ELECTRICAL METHODS 



505 



ure 302B.) Upon completion of this series of measurements, the entire 

 configuration is again moved another 1000 feet forward along the traverse 

 line, and the entire procedure repeated. The technique of making two 

 series of measurements (outward and inward) at one location constitutes 

 the occupation of one station. As explained later, the stations are spaced 

 to allow an overlap of about three stations in order to facilitate proper 

 evaluation of near-surface effects. f 



The potential-current ratios for each station are now reduced by an 

 appropriate formula * and are plotted with resistivity as ordinate and 

 electrode separation as abscissa. The near-surface resistivity anomalies 

 appear when the moving' energizing electrode traverses the inhomo- 

 geneities. Hence, the anomalies are present at the same traverse point on 

 all of the overlapping curves,** and the initial step in the interpretation 

 process is to "match" or "correlate" each of the apparent resistivity curves 

 at its respective traverse location. 



The correlation of four overlapping stations located on a short section 

 of traverse line in the Permian basin near Lovington, New Mexico, is 

 shown in Figure 303.*** The irregularities produced in the resistivity 

 profile by the near-surface inhomogeneities are quite pronounced. Promi- 

 nent anomalies which occur on all of the curves at the same traverse 

 location are marked at A, B, and C. Many minor anomalies will be seen 

 by more detailed study of the curves. 



TRAVERSE DISTANCE - FEET 



Fig. 303. — Correlation of four overlapping stations on a traverse line. 



Generally the shallow near-surface effects are very apparent due to 

 their abruptness and short lateral extent. Oftentimes, however, near- 

 surface effects exist over a considerable distance and are not easily differ- 

 entiated from the deeper effects. 



t J. J. Jakosky, "Method and Apparatus for the Electrical Exploration of the Subsurface." 

 U. S. Patent 2,211,125, Aug. 13, 1940. 



* See p. 518. 



** The shapes of corresponding near-surface anomalies which occur in curves of 

 overlapping stations are generally not identical. 



*** In practice the individual station curves are not replotted as was done for this 

 illustration, but are matched by using a light box and superposing the curves at their 

 proper traverse separations. 



