164 EXPLORATION GEOPHYSICS 



Numerical Data on Magnetic Susceptibilities. — The usefulness of 

 geomagnetic methods in obtaining information regarding subsurface con- 

 ditions depends chiefly on the relative magnetic susceptibilities of the 

 materials constituting the outermost portion of the earth's crust. Because 

 in general rocks are mixtures of numerous mineral components, the ef- 

 fective susceptibilities depend on those of the individual constituents, and 

 on the percentage of each constituent. 



As a general rule, reliable magnetic anomalies cannot be measured by 

 the usual field techniques of exploration geomagnetics unless the mag- 

 nitude of the difference between the susceptibility of the anomalous geo- 

 logic feature and the susceptibility of the surrounding media is equal to 

 or greater than plus or minus 0.00015. If the susceptibility difference is 

 positive, the anomalous subsurface feature usually possesses some ferro- 

 magnetic material. The amount of ferromagnetic materials which must be 

 present is small due to the fact that a majority of the compounds of iron 

 have very high susceptibilities. (See Figure 69.) The most familiar 

 geologic feature possessing a negative difference with respect to sur- 

 rounding media is a salt dome. 



The magnetic susceptibilities of various materials which occur in the 

 outer portion of the earth's crust are shown in Table 5. t Other values 

 appear elsewhere in the literature. % 



t N. H. Stearn, "A Background for the Application of Geomagnetics to Exploration," A.I.M.E. 

 Geophysical Prospecting, 1929, pp. 315-342. 



t L. B. Slichter, A.I.M.E. Geophysical Prospecting, 1929, p. 343. 

 F. Stutzer, W. Gross and K. Borneman: "Ueber magnetische Eigenschaften der Zinkblende 

 und einiger anderer Mineralien," Metall und Erz (1918) 15, 1, quoted by L. B. Slichter, A.I.M.E. 

 Geophysical Prospecting, 1929, p. 241. 



TABLE 5. 



MAGNETIC SUSCEPTIBILITY OF ROCKS AND MINERALS* 



MAGNETIC MINERALS Susceptibility k Remarks Reference** 



(c.g.s. units) 



Magnetite Crystals 6.3 to 24.0 Min. value is average b 



of k parallel to 3 main 

 crystal axes; Field 

 (F)=2 gauss. Max. 

 value parallel to binary 

 axis, F = 1 



Magnetite 0.04 to 2.0 Solid specimen (S) a, b 



max. value at F = 1 



Ilmenite 0.03 to 0.14 Pulverized sample (P) c, f 



min. at F = 220 

 Max. at F = 1 

 Franklinite 0.036 No data on max. d 



Pyrrhotite 0.007 to 0.028 P. min. F = 220 d, e 



Max. F = 0.6 



Specularite 0.003 to 0.004 Specular Hematite c 



P. min. F = 220 

 Max. F = 0.6 



Chromite 0.002 No data on max. f 



P. F = 220 



