1046 



Subsurface Geologic Methods 



in the United States. The unit of measurement in magnetic prospecting 

 is the gamma, which is equal to one-hundred-thousandth part of an 

 oersted. 



1 gamma = ^ oersted or 10"^ oersted 



In prospecting with the air-borne magnetometer, relative measure- 

 ments are made of the total intensity; in prospecting with magnetic-field 



,<^^' 



TOTAL INTENSITY 

 Vertical Component 



Hor.Camp.= Maximum 

 Vert.Comp- « 

 Inclinations 0* 



Hor. Comp 

 Vert.Comp. = Max 



Inclination =90 



Figure 548. Simplified diagram of the earth's magnetic field. 

 (Adapted from Jakosky.) 



balances, the vertical or horizontal components are compared. Certain 

 techniques in mining exploration involve comparative measurements of 

 the angle of inclination. 



Normal Variation of Field — It will be noted that there is a normal 

 increase in intensity of the vertical field as we approach the magnetic 

 poles until at the poles maximum value is attained. Likewise, the hori- 

 zontal component increases in intensity toward the magnetic equator and 

 reaches its maximum value there. In large-scale magnetic surveys, correc- 

 tions are applied to the field data to compensate for the normal variation 

 of the magnetic-field strength with latitude and longitude. In the state 



