ELECTRICAL METHODS 



501 



298. The separation of the energizing electrodes was 2400 feet and the 

 entire configuration was moved as a unit, with readings made each 400 feet 

 along the traverse. 



Vertical Investigations. — Vertical depth investigations are often- 

 times conducted by moving the electrodes symmetrically outward from a 

 central point along a straight line. Figure 300 illustrates the anomaly 

 usually obtained when the zone of better conductivity is located asym- 

 metrically with reference to such electrode movement. The center of the 

 electrode configuration, usually designated as the station, is located as illus- 

 trated in the figure, and electrodes 1 and 2 move outwardly in one direction 

 from this central point of reference while electrodes 3 and 4 move out- 

 wardly in equal increments of distance in the opposite direction. If the 

 subsurface is homogeneous, with the exception of the zone of high con- 

 ductivity, movement of electrodes 3 and 4 outwardly will not produce any 

 appreciable change in the apparent resistivity. Therefore, the present 

 analysis may be confined to the movement of electrodes 1 and 2. As these 

 electrodes move outward, the apparent resistivity at first decreases a small 

 amount, then rises sharply. (Compare the resistivity-distance curve in the 

 lower portion of the figure.) After reaching a maximum value, the re- 

 sistivity gradually decreases and finally takes on the normal value it would 

 have in the absence of the conductor. 



Fig. 300.— Vertical investigations and near-surface effects. 



It is clear that near-surface inhomogeneities existing along the traverse 

 to the left of the center of the configuration would yield a similar re- 

 sistivity-distance curve due to the movement of electrodes 3 and 4. Evi- 

 dently, it is dif^cult to determine the near-surface correction from data 

 obtained with this type of electrode movement, because the near-surface 

 disturbances may occur on either or both sides of the center point. 



Investigations of Asymmetrical Subsurface Inhomogeneities. — 

 The problem of ascertaining the subsurface structure when the subsurface 

 inhomogeneities are asymmetrical has been attacked by numerous methods. 

 One of the first methods for detecting anomalies produced by asym- 



