Chap. 8] MAGNETIC METHOD 377 



gravel pits they ranged from —13 to — 2O7. In valleys of granite, 

 Koenigsberger observed anomalies of from — 10 to —267; in areas of basic 

 schists, from —50 to — 2OO7; at the foot of glacio-fluvial terraces the 

 effects varied from —20 to — 3O7. 



G. Normal Fields 



Although interpretation of magnetic anomalies is based mainlv on the 

 shape of anomaly curves, it is frequently desirable to know the value of 

 the undisturbed intensity in the area under survey. In some interpreta- 

 tion problems it is necessary to differentiate between positive and negative 

 anomalies; in vector and line-of-force interpretation the sign of the anomaly, 

 and thus a fairly accurate determination of normal values, is of importance. 

 There is no standard procedure for determining normal values since they 

 depend on the type of anomaly surveyed. For large and locally limited 

 anomalies it is sufficient to select a base station in geologically undisturbed 

 terrain and to consider its intensity as the normal value. Elsewhere it is 

 usually satisfactory to use the U. S. Coast and Geodetic Survey maps. 

 Since regional anomalies are already included in their normal values, 

 anomalies due to local deviations from regional structure appear as excess 

 or deficiencies with respect to these. In extensive surveys conducted for 

 the purpose of obtaining relations between magnetic anomalies and regional 

 structure, it is better to use world charts in which only anomalies of 

 continental order of magnitude remain. 



To simply form the average of all magnetic values in an area and 

 designate this as the normal value is not reconmaended, since a given 

 geologic body may not produce a balanced number of positive and nega- 

 tive anomalies. The selection of a normal value is frequently a trial and 

 error proposition. Its final value can often be chosen only with regard 

 to the shape of the anomaly curve to be expected from theoretical and 

 geological considerations. 



V. MAGNETIC FIELDS OF SUBSURFACE BODIES 

 (INTERPRETATION THEORY) 



A. Graphical Representation of Results 



For interpretation in geologic terms, magnetic data must be so plotted 

 as to show the clearest indication of subsurface bodies. The following 

 representations are in use: (1) lines of equal anomaly; (2) profiles at right 

 angles to strike; (3) peg models, isometric maps; (4) anomalous vectors; 

 and (5) magnetic gradients. 



1. Lines of equal anomaly. For higher magnetic latitudes lines of equal 



