Chap. 9] 



SEISMIC METHODS 



491 



local labor is cheap. In most cases truck-mounted rotary drilling machines 

 are used (Figs. 9-27 and 9-28). In soft ground holes may be "washed 

 down" by the use of centrifugal pumps driven by gasoline engines (Fig. 



Oeorge E. Failing Supply Co, 

 Fig. 9-28. Failing seismic shot-hole drill; view of entire unit. 



9-29). Hole caving may be prevented by the use of Aquagel or lime, or 

 by the use of casing^® which can usually be recovered and used again. 



" A light-weight casing, made of plastic, has recently been put on the market. 



