Chap. 9! 



SEISMIC METHODS 



439 



SYMBOLS USED IN CHAPTER 9 -Concluded 



tion method, which has been found to be apphcable to depths of 20,000 

 to 30,000 feet without any apparent relation to the thickness of the re- 

 flecting formation (provided it exceeds a certain minimum value, such as 

 25-50 feet). As the principal objective of seismic methods is a depth 

 determination of elastic discontinuities, it is necessary that these properties 

 remain reasonably constant in horizontal direction. This is true for most 

 oil-bearing structures in which formations have been changed compara- 

 tively little from their original position, and it is one of the reasons why 

 seismic prospecting has been applied so extensively in oil exploration. 



Uses of seismic prospecting in mining have been few. Regions in which 

 ore bodies are found are generally folded, faulted, intruded by igneous 

 bodies, and metamorphosed. The continuity of physical properties that 

 is so prevalent in sedimentary oil-bearing regions rarely exists in mining 

 areas. Further, ore bodies generally do not differ sufficiently in their 

 elastic properties from the surrounding rocks. Exceptions are the seismic 

 determination of overburden thickness, the location of gold-bearing gravel 

 channels, and structural investigations of carboniferous regions and 

 sedimentary ores. 



Applications of seismic methods depend on the degree of contrast in 

 elastic properties of geologic bodies with respect to the surrounding media. 

 In seismic exploration, differences in the speed of elastic waves in different 

 formations are measured. These depend on certain combinations of 

 Young's modulus of elasticity, Poisson's ratio, and density. The influence 

 of density counteracts that of the modulus of elasticity. However, with 

 an increasing degree of consolidation, the modulus of elasticity increases at 

 a greater rate than does density so that formations of a greater degree of 



