510 



EXPLORATION GEOPHYSICS 



when the stations are placed close together. In areas where sufficient 

 resistivity depth variations occur and where the lithology is fairly uni- 

 form, with resultant small variations in the characteristics of the curves, 

 reliable correlations often may be made between stations which are many 

 thousands of feet apart. The latter condition prevails in many parts 

 of the Mid-Continent area and in the Permian basin area of New Mexico 

 and Texas. 



To correlate curves, the field data for each station are plotted with the 

 resistivity values as abscissas and the separations of the current electrodes 

 as ordinates on a separate sheet of transparent cross-section paper. Cor- 

 rections for surface effects are made as illustrated in Figure 303. The 

 corrected curves corresponding to the various stations are then matched 

 over a light box, i.e., correlated as a unit. The interpreter first correlates 

 the curves for the first two stations at one end of the traverse and then 

 proceeds to successive stations. The correlation of four curves is shown 



POWER ELECTRODE DISTANCE - FEET 



Fig. 307. — Correlation of resistivity curves from four adjacent stations along a traverse line. 



in Figure 307. Before evaluating the subsurface interval between stations, 

 it is necessary (a) to correct for variations in surface topography and ele- 

 vation, and (b) to determine the effective depth of measurement. 



The topography along the traverse is taken by transit and stadia rod, 

 preferably during the initial laying-out of the traverse line, and the ele- 

 vations of the electrode locations are reduced to a mean datum plane. 



The effective depth of measurement varies with the area and electrode 

 configuration used and can seldom be predicted or calculated from known 

 theoretical relationships. Usually it is determined by control data taken 

 from geologic logs of wells in the area. When work is done in areas where 

 proper well control is not available, a penetration factor is assumed, the 



