406 



MAGNETIC METHOD 



[Chap. 8 



same mine, indicating that ore bodies above the level of observation give 

 rise to negative anomalies (see below). The same phenomenon was 

 observed in the iron ore mines of Berggiesshuebel in Saxony, where meas- 

 urements were likewise made with the Schmidt balance. 



In Swedish mines Tiberg's method of determining vertical intensity as 

 well as direction and length of the horizontal disturbance vector has been 

 widely applied. The procedure was described on page 379. Another 

 method uses a magnetometer with height adjustment on a plane table so 

 that not only the regular stations in the middle of a tunnel but also points 

 horizontally and vertically in line with them may be surveyed. Results 

 of an underground tunnel survey are reproduced in Fig. 8-56, showing the 

 horizontal vectors as well as results of vertical intensity observations in 



I 



a I b 



I 



M 



a b 



(^ 



IZ 



a b 





(B- 



-ffl 



Fig. 8-57. Anomalous vectors for various ore body positions in tunnel surveys 

 I, Upper end of ore body below tunnel; II, lower end of ore body above tunnel; III, 

 lower end of ore body below tunnel; IV, upper end of ore body above tunnel. Ic, 

 vector gradient at 16; lie, vector gradient at I la. 



three levels. The need for vertical intensity measurements arises from 

 the fact that the magnetic bodies may be below or above the instrument. 



Four possible combinations are shown schematically in Fig. 8-57: 

 (I) when a south pole, or upper end, of an ore body is below the drift, the 

 horizontal anomalous vectors point toward the pole; (II) if its north pole, 

 or lower end, is above the drift, the horizontal vectors will be directed away 

 from the pole; (III) if its north pole is below, the horizontal vectors point 

 away; and (IV) if its south pole is above, the horizontal vectors will point 

 toward it. If interpretation were based on horizontal vectors alone and a 

 cross cut were driven on the strength of the vector pattern in case IV, the 

 ore body would be found; in case I it would be missed. The addition of 

 vertical intensity measurements overcomes part of the difficulty. If two 

 adits, as shown in Fig. 8-57, were always available, no mistake in interpre- 



