380 MAGNETIC METHOD [Chap. 8 



are plotted as one vector (see Fig, 8-43e). A second vector is plotted from 

 the normal inclination Iq and the normal total intensity To . The vectorial 

 difference between these two vectors is S.^^ When superimposed on geo- 

 logic sections, the vertical anomalous vectors point approximately toward 

 the poles of disturbing objects. They may be used for approximate 

 depth determinations with force-line diagrams. The accuracy of this 

 method depends on the proper selection of normal va.lues. 



Total anomalous vectors may be plotted in isometric projection from the 

 AX, AY, and AZ components as shown in Fig. 8-43/. Another method of 

 three-dimensional representation is to rotate the total about the hori- 

 zontal vector and to plot both in the same plane (see Fig. 8-43gr). These 

 procedures have the disadvantage that in plan view the resultant vector 

 does not point in the correct horizontal direction. Hence, it is better to 

 plot the horizontal vector only and to indicate the size of the vertical 

 vector by the radius of a circle about the station or by a double line 

 (Fig. 8-43/1). 



5. Magnetic gradients are obtained by dividing the intensity differences 

 of stations (or contours) to the north and east by their distance. They 

 may be measured directly with a magnetic torsion balance or inductor 

 gradiometer. These gradients are expressed in gammas per kilometer, or 

 in 10~^ (Eotvos) units per cm (SOy per km equal 5 E.U.). The direction 

 and length of the resultant gradient follows from the north and east com- 

 ponents by vectorial addition. In interpretation, magnetic gradients are 

 of advantage when torsion balance data are compared with magnetic 

 results 



B. Qualitative and Quantitative Analysis 



Most interpretation of magnetic data is of a qualitative nature. This 

 is due to several factors: (1) the magnetic method lacks depth control; 

 (2) most quantitative interpretation is indirect; (3) magnetic properties 

 of geologic formations, particularly of igneous and metamorphic rocks, are 

 subject to great horizontal and vertical variations and are dependent on 

 the thermal and mechanical history, the effects of which are difficult to 

 evaluate; and (4) magnetism is the only physical rock property which is 

 of a bipolar nature, and variability of polarization adds another unknown. 

 Quantitative interpretation is further handicapped because the proportion 

 of induced and remanent magnetism is rarely known. Fortunately, in 

 many cases, the remanent magnetism is identical in sign and direction 

 with the induced magnetization, so that interpretations may be based on 

 the assumption that the magnetization is due to induction in the earth's 

 magnetic field, that is, is dependent upon the magnetic latitude. 



** Actually lo and Ii and To and Ti are in different planes. 



