888 EXPLORATION GEOPHYSICS 



mite charges positioned above the surface of the snow. These tests showed 

 that there was very httle absorption by the uncompacted snow layer if the 

 ampHtude was very low. Under this condition probably 95 per cent of the 

 energy was transmitted, whereas if the shot energy was confined to a small 

 area, resulting in high amplitude, probably 95 per cent of the energy was 

 absorbed over the same path. Generally, in penetrating the low-velocity 

 layer, the larger the particle size, the greater may be the amplitude before 

 the absorption becomes excessive. For a mixture of particle sizes, the 

 absorption covers a wider amplitude range, but in all cases, if the amplitude 

 of the wave motion is sufficiently low, the transfer of seismic wave energy 

 from one medium to another will be relatively efficient. 



When all of the explosive is concentrated at a point and buried just 

 beneath the top surface in the unconsolidated low velocity layer, the wave 

 front has the form of a short radius bubble with very high energy per unit 

 of area, and much of the energy is absorbed in useless work. The useful 

 seismic energy is produced largely by that part of the explosion energy 

 which (1) is directed substantially downward, and (2) does not produce 

 stresses greater than the elastic limit of the material surrounding the bore 

 hole. 



If the point explosive is placed in the more consolidated strata beneath 

 the low velocity layer, much greater unit stresses can be utilized, with an 

 increased percentage of the shot energy being converted into useful seismic 

 energy. 



Many investigators have tried to devise means of developing a sudden 

 downward force upon the surface of the ground, but in most cases, just as 

 is true for the explosion in a shot hole, the area of application is so small 

 that the required amplitude is far in excess of the elastic limit of the mate- 

 rial. By employing a horizontal pattern of small explosive charges, dis- 

 tributed over a relatively large area, a wave of low intensity per unit area 

 is created, and an increased portion of its total energy will be directed 

 downward, because the force is applied to the surface of the ground by 

 means of the air pressure wave. Since the force exerted by such a wave 

 can only be normal to the surface, the seismic wave initiated under the 

 pattern of charges is directed almost entirely downward. The total down- 

 ward force is calculated at about 2,000,000 pounds upon the surface of 

 the ground for each pound of total explosive used in a properly distributed 

 pattern. 



Thus a seismic wave of high total energy is produced in the ground, 

 but since it is distributed over such a large area, the amplitude is low 

 enough to be readily transmitted by the low velocity layer. Although the 

 edge efifect around the periphery of this fiat wave front tends to develop an 

 appreciable curvature, the dimensions of the initially flat portion of the 

 wave front are such that it retains much of its directive character while in 

 transit through the low velocity layer. 



