226 EXPLORATION GEOPHYSICS 



with the overlying sedimentaries (such as may be caused by differential com- 

 paction or distortion of the sedimentaries due to lateral thrust) , (2) basement 

 rocks are not of uniform magnetic properties, as may be caused by rocks of 

 different composition or a buried hill of igneous and sedimentary rocks, or 

 (3) the structural axis of the sedimentary structure shifts with depth. 



Mapping Faults and Volcanic Formations. — The magnetic method 

 may sometimes be employed to map the locations of faults in the sedi- 

 mentary rocks. Ordinarily the magnetic location of such faults is contingent 

 upon: (1) concentration of magnetic mineralization along the fault or, (2) 

 displacement by the fault of the subsurface bed or beds which exert the 

 controUing influence on the magnetic field at the surface. 



The magnetic effect or anomaly mapped is chiefly a function of the 

 difference in depths to the controlling magnetic beds on adjacent sides 

 of the fault; i.e., the higher magnetic intensity occurs over the upthrow 

 side of the fault. The plan location of the magnetic anomaly with reference 

 to the fault is subject to the same considerations regarding displacement 

 discussed in the preceding example. Ordinarily, however, this displace- 

 ment is slight, due to the usually moderate depth to the controlling mag- 

 netic feature. 



An example of magnetic work over an irregularly-shaped laccolithic 

 mass at moderate depth (about 1500 feet) is given by Malamphy. t The 

 results obtained from the magnetic and gravitational studies are shown in 

 Figure 110. The magnetic inclination in this area is only 20° and has a 

 reversed polarity with respect to that existing in the northern hemisphere. 



The gravity gradients were calculated by the method described by 

 Barton. I The theoretical and observed gravity gradient profiles are sub- 

 stantially in agreement. 



The theoretical and observed magnetic profiles do not agree very well. 

 According to Malamphy, the observed and theoretical magnetic and 

 gravity gradient data can all be consistent only if the laccolithic mass 

 possesses "magnetic properties in excess of those that normally would be 

 induced by the earth's magnetic field alone." 



The gravitational and magnetic anomalies are due chiefly to the igneous 

 intrusions of the diabase, and to a minor extent, to the crystalline base- 

 ment rocks. 



Magnetic Anomalies Produced by Upper Magnetic Beds of Var- 

 iable Thickness. — The magnetic anomalies produced by magnetic sedi- 

 mentary beds or lava flows which overlie a substantially non-magnetic or 

 diamagnetic basement depend on the depth to the contact with the under- 



t Mark C. Malamphy, "Geophysical-Geological Study of the Sao Pedro Area, Brazil," A.I.M.E. 

 Geophysical Prospecting, Tech. Pub. 696, 1936. 



t D. C. Barton, "Eotvos Torsion Balance Method of Mapping Geologic Structure," A.I.M.E. 

 Geophysical Prospecting. 1929, p. 429. 



