9 



SEISMIC METHODS 



I. INTRODUCTION 



Seismic procedures are in the group of "indirect" geophysical methods in 

 which extraneous fields are set up and reactions of subsurface conditions 

 to such fields are measured. Seismic exploration is concerned with the 

 investigation of elastic forces. Contrary to other indirect geophysical 

 methods — such as electrical ones — these fields of elastic forces are not 

 stationary or quasi-stationary but vary and propagate with time. In 

 common with electrical methods, the depth of investigation in seismic 

 exploration may be controlled by varjdng the spacing between transmis- 

 sion and reception points.^ This gives both the seismic and electrical 

 methods great interpretational advantages over the gravitational and 

 magnetic methods, since the indications furnished by the latter two repre- 

 sent the integral effects of all masses from great depths up to the surface. 

 Direct and quantitative determinations of depths are the exceptions in 

 gravimetric and magnetic work, but they are common practice in seismic 

 and resistivity methods. A further advantage of seismic methods lies in 

 the fact that not only the depths of geologic bodies but also some of their 

 physical properties may be obtained. 



Seismic and resistivity methods are, therefore, particularly well adapted 

 to determinations of horizontal or nearly honzontal formation boundaries. 

 This does not mean that other types of geologic bodies may not be well 

 adapted to seismic exploration. Seismic refraction shooting has attained 

 great practical significance in the location of salt domes because of the 

 distinct geometric disposition and elasticity contrast existing in such 

 domes. 



Like most other geophysical methods, seismic methods depend for their 

 successful application on the size of geologic bodies to be located. Both 

 the refraction and resistivity methods depend on the thickness of forma- 

 tions compared with depth. This does not appear to hold for the reflec- 



^ With the exception of the reflection method. 



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