Subsurface Methods as Applied in Geophysics 



1047 



of Oklahoma, for example, the normal increase of intensity for the 

 vertical component toward the north is about 13 gammas per mile; toward 

 the east it is about 4 gammas per mile. 



Time Variations of the Earth's Field — Even at a fixed point on the 

 earth's surface, the magnetic intensity does not remain constant, variations 

 being observed daily, yearly, and over much longer periods of time. 

 Certain short-period and daily changes are apparently related to sun-spot 



8 9 lO I 



Figure 549. Diurnal-variation curves observed at different locali- 

 ties on the same day. Data obtained by observation of a fixed- 

 base-station instrument. (After Vacquier.) 



activity; variations of several hundred gammas within an hour are not 

 uncommon during magnetic storms. Corrections must be made for the 

 normal diurnal or daily variations of magnetic intensity with time before 

 a survey may be utilized. The diurnal curves are obtained in the field 

 either by reading a base instrument every ten or fifteen minutes or by 

 returning the field instrument to a base or check station within every two- 

 hour period. Observations made during periods of rapidly changing in- 

 tensity are of no value. (See fig. 549.) 



In summation, the magnetic-field intensity at any point at a given 



