144 



EXPLORATION GEOPHYSICS 



2, 3, and 4 are available for making any one of them usable as a check 

 station. The survey might then proceed with stations 5, 6, 7, 8 and back 

 to 4, etc. This saves time in difficult country. 



Figure 59 shows corresponding curves obtained with a base station 

 magnetometer and a field magnetometer "checked-in" at intervals of about 

 two hours. The results are in fairly good agreement, with the exception 

 of the time period between 1 P. M. and 3:15 P. M. Here the error would 

 amount to about 4.5 gammas which may be sufficient to mask or distort 

 a minor magnetic feature. 



The graph (Figure 59) also shows that the diurnal variation fluctuated 

 from +10 gammas at 7 A. M. to —14 gammas at 1 :45 P. M., i.e., the varia- 

 tion had an amplitude of 24 gammas for that particular station and date. 



7 6 9 10 II IZN I 2 3 -is 



LOCAL TIME 



Fig. 59. — Graph showing diurnal variation as given by field and 

 base station magnetometers. 



The maximum magnetic variation due to structure in this area was about 

 10 to 15 gammas, i.e., approximately % to % the diurnal variation. It is 

 evident, therefore, that in this case the diurnal variation had to be measured 

 with the same accuracy as the anomalies. 



In certain types of mining investigations where the magnetic anomalies 

 are very large, the diurnal variation may oftentimes be neglected. How- 

 ever, magnetic storms are unpredictable and may cause variations of 50 

 to as much as 500 gammas. Generally speaking it is poor policy and false 

 economy to omit the diurnal corrections. 



Continuous Recording Magnetometers, — Practically all types of 

 continuous recorders employ photographic recording on sensitized plates 

 or paper. One type of instrument utilizing photographic recording on 

 plates is illustrated in Figure 60. The magnetometer and the recording ap- 

 paratus are both mounted in a cylindrical housing, which is supported on 

 a tripod. The instrument is leveled and oriented in the same manner as 



