Chap. 8] 



MAGNETIC METHOD 



407 



tation would be possible. Positive vertical intensities with diverging 

 horizontal vectors could mean a lower end of an ore body above, negative 

 vertical intensities with diverging horizontal vectors a lower end below, 

 positive vertical intensities with converging horizontal vectors an upper 

 end below, and negative vertical intensities with converging patterns an 

 upper end above. However, such favorable conditions seldom occur and 

 the actual number of combinations of vector directions is not four but eight. 



To eliminate the remaining un- 

 certainty, intensity gradients (in 

 vertical or horizontal intensity, 

 or both) may be measured where- 

 by vector gradients, as shown in 

 the figure, may be plotted. This 

 will correctly indicate the posi- 

 tion of the pole, even if the po- 

 larization of the ore is abnormal. 



Determinations of depth or 

 distance to ore bodies may be 

 made by measuring intensities in 

 different levels of a mine, in dif- 

 ferent depths of a shaft, at the 

 surface and on a platform (see 

 Fig. 8-58), at the surface and in 

 a balloon, or by airplane in differ- 

 ent altitudes. The first method 

 was proposed by Dahlblom in 

 1899. When observations are 

 made at two points directly above 

 an ore body of considerable depth 

 extent, if d k the distance of the 

 lowe^ point o^ observation from 

 the upper pole of the ore body 

 and ii d -{- Ad is the distance of 

 the upper point from this pole, 

 the vertical intensities follow from formulas (8 -52a) and are 



W.Riddd. 

 Fig. 8-58. Magnetometer observation 

 platform. 



(above) AZ2 = ^- ^^^ ; (below) AZi = ^ ; 



Hence, from their ratio 



d = 



Ad 



y AZ2 



AZi > AZ2 . 



(8-64a) 



