150 



EXPLORATION GEOPHYSICS 



grid. Accuracy within 0.2 of a mile is sufficient for routine stations in oil 

 structure surveys, which permits traversing by the speedometer of a car. 



When an uncompensated magnetic system is used, it is necessary to wait 

 at the base station for about one-half hour until the instrument has 

 acquired the air temperature. In most magnetic work for oil structure, 

 transportation is by automobile, and care must be used to prevent the 

 magnetometer from receiving jolts which might knock the system out of 

 adjustment or damage the knife edge and bearings. 



Readings at subsequent stations are taken in turn, after the base station 

 has been occupied. Each location is recorded in the note form and usually 

 on a map of the area carried for that purpose. In the field it is necessary 

 only to record the data required in columns 1, 2, and 3 of the form, and 

 the location of the station. See sketch map (Figure 62) for location of 

 stations in the survey notes (Figure 64). 



Fig. 62. — Sketch map showing location of stations used in set of 

 field notes. 



The first step in the calculation of field notes, with the form here 

 used, is the correction of the reading due to temperature. In column 4, 

 the recorded temperature, less 20° C, is set down. For the base station 

 this is + 2.9°. The average reading is then figured from column 2 by 

 taking the mean of readings with orientation of the instrument in the East 

 and in the West positions. This is entered in column 6. 



The heading of column 6 is Mean Reading S.D. For the base this is 

 20.1. The nearest 0.1 only is carried. This average is in scale divisions. 

 The temperature correction is then applied by forming the sum of the 

 figures in column 6 and in column 5, which gives, for column 7, the 

 average reading corrected for temperature, in this case 21.0. 



In the example given, if the temperature at the base had been 17.1° 

 or 2.9° below the 20.0° base temperature, the operation of column 4 (or 

 temperature — 20° C.) would have given —2.9 for the figure in column 4. 



