112 



EXPLORATION GEOPHYSICS 



If a dip needle is oriented with its plane of rotation at right angles 

 to the magnetic meridian, it will stand vertically. This is true because the 

 horizontal component will be acting through the bearings, and only the 

 vertical component of magnetic force will be effective in aligning the 

 needle. Figure 38 shows a modern dip needle with a counterweight. 



The dip needle can be used for geophysical surveys under certain 

 conditions where relatively large anomalies exist. Its precision is about 

 one-half that of the Wilson attachment for the Brunton compass. See 

 Figure 38. 



NORMAL DIP, I* 



DRILL HOLE NO. 24 



Fig. 38 — Dip Needle — Lake Superior 

 Model; a, bearing support screw; b, counter- 

 weight; c, graduated arc; d, level; e, release 

 finger; f, release button; g, non-magnetic 

 case. (Courtesy of W. and L. E. Gurley.) 



Fig. 39. — Dip needle traverse across nick- 

 eliferous ore body at Falconbridge, Sudbury 

 District, Ontario, Canada; after Masonf. 



Figure 39 illustrates a dip traverse across the Falconbridge magnetic 

 ore-body. The main ore-body, about 30' thick under 115' of overburden, is 

 located in the Sudbury District, Ontario, Canada. 



Swedish Mining Compass. — This compass was probably the first "dip 

 needle" employed in magnetic prospecting. At present, it is used widely 

 in regions where the magnetic anomalies are very large and hence easily 

 detected. Well-known regions of this type are the Lake Superior region 

 in America and the Swedish iron ore districts. The Swedish mining 

 compass consists essentially of a pillar which is mounted on the bottom 

 of a cylindrical glass case and supports a needle point pivot. $ The needle 

 is free to move in a vertical and a horizontal plane. The rotation in the 



t M. Mason, "Geophysical Exploration for Ores," A.I.M.E. Geophysical Volume, 1939, p. 30. 

 t Eugene Haanel, On the Location and Examination of Magnetic Ore Deposits by Magneto- 

 metric Measurements (Ottawa, 1904), pp. 66-66. 



