890 EXPLORATION GEOPHYSICS 



to 1 inch have given excellent results. A backing material such as one inch 

 of sand on top of such a charge reduces the quantity of explosive required 

 by about one-third. Flat charges should be placed on a heavy cardboard or 

 beaverboard platform attached to a block of wood or a paper tube which 

 will fit onto the top of the stake to keep the explosive at a minimum distance 



Fig. 560. — A twelve charge pattern of two pound charges using a spacing of 15 feet 

 between charges. (Courtesy of Stanford Research Institute.) 



Fig. 561. — A thirteen-charge, six-pointed star pattern photographed from an eleva- 

 tion of 200 feet above ground. The spacing is 60 feet. (Courtesy of Stanford Research 

 Institute.) 



of 4 or 5 inches from the top of the stake. The cap should be placed at the 

 center of the charge. 



An equally effective charge in producing a flat wave front directed verti- 

 cally downward is a cylindrical charge having a length-to-diameter ratio 

 of six to one or greater, mounted in a vertical position, and with the cap 

 in the middle or in the top end of the stick. This charge can be mounted on 

 a wooden spike on top of the stake. This is a very convenient type of charge 



