Chap. 8] 



MAGNETIC METHOD 



375 



frequently observed on developed fields. The influence of all wells in the 

 Healdton field was theoretically sufficient to account for its negative 

 anomaly ( — 2OO7). 



Table 43" 



COMPARATIVE VERTICAL ANOMALIES OF A SHORT AND LONG 



STRING OF WELL CASING 



" After Barret. 



12. Derricks, electric line towers, and mine shafts produce negative mag- 

 netic anomalies, being above the plane of observation. Depending on size 

 and amount of steel involved, the fields range from several hundred to 

 several thousand gammas at nearby points. 



F. Terrain Anomalies 



In magnetic prospecting, terrain interference is not nearh^ so important 

 as in other lines of geophysical exploration. It is due to (1) a geo- 

 metric effect resulting in a change of vertical distance of magnetic 

 bodies from the plane of observation, (2) magnetic effects caused by 

 surface rocks. 



1. The geometric effect is of importance when changes in elevation along 

 magnetic profiles are comparable with the distance of observation points 

 from the magnetic body. For a three-dimensional body magnetized by 

 induction (northern magnetic latitude), whose vertical intensity is nor- 

 mally at a maximum above the body and whose horizontal intensity has a 

 maximum and minimum to the south and north, the appearance of the 

 anomaly curves is almost reversed if the slope is steep enough,^^ since the 

 lower pole of the body comes closer to the surface. For uniform slopes 

 the changes in relative position of instrument and subsurface poles may be 



'"^ See Haalck, op. cit. 



