INTRODUCTION 11 



developments, although a large number of smaller ore bodies, aggregating 

 a most important economic value, have been located. In America there 

 have been few major mineral discoveries since 1915, and in Europe none 

 since 1850. 



Recognition of the influence of subsurface structure on the current 

 distribution at the earth's surface was followed by the use of electrical 

 methods for structural mapping, with many successful applications to oil 

 prospecting and civil engineering problems. Deep structural mapping by 

 electrical methods has been most successful in the Permian basin of New 

 Mexico and West Texas. There the geologic section comprises one to two 

 thousand feet of recent depositional material, underlain with a salt and 

 anhydrite section of variable thickness from a few feet to over a thousand 

 feet. Beneath the anhydrite section lie the sandstones and limestones con- 

 taining the petroliferous source beds. Commercial accumulations of petro- 

 leum occur in closed structures and stratigraphic traps. 



The variable thickness of the anhydrite section has complicated suc- 

 cessful application of seismic and gravitational methods, while the electrical 

 methods have proved to be more applicable due to the large differences in 

 electrical conductivity of the lithologic units constituting the section. 



Electrical logging for determining the character and thickness of the 

 strata penetrated by a drill hole was developed commercially by C. and M. 

 Schlumberger in 1928. This technique with modifications has now become 

 an accepted step in oil well drilling and structure correlation. In explora- 

 tion work for mapping of subsurface structure the electrical logs are 

 correlated between drill holes. 



Popular interest in radio has focused attention on electrical methods of 

 prospecting, and the rapid progress in radio communication has led to 

 repeated attempts to locate ore bodies and oil by radio. To date, these 

 methods have not been of commercial value, due to the poor penetrating 

 power of high frequency currents and electromagnetic fields. Recent 

 theories linking near-surface mineralization with deeper-seated petroleum 

 deposits and structural conditions have revived interest in resistivity 

 measurements for shallow stratigraphic studies. At the present time the 

 economic usefulness in petroleum exploration of these shallow penetration 

 methods is controversial. 



Seismic Methods. — Perhaps the person most worthy of being called 

 the first seismologist was John Michell. In 1761, Michell published a paper 

 in which he stated that the motion of the ground produced by earthquakes 

 was transmitted as elastic vibrations through the earth's crust. In the same 

 memoir, Michell suggested that observations on the time of shock at 

 several places would permit the determination of the place of origin of the 

 earthquake. As early as 1855, a number of seismographs using electro- 

 magnetic recording had been constructed in Italy by Palmieri. Robert 

 Mallet, the first investigator to suggest the term "seismology" and the 

 discoverer of the refraction method, did much to create a widespread 



