1048 



Subsurface Geologic Methods 



time consists of the field due to subsurface geologic bodies superimposed 

 on the terrestrial magnetic field for that latitude and longitude. 



Magnetic-Prospecting Instruments 



None of the instruments used in magnetic prospecting today measures 

 the magnetic-field intensity; without exception they compare intensities 

 from point to point over the surface of the earth. The most widely used 

 instrument, both in oil and mining prospecting, is the Schmidt-type 

 magnetic-field balance, which measures variations of the vertical com- 

 ponent of the magnetic field. 



The instrument consists essentially of a pair of magnetized blades 



S S, 



p s pole strength of magnet 

 V= vert, component of field 

 c= center of gravity of system 

 m= moss of system 

 g = force of gravity 



Figure 550. Schematic diagram of compensated-vertical-magnetometer system showing 

 position of latitude, temperature, and sensitivity-adjusting weights. 



pivoted on a horizontal quartz-knife edge. A magnetic couple, depending 

 on the strength of the vertical magnetic field, tends to rotate the blades 

 about the horizontal axis. The magnetic couple is opposed by a gravity 

 couple acting in the opposite direction. By observing the equilibrium posi- 

 tion, where magnetic couple equals gravity couple, between two points 

 on the earth's surface, we are able to determine the difference in vertical 



