6 INTRODUCTION ' [Chap. 1 



ing there are virtually ideal for geophysical exploration. At present 

 seismic and gravitational methods for mapping oil structures dominate 

 the field; electrical well-logging is widely used for the purpose of correlat- 

 ing formations by their resistivity and for identification of oil sands. It 

 has been estimated that the oil industry spends between 15 and 20 million 

 dollars annually for geophysical field work and laboratory research. 



Compared with the oil industry, the mining industry has made relatively 

 little use of geophysical exploration although there have been more pub- 

 lished accounts of mining surveys. Various reasons account for this lack 

 of geophysical activity. (1) The small size of the average ore body makes 

 it impossible to cover systematically township after township, as in oil 

 surveying. (2) Large industrial groups capable of financing extensive re- 

 search and exploration programs are few. (3) In mining areas geology is 

 frequently known from outcrops, so that a determination of subsurface 

 structure so important in oil exploration is less necessary. (4) Structural 

 relations and dispositions of ore bodies are usually complex, making inter- 

 pretation of geophysical data more difficult. (5) Many geophysical 

 methods are adversely affected by the rugged topography prevalent in 

 mining districts. (6) Seismic methods, at present most prominent in oil 

 exploration, have found little application in mining because dynamo- and 

 contact-metamorphic agencies have obliterated original differences in elas- 

 ticity between formations. (7) Transportation in mining regions is diffi- 

 cult and inadequate. In spite of these handicaps, the application of 

 geophysics in mining is often more fascinating to the geophysicist than in 

 oil because of the greater variation in method and procedure. Not only 

 may structural investigations be made, but the ore itself may produce 

 indications; further, associations of the sought but ineffective mineral 

 with noncommercial but physically effective minerals may be utilized. 

 Generally speaking, the planning and execution of a geophysical survey in 

 mining and oil are quite different; further details are presented in 

 Chapter 5. 



A third major field for geophysical work is that of engineering, encom- 

 passing civil engineering, engineering geology, and allied fields such as 

 military, structural, gas, and pipe-line engineering, and the like. Geophys- 

 ics is being applied to problems involved in dam-site and tunnel investiga- 

 tions, determination of foundation conditions in highway and railroad 

 construction, location of construction materials for highway and railroad 

 work, water location, detection of corrosion and leakage in gas and water 

 pipes, investigation of building and road vibrations, and so on. Military 

 engineering has utilized geophysics in similar ways; Chapter 6 covers these 

 topics in more detail. 



