492 



SEISMIC METHODS 



[Chap. 9 



Shot holes are tamped with water. Most reflection parties have a water 

 truck following the drilling truck. Drilling costs may range from 25 cents 

 to over a dollar per foot, depending upon conditions. Seismic-hole drilling 

 may be contracted at $1000 to $1500 per month. Charges are generally 

 fired from a shooting truck set up as close to the shot point as practicable. 



5)rM- 



■'/■///JF 



5uch'oti 

 Hose 



Fig. 9-29. Evinrude centrifugal pump for drilling shot holes in swamps. 



Z ^.^ 1 



In earlier refraction shooting 6nmd AmplHudt 

 some companies fired charges /" 



by radio from great distances 

 but this practice is now aban- 

 doned. 



It has been demonstrated ' 



that no changes in travel times 

 occur when charges are 

 changed as much as ten times 

 in energy, that is, in quantity 

 or in strength. As shown in 

 Fig. 9-30," the ampHtudes 

 generally increase in propor- 

 tion to the square root of the charge. 



3. Energy transmission and absorption. It is difl&cult to determine 

 accurately the relation between distance and record amplitude, because 

 such measurements make it necessary to move either shot points or ob- 

 servation points. If the shot point is moved, the energy transferred to 

 the ground changes with local conditions at the shot point. If the re- 



400 600 1200 1600 2000 l^U 2800 



dnms Chtr^e 



Fig. 9-30. Variation of amplitude with charge 

 (after Rixman). (1) Shots fired in dry sand; (2) 

 shots fired in moist sand; ai, ground amplitude 

 in microns. 



"F. Rixmann, Zeit. Geophys., 11(4/6), 197-207 (1935). 



