856 



EXPLORATION GEOPHYSICS 



regions where the upper low velocity layer does not permit effective 

 transmission of energy, where extraneous energy is excessive, and where 

 the subsurface section is too complex to permit of an analysis, such as 

 in severely fractured zones. 



With time, as the more likely and readily investigated regions are 

 explored, reflection shooting will be forced into greater refinements. The 

 reflection shooting of the future will require greater depth of penetration, 

 greater accuracy in detecting the low relief structural features previously 

 overlooked, and more effective analysis to interpret data from complex 

 structural and stratigraphic conditions, such as overlaps, stratigraphic 

 traps, etc. 



GENERATION OF SEISMIC WAVES 



Various methods are utilized in seismic exploration work for initiating 

 elastic waves. The methods may be classified as (1) mechanical and (2) 

 explosive. (Within their depth ranges, essentially similar seismograms are 

 obtained from either source.) 



i'p/a^ 





Fig. 534. — Impact stake suitable for shallow seismic 

 investigations. 



Generation of Seismic Waves by Mechanical Means. — Mechanical 

 methods are occasionally employed for shallow mining and bedrock in- 

 vestigations, chiefly to avoid the danger and liabilities incident to the use 

 of the more convenient and efficient explosive methods. 



Mechanical methods are usually of the impact type, utilizing a heavy 

 weight dropped from a known height or the blow of a heavy sledge. For 

 general bedrock determinations and similar shallow studies, a ten-pound 

 sledge and an impact stake of the type shown in Figure 534 may be 

 employed. The purpose of the large surface of the stake is to provide 

 a means for relatively efficient transmission of energy from the hammer 

 to the soil. In regions where rocks outcrop, the impact stake may be 

 omitted and the blow delivered directly to the surface of the rock. 



