106 EXPLORATION GEOPHYSICS 



distribution of magnetic vertical intensity is shown in Figure 33. The 

 contours are made as smooth and regular as possible, in order to express 

 the change in vertical intensity throughout the United States as a function 

 of latitude and longitude only. 



Latitude and Longitude Correction. — The contours show the value 

 of vertical intensity in gauss. The change in gammas per mile north-south 

 and east-west, used in field surveys with prospecting magnetometers is 

 computed, for a given area, from such a map. This latitude and longitude 

 correction (L. and L. correction) is for the central part of the state of 

 Colorado 11.8 gammas per mile + to the north and 4.7 gammas per mile 

 + to the east.* From a base station the change would be — to the south 

 and — to the west. In the eastern part of the United States the contours 

 of vertical magnetic intensity trend in a NE direction which makes the 

 east-west gamma change per mile — to the east and + to the west. 



A similar map for the variation in horizontal intensity is used to calcu- 

 late the L. and L. correction, for field work with a horizontal 

 magnetometer. 



Time Variations. — The earth's magnetic field undergoes three periodic 

 variations with time. These are known as secular, annual, and diurnal 

 variations. 



Secular variation is a change with a periodicity of about 960 years. 

 Records of measurements of any of the magnetic elements do not extend 

 far enough back to permit an accurate computation of secular change. It is 

 known, however, that declination changed from 8° East in London in 1650 

 to 2° West in 1700, to a maximum of about 26° West in 1800 and then 

 decreased to some 20° West by 1900. The inclination changed from about 

 70° to 75° to 71° to 64° at the above dates, respectively, at London. The 

 rate of change is of the order of 2 minutes per year for declination. This 

 change is ordinarily not of importance in magnetic surveys, except when 

 a survey is being checked after a long period of years. For example, a line 

 1000 ft. long, run due north by compass from Boston in 1925 would have 

 its north end 125 ft. farther west than a similar line run in 1785. 



An annual change in the magnetic elements has been observed, having 

 a periodicity of one year. The amplitude of the deviation for declination, 

 in London, is about 2.25 minutes, with a maximum in an easterly direction 

 in August and in a westerly direction in February. This annual variation 

 for North America amounts to about 1 minute and may be considered 

 negligible in general for the application of geomagnetics. 



Diurnal variation is of importance in magnetic surveys and must be 



* The latitude change in gammas per mile for a given area can be determined from 

 the above chart (Figure 2i2) by scaling the distance in miles between contour lines and 

 dividing that distance by the number of gammas between contours. For the north- 

 south correction the distance is scaled in a north-south direction, and for the east-west 

 correction the distance is scaled east-west. 



