Chap. 8] 



MAGNETIC METHOD 



405 



magnetic or abnormally polarized rocks on tunnel and shaft faces. Before 

 an underground survey is contemplated, it is good practice to go through 

 the workings with a compass and determine the spots of irregular attraction. 

 Magnetic measurements underground have been made chiefly with Swedish 



abovt ntrmtl Mntr 



JOikm 



Ka 



Fig. 8-56. Results of underground observations, by Tiberg method (from Berg 



Catalogue). 



instruments. However, the more sensitive Schmidt balances have been 

 used in a few cases. Haalck^"* has published the results of a number of 

 vertical intensity observations in a drift from which rails and pipes had been 

 removed. Two negative anomalies of about —200 7 were observed. 

 These were due to lenses of serpentine encountered in higher levels of the 



1" Handb. Exper. Phys., 26(111), 386 (1930). 



