4 INTRODUCTION [Chap. 1 



from corresponding procedures in geophysical science. While the objects 

 of these earlier studies were the broad regional features, present geophysical 

 methods aim at the location of local geologic structures and mineral de- 

 posits. This development resulted from a substantial increase in accuracy 

 and reliability of field methods. It has been coincident with the advances 

 made in physics generally and in electricity in particular and received its 

 impetus from the need for raw materials during and after the World War. 

 Geophysical exploration may be called the application of the principles of 

 geophysical science to (commercial) problems of smaller geologic scale. 



Experience has demonstrated that most subsurface structures and 

 mineral deposits can be located, provided that detectable differences in 

 physical properties exist. The main properties exhibited by the more 

 conmion rocks and formations are: density, magnetism, elasticity, and 

 electrical conductivity. This entails four major geophysical methods: 

 gravitational, magnetic, seismic, and electrical. 



11. GEOPHYSICS AS A TOOL FOR DETERMINING GEOLOGIC 



STRUCTURE; EXCEPTIONS; INDIRECT MINERAL 



LOCATION 



The first objective of geophysical exploration is the location of geologic 

 structures; as a rule, information regarding the occurrence of specific 

 minerals is obtained only in an indirect manner. 



The geophysicist measures, at the earth's surface, anomalies in physical 

 forces which must be interpreted in terms of subsurface geology. In 

 many cases he has to be content with a general statement that a given 

 area is structurally high or low (as in oil exploration), or that a zone of 

 good or poor conductivity exists (as in mining). In some instances, how- 

 ever, an appreciation of the geologic possibilities and a background of 

 experience obtained by working in similar areas makes it possible to in- 

 terpret surface anomalies more specifically. Assume, for example, that 

 an iron ore deposit has been traced by magnetic instruments. When 

 surveying adjacent properties, one would, therefore, be justified in attribut- 

 ing large magnetic anomalies to the same ore. In a different area, how- 

 ever, large magnetic anomalies may result from entirely different geologic 

 bodies, such as intrusions of igneous rocks or contact-metamorphic zones. 



Other definite geophysical indications are: gravity minima on salt 

 domes, magnetic highs on basement uplifts, seismic refraction travel-time 

 curves typical of salt domes, electrical indications characteristic of sulfide 

 ore bodies, and so on. In such cases interpretation of findings in terms of 

 definite mineral deposits has been very successful. On the other hand, a 

 geophysicist unacquainted with geologic possibilities may carry interpre- 



