20 EXPLORATION GEOPHYSICS 



will indicate, first, whether use of a geophysical method is technically 

 justified and second, whether its use is economically sound. Technical 

 justification depends primarily on the possibility of coordinating the known 

 and/or probable subsurface geological conditions in any area with the data 

 which can be supplied by a specific geophysical method. Some of the 

 methods are uniquely applicable to certain geological and terrain condi- 

 tions. The selection of a suitable method requires, therefore, a study of 

 the theoretical applicability of the various methods to the specific problem. 



Economic justification for the use of geophysics in any case is mainly 

 to decrease the financial risk of subsequent exploration. Due to the com- 

 plexity of geological conditions in different areas, however, strict standard- 

 ization of cost is not possible in geophysical work. Generally, the cost can 

 be evaluated only after proper study has been given the geological and 

 other technical considerations. The interrelation of the cost factors and 

 the technical factors may be expressed by defining the purpose of a geo- 

 physical survey as the securing of maximum geologic or subsurface infor- 

 mation at minimum cost. 



Theoretically, the applicability of geophysical methods of exploration 

 may extend to any area in which the subsurface geological relations are 

 obscure and where additional subsurface information may serve to reduce 

 the financial risk or engineering hazard involved in future exploration. 

 Practically, specific fields of application as well as the choice of proper 

 methods to employ depend upon a great many considerations. Some of the 

 more important may be listed as follows: (1) amount of surface and/or 

 subsurface geological information already available, (2) local geology 

 (probable type of structure, etc.), (3) purpose of survey, (4) depth of 

 investigation required, (5) terrain conditions and geographic location, 

 (6) theoretical applicability of specific methods, (7) cost of geophysical 

 survey, (8) cost of alternative means of securing required information 

 (core drilling, etc.), (9) local land ownership, and (10) geographic loca- 

 tion of the area and available means of transportation. These various items 

 are discussed in greater detail in following sections, which outline the spe- 

 cific applications of the several methods in the fields of petroleum geology, 

 mining geology, civil engineering, and water supply engineering. 



Classification of Exploration Methods. — In the following tabula- 

 tion the various methods are listed in the generally recognized order 

 of increasing resolving powerf : i.e. their ability to obtain specific data 

 diagnostic of subsurface conditions. The term resolving power relates 

 only to the general use and applicability of the methods, because a method 

 which usually has a relatively low resolving power might under specific 

 favorable circumstances become a method of superior resolving power. 

 For example, magnetic methods, while generally of low resolving power 



t See also E. E. Rosaire, "On the Strategy and Tactics of Exploration for Petroleum," Journal 

 of the Society of Petroleum Geophysicists, Vol. VI, No. 1, July, 1935, pp. 11-26. 



