376 MAGNETIC METHOD [Chap. 8 



readily calculated. In the interpretation of the results in irregular 

 terrain, it is necessary to consider the relative position of each station 

 with respect to subsurface bodies. In this case it is advisable to obtain 

 additional data by observations at every station at two heights of the 

 instrument^* or by the use of a compass variometer^' which is very sensitive 

 to rapid space changes of intensity. 



2. Magnetic interference may occur even on a fiat terrain surface by 

 locally different weathering, by magnetic rocks in glacial drift, by surface 

 float near iron and nickel deposits, by magnetite stringers in igneous rocks, 

 and by outcrops magnetized by lightning. It is difficult to give even 

 approximate general figures for these effects. On the other hand, the 

 influence of topographic features of simple geometric shape may be es- 

 timated*" by assuming uniform magnetization in the earth's field. If the 

 susceptibility is k, the greatest vertical intensity field of a hill station 

 (calculated on the assumption of an elongated ellipsoid) is 



AZ = +47r/cZo . (8-51a) 



The approximate anomaly near the bottom of a vertical wall (quarry 

 station) is 



AZ = -ItkZo. (8-516) 



The vertical field in an elongated depression between walls of uniform 

 slope on either side depends not on the absolute differences in elevation 

 between summits and valleys, but on the degree of slope. For gentle 

 slopes 



AZ = -2irKZo (8-51c) 



approximately, while for steep slopes 



AZ = -Stt/cZo . (8-51(i) 



The anomaly in the interior of a sphere (measurements in caves, tunnels) is 



AZ = -Itt/cZo. (8-51e) 



Koenigsberger*^ confirmed these theoretical values by numerous ob- 

 servations. The greatest difference between summit and valley values 

 in gneiss was I3O7; in gneiss quarries the anomalies reached — 50y; in 



''"' A. Nippoldt, Magnetische Mutung, p. 12 1930. 



" See A. F. Hallimond, Min. Mag., 41(1), 16-22 (July, 1929). Nippoldt, A. Mag- 

 netische Mutung, p. 66. The formulas for platform observations or for different 

 elevations of mine shafts apply (see pp. 407-408). 



" Phys. Zeit., 176-179 (1907); A. Nippoldt, Magnetische Mutung, pp. 68-70, 



*« Koenigsberger, Gerl. Beitr., 20(3/4), 293-307 (1928). 



" Ihid. 



