378 MAGNETIC METHOD [Chap. 8 



vertical intensity anomaly are best suited. Their interval should be two 

 to three times the probable error of the survey; greater intervals are pre- 

 ferred for large anomaly gradients. "Isanomalic" maps are comparable 

 to contour maps; it is helpful to shade areas of equal anomaly, darkest 

 shades corresponding to areas of greatest anomalies. The use of two 

 different colors will be of assistance; red may be used for positive, and 

 blue or green for negative anomalies. 



2. Profiles at right angles to strike show magnetic values as ordinates and 

 distances as abscissas above a known or inferred geologic section. For 

 geologic features causing negative anomalies, inverted ordinates are used. 

 Where a simple geometric relation between magnetic values and subsurface 

 contours can be assumed, the scale of the magnetic ordinates may be made 

 identical with the vertical scale in the geologic section. Parallel profiles 

 across the same geologic feature may be cut of celluloid strips and mounted 

 on a map (see Fig. 8-72). If horizontal intensity measurements have 

 been made in the plane of the magnetic meridian, the anomalies must be 

 projected upon a direction at right angles to the strike. This projected 

 anomaly is hereinafter designated by the symbol Hp . The conversion may 

 be avoided by orienting a horizontal magnetometer at right angles to the 

 strike or in a profile direction at approximately that angle. 



3. Peg models, isometric maps. Magnetic anomalies may be represented 

 on maps by pegs of proportionate length (see Fig. 8-71). For areas of 

 small magnetic gradients, better visualization of the magnetic relief can 

 be obtained in this manner. However, negative magnetic anomalies do 

 not show to advantage. Isometric maps are prepared by using the 60° 

 axis for the north, the 120° axis for the east coordinate of the map, and 

 the 0° or vertical axis for the representation of anomalies. A difficulty 

 with isometric maps is that large anomalies may obscure adjacent features 

 and that negative anomalies cannot be shown advantageously. 



4. Anomalous vectors. Interpretational advantages are often gained by 

 representing the results in the form of anomalous vectors, since they are 

 tangent *,o the lines of force radiating from the poles of the disturbing 

 objects. Their usefulness, however, depends upon a careful selection of 

 the normal value. 



Horizontal vectors may be plotted from anomalies in declination and 

 horizontal intensity. A convenient method is to calculate the north com- 

 ponent X and the east component Y from the observations, to subtract 

 Xo and Yo , and to plot AX and AY. The anomalous vector R is the 

 resultant of the two (see Fig. 8-43a). In another method the normal Ho 

 is plotted in the direction 5o , and the observed H is plotted at the observed 

 angle (see Fig. 8-436) ; the horizontal anomalous vector R is equal to the 

 vectorial difference of Ho and H. The Tiberg method of the parallelogram 



