18 



METHODS OF GEOPHYSICAL EXPLORATION 



[Chap. 2 



which allows for errors of closure when checking back to a base station; (3) a 

 correction for daily variation which may be determined by visual observa- 

 tion or recording of a second magnetometer ; (4) a planetary correction, which 

 eliminates the normal variations of the earth's magnetic field with latitude. 



Mafnefic 

 Vertical Intensify 

 Anomalij 



. / / / / 

 Lines (^ equal Vert. Infens. AnomaJ^ 



'/III 



drwi/y 



v/yr^^^i^//A!:!:^y^^//>i^j^y///}!^t^iy^> 



Fig, 2-8. Vertical magnetometer on traverse across buried granite ridge. 



Fences, bridges, pipe lines, tanks, derricks, well casings, and the like, are 

 a serious handicap to magnetic exploration and must be kept at sufficient 

 distance, as it is difficult to correct for them. 



Interpretation. Magnetic results are generally represented in the form 

 of lines of equal magnetic anomaly ("isanomalic" lines^ or in the form 



» The name "isogams" as applied to magnetic lines is a misnomer. 



