MAGNETIC METHODS 



151 



This would have made a temperature correction of —0.9 S.D., to have 

 been appHed to the average reading. This is correct in that a lower tem- 

 perature would have increased the reading, necessitating the subtraction of 

 a temperature correction to put the corrected reading on the 20° C. base. 



In column 8 is recorded the corrected reading for the station, in gam- 

 mas. This is obtained by multiplying the scale reading (as corrected for 

 temperature) of column 7 by the scale value of the instrument. In this 

 problem E = 27.2 gammas/^"./}. For the base station 21.0 x 27.2 = 572 

 gammas for column 8. 



Concerning column 9, on certain stations of a survey the magnetic 

 anomaly may be so great as to cause the scale to run out of view in either 

 the plus or minus direction. When this occurs, place an auxiliary magnet 

 of known moment directly under the instrument, by means of the holder 

 and extension rod attached to the tripod. This magnet is set at a measured 

 number of centimeters from the system. The magnet can be adjusted to 

 bring the scale back into view so that a reading can be obtained. 



The amount of the field thus added (or subtracted) is recorded in 

 column 9, using the formula F — 2M/r^ where F = field in gammas, M = 

 moment of auxiliary magnet in gammas, and r = the distance in cm. from 

 the magnetic system to the center of the auxiliary magnet. This procedure 

 is not often necessary but can be ap- 

 plied for a few stations to save re- 

 adjusting a system by means of the 

 latitude screw. No auxiliary magnet 

 was used in the sample notes. 



The total amount of the daily 

 variation in column 10 is determined 

 by finding the difference in the reading 

 at the base when it is first occupied 

 and when it is occupied at the end 

 of a series of stations. In this case 

 572 — 548 = 24 gammas, which is 

 considered as minus because the read- 

 ing at the base was less at the end of 

 the run than at the beginning. The 

 amount of this daily variation to be 

 applied to each station may be deter- 

 mined from a graph with time plotted 

 on one axis and total change on the 

 other. See Figure 63. 



It is assumed that the change in reading as shown by the base station 

 checks is linear between points and represents the daily variation. This 

 assumption is not strictly true, but, lacking a base station instrument with 

 which to record the true daily variation, as is sometimes used, it is the 

 procedure followed. For a large enough number of points, a smooth 



572 



546 



24 



TOTAL 

 DAILY 

 J.15 VARIATION 



■ZAf 



3:10 3-25 3:40 STATION TIME 



Fig. 63. — Graph showing a portion of the 

 daily magnetic variation, from base checks, 

 for sample set of notes. 



