ELECTRICAL METHODS 507 



measurement are plotted as shown in the figure and are correlated by the 

 method outlined on p. 509. 



ANALYSIS OF RESISTIVITY DATA 



Various interpretative procedures have been developed for inferring 

 the subsurface structure from resistivity data. The mathematical methods 

 have been discussed by Tagg, Roman, t Hummel, t Watson § and others, tt 

 The method of Tagg has already been considered. Apart from the mathe- 

 matical methods, the interpretative procedures commonly employed may 

 be classified into three groups: (1) rule of thumb or visual interpretation; 

 (2) curve correlation methods which often can be applied successfully in 

 many cases of complex structure where the more basic mathematical 

 analysis would be either impractical or impossible; (3) small scale 

 experiments. 



Workers in electrical geophysics are divided into two general schools 

 of thought regarding the technique of interpretation. The mathematically- 

 minded school holds that the "rule of thumb" methods are not sufificiently 

 precise or scientific, while the self-styled "practical" school holds that the 

 mathematical parameters cannot be evaluated with sufficient accuracy under 

 normal field conditions to justify the assumptions necessary for a rigid 

 mathematical basis of interpretation. This school bases its interpretation 

 upon visual inspection of the depth-resistivity or traverse-resistivity curves, 

 and upon the experimentally proven fact that the major trend breaks may 

 be caused by changes in the subsurface. 



As in most controversies of this nature, both schools are correct in their 

 contentions under certain conditions. Experience is essential for correct 

 interpretation of the electrical data. 



Rule of Thumb Theorems for Determining Effective Depth of 

 Measurement. — In an attempt to simplify the calculations necessary 

 to interpret field data, many empirical rules have been devised. The 

 literature of electrical prospecting contains numerous illustrations of the 

 application of such rules to actual problems. Many of these rule theorems 

 are valid for the particular problems for which they were derived. How- 

 ever, the blind application of such rules to general exploration problems 

 usually results in errors because the conditions under which the empirical 



t Irwin Roman, "How to Compute Tables for Determining Electrical Resistivity of Underlying 

 Beds and their Application to Geophysical Problems," U. S. Bur. of Mines, Tech. Pub. 502; "Some 

 Interpretations of Earth Resistivity Data," A.I.M.E. Geophysical Prospecting, 1934, pp. 183-201. 



t J. N. Hummel, "Theoretical Study of Apparent Resistivity," A.I.M.E. Geophysical Prospect- 

 ing, 1937, Tech. Pub. 496; "A Theoretical Study of Apparent Resistivity in Surface Potential 

 Methods," A.I.M.E. Geophysical Prospecting, Tech. Pub. 418, 1931. 



§ R. J. Watson, "A Contribution to the Theory of the Interpretation of Resistivity Measure- 

 ments Obtained from Surface Potential Observations," A.I.M.E. Geophysical Prospecting, Tech. 

 Pub. 518, 1934. 



tt D. O. Ehrenburg and R. J. Watson, "Mathematical Theory of Electrical Flow in the Strati- 

 fied Media," A.I.M.E. Geophysical Prospecting, Tech. Pub. 400, 1931. 



S. Stefanesto, in collaboration with C. and M. Schlumberger, "Sur la Distribution Electrique 

 Potentielle Autour D'Une Prise de Terre Ponctuelle Dans un Terrain A Couches Horizontales, 

 Homogenes et Isotropes," Le lournal de Physicue et le Radium, Vol. 1, 1930, p. 132. 



C. L. Pekeris, "Direct Method of Interpretation in Resistivity Prospecting," Geophysics, Vol. 

 V, No. 1, January, 1940, pp. 31-42. 



