4 EXPLORATION GEOPHYSICS 



alone was 10,178 feet.J The cost of the exploratory drilHng to these deep- 

 seated oil deposits is a prohibitive gamble without reliable data as to the 

 existence of favorable geologic structure. 



The estimated proved oil reserves in the United States are in excess 

 of those known ten years ago, in spite of the fact that more petroleum has 

 been produced than was previously assumed to exist. An estimated total 

 of about 60,000 million barrels of oil have been discovered ( 1947) , of which 

 about 36,000 million barrels have been produced, leaving 24,000 million 

 barrels in the ground as proved oil reserves. § Of this reserve, the state of 

 Texas has about 50 per cent, California 13 per cent, Oklahoma 4 per cent, 

 Louisiana 6 per cent, and New Mexico 4 per cent (approximate figures), 

 with the balance chiefly in Kansas, Wyoming, Illinois, Arkansas, and 

 Pennsylvania. It is probable that many new fields will be found to augment 

 this supply materially. Exploration geophysics has played an important 

 part in the discoveries of the last 25 years, and it will be only by the 

 improvement of present methods or the development of new methods that 

 the recent rate of discovery can be maintained. 



Each geophysical method has had its period of initial trial, followed by 

 its peak commercial application over a period of a few years. As the areas 

 amenable to a particular method have been studied and covered, the appli- 

 cation or popularity of that method decreased and it was replaced by other 

 methods. The succession of one method after another sometimes followed 

 rapidly. At other times there has been considerable lapse of time between 

 peak applications of various methods. Each method has had a limited 

 period of maximum commercial activity, because it either eventually cov- 

 ered the most amenable areas, or else was succeeded by a newer technique 

 having economic advantages. 



HISTORY OF DEVELOPMENT 



Magnetic Methods, — Probably the first geophysical instrument had 

 its beginning in the discovery that a lodestone, or a piece of a certain kind 

 of iron that has contacted a lodestone, will orient itself approximately in 

 a north-south direction. Early Chinese literature indicates that this orienta- 

 tion property of lodestone or magnetized iron was known and utilized some 

 time during the period from 2637 B.C. to 121 A.D. Peregrinus in 1269 

 discovered the "magnetic poles" and named them North and South. His 

 investigations further proved that unlike poles attract and like poles repel 

 each other. He also made the important discovery that fragments of 

 broken magnets behaved as the original magnets. In 1492, on his voyage 

 to America, Columbus noted that his compass deviated from astronomical 

 north. Hartmann in 1544 discovered the inclination of the compass. In 

 1581, Robert Norman proved that magnetization of a steel needle had no 

 effect upon its weight. This prepared the way for further studies of 



t Frederic F. Lahee, "Statistics of exploratory drilling in 1947," Geophysics, June, 1948. 

 § Ihid, p. 865. 



