156 EXPLORATION GEOPHYSICS 



by inspection.* The small closed contours representing curves of greatest 

 magnetic strength are usually shaded outwardly, and are termed magnetic 

 "highs." The contours representing curves of lowest field strength are 

 usually shaded inwardly, and are termed magnetic "lows." (Figure 65.) 



Illustrations of magnetic anomaly profiles and isanomalic contour maps 

 are given in the section dealing with results of surveys. 



Fig. 65. — Method for representing magnetic high 

 contours (A), and magnetic low contours (B). 



'^Normal^' Values of the EartK's Intensity Components 



Interpretation of magnetic anomaly curves is sometimes facilitated 

 by knowing the "normal" intensity components in the area under 

 investigation. 



A precise definition of the normal intensities cannot be given. The 

 procedure for arriving at the normal values for a given area is an approxi- 

 mation process. Consideration must be given the shapes of the anticipated 

 geological features and their relative magnetic effects. In regions where 

 large anomalies exist, the normal values are the intensity components 

 measured at a station removed from the anomalous area. 



Summary 



Preliminary 



In Laboratory 



Determining temperature coefficient of instrument 

 Determining magnetic moments of auxiliary magnets 



In Field 



Adjusting for latitude 



* In many surveys the method of least squares is applied before drawing final 

 isanomalic contours. Barton t and Roman t describe the application of this method 

 to magnetic prospecting. 



t D. C. Barton, "Control and Adjustment of Surveys with the Magnetometer or the Torsion 

 Balance," Bull. A.A.P.G., Vol. 13, 1929, pp. 1163-1183. 



t I. Roman, "Least Squares in Practical Geophysics," A.I.M.E. Geophysical Prospecting, 1932, 

 pp. 460-506. 



