CHAPTER I 



INTRODUCTION 



Geophysics is a study of the physics of the earth with special reference 

 to its physical properties, structure, and composition. It is commonly 

 divided into two general branches. The first includes the subdivisions of 

 the physical sciences such as electricity, magnetism, chemistry, heat, elas- 

 ticity, etc., while the second branch pertains to the study of the three major 

 components of the earth: solid (lithosphere), liquid (hydrosphere) and 

 gas (atmosphere). The wide diversity of subject matter embraced in the 

 field of geophysics may be more thoroughly appreciated by reference to 

 Table l.f 



Exploration geophysics is the art of applying the physical sciences to 

 the study of the structure and composition of those layers of the earth 

 which are sufficiently shallow to be exploited by man. This book is pri- 

 marily concerned with the application of geophysics to the outer portion 

 of the earth's crust, especially in the solution of problems of structural 

 and economic geology. This application may embrace practical engineering 

 techniques at one extreme and the methods of mathematical physics at 

 the other. 



Exploration geophysics has evolved from the practical use of the knowl- 

 edge accumulated by many investigators working independently in ditferent 

 parts of the world. Although it is a comparatively young science and 

 until 25 years ago no authoritative English text on the subject had been 

 compiled, vague indications of its beginnings may be said to exist in ancient 

 Chinese and in medieval literature. For centuries the divining rod was the 

 most common device used in the attempt to locate valuable substances 

 concealed underground. The success of this method actually depended 

 more upon the law of averages than on the psychic powers of the man 

 with the hazel twig, who was certain to be right once in a while, particularly 

 when looking for water. Commercial application was therefore rather 

 limited, although, as one writer states, the divining rod has been used in 

 the quest for "water, minerals, witches, criminals, Protestants, hidden 

 treasure, lost animals, and the points of the compass."| Any exhaustive 

 study of this method of exploration would properly belong to the field of 

 psychology. 



However, many of the fundamental laws and theories of present-day 

 geophysics were developed by scientists of the past few centuries, most of 

 whom probably had no thought of the direct application to economic geol- 



t B. Gutenberg, "Geophysics as a Science," Geophysics, Vol. 2, July, 1937. 

 i A. S. Eve and D. A. Keys, Applied Geophysics, p. 7. 



