Chap. 1] INTRODUCTION 5 



tative analogies too far into unknown territory. Failures resulting from 

 such procedure are forceful reminders that geophysics does not locate 

 specific deposits, but furnishes only a physical indication which must be 

 interpreted conservatively in geologic terms. 



In this connection a word may be said about the present status and 

 future possibilities of methods for direct location of oil, gold, or water. 

 At present there is no established direct means for finding oil; it is located 

 indirectly by mapping geologic structures which, from experience, are ex- 

 pected to be favorable for the accumulation of oil. Gold in placer chan- 

 nels may be located indirectly by tracing magnetic black sand concentra- 

 tions, provided that the geologic association of gold with black sand h&s 

 been established. Water is difficult to locate; the indications require 

 careful interpretation in the light of the electrical characteristics and dis- 

 position of near-surface beds. On the other hand, recent developments 

 along the line of direct methods indicate definite possibilities. The best 

 chances for direct oil location are in the fields of electrical prospecting and 

 soil and gas analysis. Electrical induction methods show promise of suc- 

 cess in locating placer gold concentrations and water. 



As indicated above, the greater number of applications of geophysical 

 methods are of an indirect nature. If a mineral, rock, or formation does 

 not have any distinguishing physical properties, another mineral or geologic 

 body may be utilized which has such properties and bears a known rela- 

 tion to the ineffective body. The location of oil by the mapping of struc- 

 tures which provide a trap for oil (anticlines, salt domes, faults, and buried 

 hills), and the location of ore bodies by determining associated structure 

 are examples of indirect procedure. 



III. MAJOR FIELDS OF GEOPHYSICAL EXPLORATION 



Since geophysical exploration is the determination of subsurface geologic 

 structure by means of surface physical measurements, it is applicable in 

 industrial fields where a knowledge of geologic conditions is essential. It 

 is understood that such applications are advisable only where structures 

 and ore bodies are not exposed, as most geophysical measurements are 

 more expensive than surface geological surveying. 



At present the greatest use of geophysical prospecting is made in oil 

 exploration. In this country relatively few oil areas exist where geologic 

 structure is exposed at the surface; in the majority of cases, the deeper 

 formations are concealed by (frequently unconformable) younger strata. 

 This is particularly true for the entire Gulf coast, the Midcontinent, the 

 Great Plains, Western Canada, and a part of California. The Gulf coast 

 has seen the most extensive geophysical activity because conditions exist- 



