and upsetting the galvanometer whose geophone connector lay in the 

 water-saturated material. This can be avoided by placing connectors on 

 the head of the geophone, off the sand, or by using waterproof connectors 

 to the takeout cable. 



Seismic energy was provided by 60 percent gelatin explosive detonated 

 by an instantaneous electric blasting cap. Blasting caps of the seismic 

 variety are recommended due to the larger amount of current required to 

 fire them. Amounts of explosive varied from 1 ounce to 3 pounds; the 

 smaller amounts were used in areas of shallow bedrock and on clam flats; 

 the larger charges were required in dry dune sands. Blasting caps alone or 

 sledge hammer shock sources will not work in coastal sands. 



Explosive charges are always buried as deeply as possible to improve 

 energy transmission. This usually means a shovel-dug hole 2 to 3 feet deep 

 on the beach or a bar-punched hole in the marsh peats. The charge should 

 be placed in or near the water table because this allows a reduction in the 

 amount of explosive employed. Dry sand is an energy absorber which sup- 

 presses energy propagation unless the explosion can be coupled to the water 

 table. 



The ecological effects of explosives in the shore environment require 

 some comment. No more explosive should be used than is required to yield 

 a sharp time break at a particular gain setting, determined by existing 

 background noise. Therefore, three variables are related: background 

 noise, gain setting, and amount of explosive. They should be considered 

 in that order. Naturalists will often express a fear of large scars left 

 by explosion seismology. Such fears are unnecessary because charges up to 

 1/2 pound can be detonated 3 feet down in marsh peat without disturbing 

 the surface. Seismic charges in dune sand or other dry vegetated areas 

 cause little or no disturbance. Even the larger charges required to pass 

 energy in the loose material rarely leave a scar. Seismic profiles sea- 

 ward of the high-tide mark on beaches or on intertidal sand bodies without 

 shellfish can be run without undue concern for amount of explosives used. 

 The incoming tide erases the craters . 



Shellfish flats do present a problem. Shell fishermen should be in- 

 formed about the nature of the work. Profiles should be located over areas 

 of sparse shellfish population; gain is then increased and charges as small 

 as 1 ounce can locate a bedrock profile as deep as 40 feet. 



Safety is always foremost when using explosives. Only qualified ex- 

 plosive experts should handle and load explosives, or wire the shot holes. 

 The crew should be large enough to keep onlookers from the blasting area, 

 an acute problem in popular beach areas. Blasting caps and dynamite should 

 be stored and transported in separate magazines. If possible, these ex- 

 plosives should be transported in a vehicle used solely for that purpose, 

 rather than in one carrying personnel or seismic equipment, particularly 

 sparking (iron or steel) tools. 



