498 EXPLORATION GEOPHYSICS 



MEASUREMENTS OF NEAR-SURFACE INHOMOGENEITIES 



In any geophysical method it is necessary that the anomahes in the 

 measured physical property be of sufficient magnitude to manifest them- 

 selves in an unequivocal manner, despite any irrelevant variables which 

 may be included in the measurements. In measurements of potentials, the 

 effect of a near-surface disturbing factor increases rapidly with the depth 

 of measurement. Very small lateral variations in the conductivity of the 

 ground near the surface will produce effects comparable to those produced 

 by structural variations of large magnitude at some depth below the surface. 

 The lateral variations may be demonstrated by maintaining the current 

 electrodes in a fixed position and measuring the E/I ratios as the potential 

 electrodes are moved to different locations. Frequently the individual read- 

 ings deviate fifty per cent or more from the computed theoretical values. 

 Structural changes, however, seldom cause a variation in the computed 

 resistivity value of more than a few per cent, except in unusual cases. It 

 is evident, therefore, that the near-surface variations tend to mask the 

 absolute resistivity variations that are associated with changes in the path 

 of current flow caused by subsurface structural changes. 



The greatest lateral variations occur in the aerated layer and are asso- 

 ciated primarily with ground water movements. Local zones of higher 

 resistivity are usually associated with descending fresh water movement. 

 The more highly conductive areas are usually found in regions where 

 ascending solutions and concentration of mineralization occur — due to 

 surface evaporation — and in regions where local moisture content is in- 

 creased by surface or near-surface impoundment of rain or drainage water. 

 These conditions often manifest themselves visibly by alkali beds and 

 changes in vegetation. Frequently, however, visual inspection fails to 

 account for certain shallow anomalies, and therefore it is necessary to 

 conduct the field work in a manner which will permit differentiating be- 

 tween the near-surface and the "deep" variations. 



Lateral Investigations. — Lateral investigations are usually con- 

 ducted to determine lateral variations associated with fault zones, contacts 

 of different formations, variations in depth to "marker" beds, etc. Lateral 

 resistivity measurements generally are carried out by moving the electrode 

 configuration as a unit along a traverse line that crosses the area. The 

 variations in resistivity are plotted against the traverse distance.* 



The manner in which near-surface inhomogeneities affect the apparent 

 resistivity values as the electrodes move over the area may be illustrated 

 by referring to Figure 297. Positions of the electrodes are shown with 

 reference to a zone of relatively high conductivity. The energizing or 

 power electrodes are designated by 1 and 4. The electrode configuration 

 shown is the conventional Wenner arrangement, wherein the separation 

 between adjacent electrodes is equal. The lines of current (not shown in 



* Measurements of this type are often termed "constant depth" traverses. 



