850 EXPLORATION GEOPHYSICS 



in several closely spaced shot-points have been used to achieve sufficient 

 energy. (See section on Multiple Shot-points.) This procedure is seldom 

 employed because very small charges are difficult to load, causing time 

 delays. 



The depth of the shot is determined by the character of the subsection 

 and must be below the low velocity, or so-called weathered zone, because 

 absorption of energy is very great in the low velocity zone. The base of this 

 zone frequently coincides with the top of the water table. A preliminary 

 idea of the necessary depth for shot-holes in a new area can be obtained, 

 therefore, from local water well records. When the medium immediately 

 below the low velocity zone is stratified, it is preferable to seek out that 

 stratum which yields highest efficiency of transmission of energy and to 

 load the explosive at that depth. Shale usually transmits the shot energy 

 more efficiently than sand or unconsolidated material, while the less porous 

 rocks are still more effective. 



The usual procedure for ascertaining the optimum shooting depth in a 

 new area is: (1) to determine the depth of the low velocity zone from 

 shallow refraction shooting, (2) to shoot light shots up-hole to obtain an 

 up-hole low velocity diagram, (3) to shoot at various depths to obtain 

 sample records. The drilled hole depth must exceed the depth of the bottom 

 of the low velocity zone by an amount sufficient to insure that the top of 

 the dynamite string is below the base of this zone and that the area of 

 shattering is within the "consolidated" zone. Additional leeway is given 

 when doubt exists as to the exact thickness of the low velocity zone. 



Sufficient head of water must be maintained above the charge in order 

 to keep the explosive from expending an excess of its energy up the hole. 

 (The shooting truck usually carries a tank of water for this purpose.) 

 Sometimes the subsurface water will maintain a sufficient head that no 

 extra water need be added to the hole. 



An outline of optimum shot-point procedure for efficiency, safety and 

 quality of data is presented by Williams,f together with a review of some 

 of the characteristics of seismic explosives. 



Seismometer Disturbance 



The seismometers are generally buried in shallow holes or pits which 

 are dug either with a post-hole auger or with spades. The depth of the 

 hole should be at least sufficiently great to allow the top of the instrument 

 to be flush with the ground in order to minimize wind disturbances. 



The recent trend in reflection shooting is toward a large number of 

 recording channels and the use of several seismometers on each channel. 

 This has necessitated the use of numerous seismometers and has given an 

 impetus to placing the seismometers on the surface of the ground to con- 

 serve time and labor. When the seismometers are to be placed on the surface 



t F. J. Williams, "Notes on Shot-point Procedure," Geophysics, Vol. XI, No. 4, Oct., 1946, 

 pp. 443-456. 



