6 EXPLORATION GEOPHYSICS 



world. The purpose of the earHer investigations was to obtain information 

 on the distribution of the earth's magnetism with particular reference to 

 navigation problems, any geological conclusion being purely secondary. 

 An impetus to practical geomagnetic investigations for exploration pur- 

 poses was given by the construction of the Schmidt field magnetometer in 

 1915. This was the first rugged and portable instrument capable of detect- 

 ing local magnetic anomalies of small magnitude. 



Gravitational Methods. — The variation of gravity at different loca- 

 tions on the surface of the earth was discovered by Jean Richer in 1672, 

 when he noted that a clock whose pendulum was calibrated to beat seconds 

 in Paris lost about two and one-half minutes a day in Cayenne, French 

 Guiana. It was known that the time of swing of a simple pendulum is a 

 function of its length and the pull of gravity. Richer concluded that since 

 the length had not altered, the variation in time must be due to the fact that 

 the gravitational value in Cayenne differed from that in Paris. This expla- 

 nation indicated that the gravitational attraction might be expected to vary 

 at diflferent locations on the earth and suggested a method of determining 

 this variation, namely, the method of pendulum observations. 



The first application of this method to a study of the shape of the earth 

 was made by another Frenchman, Bouguer. Working on the measurement 

 of a terrestrial arc, he led an expedition to Peru (1735-1743) during which 

 he made original use of the invariable pendulum idea and achieved the 

 first relative gravity measurements. His name designates corrections and 

 anomalies employed in making such measurements today. f 



The next important advance was the invention of the reversible pendu- 

 lum by H. Kater in 1818. This type of pendulum is still employed for 

 precision measurements of absolute gravity. The difficulty of calibrating 

 a reversible pendulum accurately and the tediousness of the extreme pre- 

 cautions required in determinations of the absolute gravity led to the use 

 of the so-called invariable pendulums. Invariable pendulums are suitable 

 for determinations of the relative gravity at a large number of stations. 

 This type of pendulum comprises a bob rigidly attached to a "massive" rod, 

 in place of the "weightless" rod of the simple pendulum. The type of 

 invariable pendulum most widely employed at present is patterned after 

 that of the Austrian geodesist, von Sterneck. 



Sir John Herschel, in a book published in 1833, suggested the first 

 gravity meter wherein the relative displacements of a bob, suspended by a 

 spring, would serve as a measure of the relative pull of the earth, i.e., the 

 gravity, at various locations. 



During the latter part of the 19th century and the early part of the 

 present century, various gravity meters having an accuracy higher than that 

 of the Herschel instrument were proposed. The Threlfall-Pollack gravity 

 meter and the other meters proposed prior to 1918 were not widely used, 

 due to the fact that their accuracy was even less than that of the pendulum. 



t E. A. Eckhardt, "History of gravity methods of prospecting for oil," Geophysics, V, 3. pt. 1, 

 July, 1940. 



