Chap. 2] 



METHODS OF GEOPHYSICAL EXPLORATION 



Some of the minor geophysical exploration or detection methods make 

 use of the elastic properties of the surface soil (dynamic soil testing) and 

 of the water and atmospheric air (acoustic detection). Other methods 

 involve the detection of thermal effects (geothermal well testing), the detec- 

 tion of gases, and the mapping of radioactive radiations (see Table 2). 



Table 2 

 SUMMARY OF MINOR GEOPHYSICAL METHODS 



In the following sections, a summary of both major and minor geo- 

 physical methods is presented, with special reference to general principles 

 involved, instruments used, corrections applied, and interpretation pro- 

 cedure. 



II. GRAVITY METHODS 



General. Variations in the gravitational field may be mapped by the 

 pendulum, gravimeter, and torsion balance. The pendulum and gravim- 

 eter measure relative gravity, whereas with the torsion balance, the 



