MAGNETIC METHODS 183 



necessary space factor for penetration of the magnetic flux. Another dis- 

 advantage lies in the use of relatively high frequency current which intro- 

 duces a component due to the specific conductivity of the specimen. For 

 instance, a sheet of any non-magnetic metal placed near the coil will cause 

 a change in the inductance greater than the normal changes encountered 

 due to susceptibility. For this reason it is not suitable for measuring the 

 magnetic properties of highly conductive shales, slates or material from 

 mineralized zones. The method works fairly consistently on those earth 

 materials having a relatively high specific resistivity. 



In the foregoing discussion of instruments and methods for the quanti- 

 tative measurement of rock magnetization the objective has been to cover 

 typical procedures. Preference has been given to those methods which 

 could be employed conveniently in the field, in order to encourage such 

 measurements. Limitations of space, in a work of this kind, do not permit 

 the consideration of a number of other excellent and efficacious techniques 

 for susceptibility measurements. 



It is most advantageous to know the values of the magnetic susceptibility 

 of typical rocks and formations that relate to a given magnetic problem. 

 Such information minimizes one of the unknown factors often present in 

 the interpretation of magnetic data. 



The findings of W. H. Fenwickf in a study of induced and remanent 

 magnetization of rocks are in point here. He shows that the remanent mag- 

 netization can have a decided efifect on the character of a magnetic anomaly. 

 It may result in either higher or lower values of vertical or horizontal 

 intensities than are obtained if induced magnetization only is considered in 

 an analysis. 



THEORETICAL ANALYSIS OF MAGNETIC DATA 



As stated previously, magnetic anomalies are produced chiefly by dif- 

 ferences in the magnetic permeability of the rocks and formations com- 

 prising the earth's crust. In addition to the variation in permeability, 

 remanent magnetism, such as is possessed by lodestone and a few other 

 materials occurring in nature, may contribute to the anomalies. The prob- 

 lem of interpretation consists in inferring the position (or attitude), depth, 

 configuration, and general character of the subsurface body or structure 

 from the magnetic anomaly observed at the surface of the earth. 



The usual treatment of magnetic data is almost exclusively empirical. 

 Deductions and inferences are drawn in a qualitative manner from the 

 sizes and configurations of the magnetic anomalies. (Compare p. 206.) 

 However, the empirical treatment of magnetic data is based in part on a 

 knowledge of the theoretical anomalies produced by certain inhomogene- 

 ities. Furthermore, a theoretical type of analysis is applicable in certain 

 cases. Hence, it is advantageous to consider the theoretical procedure 



t W. H. Fenwick, "Two Neglected Factors in the Interpretation of Magnetic Anomalies," 

 Unpublished Thesis, Library, Colo. Sch. of Mines, Golden, Colo., 1938. 



