206 EXPLORATION GEOPHYSICS 



EMPIRICAL METHODS OF INTERPRETATION: 

 CORRELATION WITH KNOWN GEOLOGY 



In the practical application of magnetic measurements to the solution 

 of subsurface problems of economic geology, theoretical calculations can 

 be carried out only for relatively simple bodies, e.g., two-dimensional 

 bodies, spheres, etc. However, the usual configurations of geologic bodies 

 rarely produce the simple magnetic effects equivalent to those produced 

 by simple geometric forms. The interpreter, therefore, must resort to 

 other aids besides direct theory when solving his field problems. The 

 three most effective aids are : first, and most important, adequate geologic 

 or subsurface control ; second, magnetic studies over known geologic con- 

 ditions in the immediate area; and third, small scale model experiments. 



The usual interpretative technique is almost exclusively empirical. 

 Deductions and inferences are drawn qualitatively from the sizes, shapes, 

 and configurations of the magnetic anomalies and are coordinated with 

 the known regional and local geology. 



In many problems where prior subsurface development work has been 

 done, it is advantageous to carry out magnetic studies over known sub- 

 surface features or producing fields and utilize the results for interpre- 

 tative control. For instance, in magnetic studies of the Sparta- Wilcox 

 trend in central Louisiana, control studies were conducted over the Eola 

 and Cheneyville structures, and the control used as a guide in interpre- 

 tation. Similar studies have been employed in mapping the extension of 

 the Kettleman structure in California. The magnetic results obtained 

 over the Hobb's Field in Lea County, New Mexico (see Figure 106) have 

 been used as interpretative control for numerous studies made in that 

 area. The rapidity and low cost of magnetic work allows this type of 

 interpretative control to be economically feasible. The combined recon- 

 naissance and detail work in this type of study seldom cost more than 

 23/2 to 5 cents per acre. 



Model Experiments. — Model experiments are of value in deter- 

 mining the relationship between the directions of the magnetic vectors in 

 space and the shape of a magnetic structure. The operating principle is to 

 construct small-scale models having relative dimensions and geometric dis- 

 position similar to those assumed for the anomalous geologic bodies. 



Experiments on model ore bodies have been reported by Hotchkiss, t 

 Keys, t and Jenny. § 



Hotchkiss investigated the magnetic effects as a function of the dip 

 and strike of model formations. One of the experimental arrangements 

 consisted of a drawing board with a sheet of tin suspended by a wood 

 frame from its side. The sheet was positioned at various distances from 



t W. O. Hotchkiss, Mineral Land Classification, Wisconsin Geological and Natural History Sur- 

 vey Bulletin, No. 44, Madison, 1915, Ch. IV, "Magnetic Observations," pp. 112-126. 



t D. A. Keys, A.I.M.E. Geophysical Prospecting, 1932, pp. 205-208. 



§ W. P. Jenny, "Experimental Interpretation of Magnetic and Gravimetric Anomalies," Terr. 

 Mag., 40 (1), 1935, p. 72. 



