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INTRODUCTION 



I. SIGNIFICANCE OF GEOPHYSICAL EXPLORATION 



Geophysical exploration may be defined as prospecting for mineral 

 deposits and geologic structure by surface measurement of physical quantities. 



Geophysical exploration does not rely on magic or on any other super- 

 natural procedure. It makes use of phenomena which can be interpreted 

 fully through the fundamental laws of physics, measured, and verified 

 by anyone as long as suitable instruments are used. A psychological 

 reaction of the individual does not enter. Therein lies the difference 

 between geophysical exploration and the "divining rod" whose scientific 

 merits have never been established. There may be persons who can 

 "sense" the presence of subsurface geologic anomalies; however, if they 

 are so distinguished, they should have no need to surround their ability 

 with a veil of mysterious devices. Experience has shown that the divin- 

 ing rod, contraiy to geophysical instruments, will rarely give identical in- 

 dications at the same place or for different operators. 



Geophysical exploration may be considered an application of the prin- 

 ciples of geophysics to geological exploration. Derived from the Greek 

 i) yri and i) (pmis, the word geophysics means "physics" or "nature" of 

 the earth. It deals with the composition and physical phenomena of the 

 earth and its liquid and gaseous envelopes; it embraces the study of 

 terrestrial magnetism, atmospheric electricity, and gravity; and it includes 

 seismology, volcanology, oceanography, meteorology, and related sciences. 



The foundation to the development of most geophysical exploration 

 methods was laid by geophysical science. In the past century systematic 

 efforts were begun in all parts of the world to study earth's phenomena, 

 such as gravity, magnetism, earthquakes, and volcanism. Such studies 

 were expected to give information in regard to the constitution of the 

 earth's interior. Magnetic and gravitational surveys were organized by 

 government and state agencies, and observatories for recording meteoro- 

 logic, oceanographic, and earthquake phenomena were established. 



Practically every geophysical exploration method has been developed 



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