MAGNETIC METHODS 



107 



15 GAMMAS 



Fig. 34. — Graph of the average daily variation in 

 vertical magnetic intensity. Useful in showing at what 

 time of day the rate of change is the greatest, and 

 hence, when base checks are most necessary. 



corrected for. For a typical day based on an average of many days, the 

 average variation in vertical intensity is shown in Figure 34. 



The total variation covers a 

 range of about 20 gammas. ""' 



The type curve cannot be used 

 for making daily variation cor- 

 rection in field surveys. A 

 given day may be entirely dif- 

 ferent in the character of its 

 variation. 



In practice a reading at the 

 base station is taken when it 

 is first occupied ; several times 

 during the day the base is re- 

 occupied and readings taken. 



Then, assuming the change to be linear between the times of readings, a 

 daily variation or diurnal curve is plotted. From this curve a connection is 

 made for each station read during the interval between readings at the base 

 station. It is also possible to have a second magnetometer permanently at the 

 base station for reading at intervals of ten to twenty minutes, or to use a 

 continuously recording magnetometer, in order to obtain an accurate daily 

 variation curve. 



Magnetic Storms. — -Magnetic storms are aperiodic and quite unpre- 

 dictable variations in the earth's magnetic field. They are somewhat 

 analogous to air pressure changes as evidenced on a barometer. Magnetic 

 storms may occur very suddenly and cause such a rapid change in magnetic 

 force (up to 500 gammas in less than one half hour) as to necessitate sus- 

 pension of field operations. 



Magnetic storms are of three types: (1) oscillatory, (2) sudden com- 

 mencement and (3) bay type. 



The oscillatory magnetic storm is evidenced by a pulsating variation 

 which may be of the order of 10 gammas in horizontal intensity and of a 

 periodicity of from 2 to 10 minutes. Such a pulsation may last about 

 two hours and may take place on what is otherwise a magnetically quiet 

 day. The back-and- forth magnetic pulsation would afTect the other mag- 

 netic elements, as in the case of all magnetic storms, to some degree, 

 perhaps to the extent of 5 minutes in declination. 



The sudden-commencement type of magnetic storm is of importance 

 in magnetic field work because it may cause large variations in a most 

 erratic manner and in a very short time. Such magnetic storms appear to 

 occur all over the world at approximately the same time, or within less 

 than ten minutes. They may last a few hours or may even run for a day 

 or more. 



When severe, this type of storm may cause an abrupt and sudden 

 change in magnetic force up to as much as 500 gammas in the vertical 



