158 



EXPLORATION GEOPHYSICS 



Accuracy to Which the Work is Conducted 

 Depends upon anomalies in the region 



Accuracy of Data Depends on 



Errors in scale reading : 2 to 4 gammas 

 Temperature effects : 1 to 3 gammas 

 Diurnal variations : 2 to 5 gammas 

 Regional variation 



Final Interpretation Includes 

 Checking scale value 

 Checking computations 



Making corrections and converting readings to gammas 

 Correcting for regional gradient if necessary 

 Plotting of magnetic data 



Correlating with areal and subsurface geology 

 Correlating with other geophysical work 

 Predicting probable subsurface conditions 



Special Types of Magnetic Instruments 



Kohlrausch Magnetometer, — The Kohlrausch magnetometer con- 

 sists essentially of a circular, glass-covered case which houses a pivoted 



horizontal swinging magnetic 

 needle4 The case is supported 

 by a vertical rod which is co- 

 axial with the pivot of the 

 swinging needle. (Figure 66.) 

 An auxiliary bar magnet is also 

 supported by the vertical rod. 

 The distance d between the bar 

 magnet and the needle is ad- 

 justable, thereby controlling 

 the sensitivity. The bar magnet 

 may be rotated either clockwise 

 or counterclockwise, the rota- 

 tion to either side being lim- 

 ited by stops. The vertical rod 

 is supported on a three-legged 

 base which is provided with 

 screws and a small bubble level 

 so that the rod can be orient- 

 ated in a vertical position. 

 The magnetometer is first set up at a base station with the needle and 

 magnet orientated in the direction of the magnetic meridian. The bar 

 magnet is rotated until the compass needle is deflected 90° and one of the 



J A. S. Eve and D. A. Keys, Applied Geophysics (Cambr. Univ. Press, 1938), pp. 31-35. 



Fig. 66. — Kohlrausch magnetometer. C, pivoted 

 magnetic needle; P, supporting pillar; M, bar magnet; 

 L, leveling screws; d, sensitivity adjustment. 



