MAGNETIC METHODS 143 



Corrections for Daily Variations 



Base Check Method 



The irregular variations in the strength of the earth's magnetic field 

 necessitate corrections for each of the various stations occupied during 

 the course of a survey. These corrections are always the negative of the 

 magnetic variations; e.g., if the magnetic intensity in the area increases 

 at a certain time, the amount of increase will be subtracted from the mag- 

 netic reading made at that particular time. In general, the annual and 

 secular variations are of interest in geomagnetic explorations only when 

 surveys are conducted over a considerable period of time or when a lapse 

 of several months occurs in extending or checking a given survey. The 

 diurnal variations, however, are of considerable importance. 



Corrections for magnetic variations during the progress of a survey 

 are made by recording the variations of the magnetic intensity at one or 

 more base stations. The ideal procedure is to employ a continuous re- 

 cording magnetometer at the base station. This ideal procedure is approxi- 

 mated by setting up one instrument at a fixed station and taking inter- 

 mittent readings. In many surveys where a separate instrument cannot 

 be utilized for these fixed or base station measurements, results of lower 

 (but often adequate) accuracy are obtained by readings made every hour 

 or two at a central base station. 



Intermittent Readings: Diurnal Variation. — In practice, intermittent 

 readings of the diurnal variation frequently are made by setting up a mag- 

 netometer at a base station in the area under investigation and having an 

 observer read the magnetic field strength at this station at intervals of 15 

 to 20 minutes. The readings should be started about an hour prior to 

 the other field work and extend for a similar period of time beyond the 

 other field measurements in order to establish general trends. Any varia- 

 tions in the field instruments are immediately apparent when check read- 

 ings are taken at the base station. The two instruments must, of course, 

 be mounted at a sufficiently large separation to insure that the mutual 

 attraction between the two magnet systems is negligible. 



If an extra instrument is not available for use as a base station mag- 

 netometer, usable results may be obtained by "checking-in" at the base 

 station a number of times during the day. In this procedure, the operator 

 returns to the base station at regular intervals and takes readings with 

 his field magnetometer. These readings, when corrected for temperature 

 changes, give a fair approximation of the diurnal variation. In this 

 method, any change in instrument calibration will introduce an error 

 which cannot be differentiated from diurnal variations. 



An alternative field technique involves checking back on a previously 

 occupied station rather than the base station. Occupy base station A, then 

 stations, 1, 2, 3, 4 and back to A. Thus, corrected values for stations 1, 



