ELECTRICAL METHODS 



499 



the sketches) are concentrated in the zone of high conductivity and the 

 equipotentials are distorted so as to be displaced away from this zone. 



Referring to diagram A, as the electrode configuration approaches the 

 zone of high conductivity, the distortion of the equipotentials away from the 

 conductor decreases the potential difference between 2 and 3. Hence, 

 because the apparent resistivity is proportional to {V2— Vz) /I, the 



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TRAVERSE DISTANCE OF LEADING ELECTRODE, N9 1 

 Fig. 297. — Lateral investigations and near-surface effects. 



apparent resistivity is diminished with respect to the value it would have 

 in the absence of the conductor. (Compare the resistivity-distance curve 

 in the lower portion of the figure.) For the electrode positions shown 

 in sketch B, the potential drop between 1 and 2 is very small due to the 

 short circuiting effect of the conductor. Hence, the potential between 2 

 and 3 is large and the apparent resistivity has a maximum value. At the 

 electrode positions shown in C, the potential difference is measured across 

 the conductor and hence is very small. Thus for this position of the poten- 

 tial electrodes, the apparent resistivity is a minimum. In diagram D, the 

 relative positions of the potential electrodes are the same as in 5. Hence, 

 the apparent resistivity again reaches a maximum. As the configuration 

 continues its movement, the curve at first decreases slightly and then coin- 

 cides with the normal regional resistivity profile.* This type of anomaly 



* The normal regional resistivity profile or the regional resistivity profile is a 

 smoothed-out plot of resistivity versus distance for a given region. The best fitting 

 line through the regional resistivity profile is sometimes called the regional gradient. 



