Chap. 8] 



MAGNETIC METHOD 



295 



Further, in order to be detectable, the size of geologie bodies has to increase 

 in proportion to. depth. 



The magnetic field of the earth and of geologic bodies is uniquely defined 

 by the magnitude and direction of the total intensity vector. In practice 

 it is preferable to resolve the field into its components which, in the 

 direction of the vector, are the horizontal and vertical intensities. In high 

 or intermediate latitudes of the northern and the southern hemispheres, 

 measurements of the vertical component are most frequently made but are 

 occasionally supplemented by horizontal intensity observations in the 

 magnetic meridian or at right angles to the strike of geologic bodies. 

 Fig. 8-1 is a vector diagram of the earth's magnetic field for both hemi- 

 spheres. T is the total intensity, H is the horizontal and Z the vertical 

 component; D is the declination and I the inclination. H may be resolved 

 into X (the north) and Y (the east) components. The following relations 

 exist between these components: 



X = HcosD; Y = HsinD; Z = H tan I = T sin I; 



H = VX2 + Y2 = T cos I; 



T = VX2 + Y2 + Z2 = VH^ + Z2 = H/cos I 

 tan I = Z/H = Z/VXM^"^; tan D = Y/X. J 



The unit of measurement in magnetic exploration is the gauss^ (F) 

 (cm~ -g sec."^) which, by definition,^ is numerically equal to 1 dyne. 



1 See footnote 3, p. 298. 



^ Field intensity equals force on unit magnetic pole. Another definition for the 

 gauss unit which follows from the discussion on p. 298 is the field strength, or mag- 

 netomotive force per unit of length, inside a long coil with 0.796 ampere turns per 

 cm. According to the definitions introduced between eq. (8-3) and (8-4), field 

 intensity in gauss also equals the number of magnetic lines of force per square cm 

 in air. 



(8-1) 



