MAGNETIC METHODS 219 



anomalies for a series of structures which would give the results obtained 

 from the field measurements, and to select from this series the structure that 

 seemed most likely from the geological standpoint. 



W. P. Jenny,t in discussing geological and physical problems in the interpretation 

 of regional and local anomalies, states a simplification that is often valuable : "The 

 geological interpretation of regional and local anomalies is based on the fact that the 

 rocks which constitute the outer crust of the earth vary greatly in their magnetic 

 susceptibility. The average magnetic susceptibility of basic igneous rocks and of some 

 magnetite-bearing crystalline schists is generally assumed to be from 10 to 100 times 

 stronger than the average susceptibility of sedimentary deposits, granites, and the 

 majority of metamorphosed rocks (according to H. Reich, 1930)." 



However, it is fundamentally wrong to assume that the magnetic anom- 

 alies are almost exclusively due to the igneous rocks. In most oil regions, the 

 igneous or basement rocks are buried at depths ranging from 3,000 to over 

 20,000 feet. Since the magnetic efifect of local structures is about inversely 

 proportional to the square of their depth, any small magnetic variation in 

 the near surface sedimentary rocks may produce anomalies comparable to 

 the larger magnetic efifect of the basement rocks. 



A discussion of the importance of the sedimentary beds and their magnetic effects 

 has been given by WeissJ who writes : "The application of the magnetic method in the 

 Witwatersrand area has met with considerable success, especially in the locating of Post 

 Karoo dykes and in determining the approximate location of the sub-outcrop of three 

 strongly magnetic shale beds in the lower Witwatersrand system : the Water Tower 

 shales, contorted beds, and West Rand Shales. By means of these magnetic key-beds, 

 it is possible to delineate indirectly the zone of the approximate position of the main 

 reef. The above three magnetic markers can be detected even if they are covered with 

 1,000 to 2,000 feet of younger sediments, and consequently the magnetic method is of 

 great value in prospecting areas where Karoo beds and dolomite rest unconformably 

 on the Witwatersrand system." 



A further discussion of the magnetic effects of sedimentary beds has been presented 

 by Lynton,tt who says: "General conditions for the use of magnetic geophysical meth- 

 ods in California are good, as there is marked variation in the magnetic susceptibility of 

 the sedimentary rocks of economic interest. In Tertiary rocks, the magnetic susceptibility 

 varies from 14 x 10"^ in the Sagus of the Upper Pliocene to 412 x lO"*^ in the vivianitic 

 sandstone of the AlcKittrick group of the Pliocene. This variation is sufficient to give 

 a definite magnetic contrast at several horizons. Magnetic marker beds, such as this 

 vivianitic sandstone, beds of volcanic tuff, and interbedded basaltic flows, extending 

 throughout considerable areas, have been found, which are sufficiently thick and mag- 

 netic to cause anomalies of several hundred gammas at surface exposures and recog- 

 nizable indications under deeper cover." 



Rieber§ brings out that : "Certain strata in the more recent fresh-water deposits 

 of the Tulare Series in the San Joaquin Valley, California, which contain material 

 only slightly more magnetic than the adjacent strata, show sufficient magnetic anomalies 

 in the vicinity of known faults and folds to give promise of extensive usefulness of 

 magnetometer surveys in the exploration for oil." 



t W. P. Jenny, The Science of Petroleum, Vol. 1, Oxford University Press, 1938. 



t O. Weiss, "The Application and Limitation of Geophysical Prospecting Methods in the Wit- 

 watersrand Area", Jnl. Chem., Met. and Min. Society of S. Africa, Vol. 34, pp. 321-361, 1934. 



tt F. D. Lynton, "Some Results of Magnetometer Surveys in California," A.A.P.G. Bull., Vol. 

 15, pp. 1351-1370, 1931. 



§ F. Rieber, "A New Micro-Magnetometer," A.I.M.E. Geophysical Volume, 1929, pp. 401-415. 



