carrying equipment and supplies. 



(b) Helicopter pulling geophone cables. 



(c) Portable drilling unit in operation. 



(d) Helicopter carrying disassembled portable drill, ready to take off for next drilling location. 



Amount and Depth of Explosive 



For economic reasons the charge of dynamite is made as small as consis- 

 tent with adequate refraction or reflection energy.* Especially in reflection 

 shooting, other reasons also may dictate this rule. With increasing charge, 

 the useful wave energy does not rise proportionately to the magnitude of 

 charge, due to increased mechanical pulverization of the earth in the vicinity 

 of the shot-hole. Also, the energy of the surface waves generally increases 

 more rapidly with the amount of dynamite than does the refracted or 

 reflected energy. The usual experience is that a charge that exceeds some 

 certain amount has scarcely any advantage and may be a definite detri- 

 ment. However, sufficient dynamite must be shot so that adequate ampli- 

 tude of ground movement due to reflection or refraction energy is pro- 

 duced at the seismometers. It also is essential that the reflection or 

 refraction energy be greater than the extraneous energy in the recorded 

 band of useful frequency. In certain special cases simultaneous explosion 



* See pages 857 to 863 for detailed discussion of explosives and their characteristics. 



