514 



EXPLORATION GEOPHYSICS 



primarily upon the effective volume included in the zone of measurement 

 rather than upon depth. Part B shows a flat lying structure, the weighted 

 or mass effect of which is equivalent to that of part A. Thus, both types 

 of structure give comparable electrical anomalies, despite the fact that 

 their reliefs are different. Analysis of resistivity data, therefore, must 

 proceed from the viewpoint of a weighted average, rather than from the 

 viewpoint of a direct measurement.* 



2. Variations in Effective Penetration. — The effective depth of 

 current penetration is dependent to a large extent on the relative con- 

 ductivities of the strata which constitute the volume included in the path 

 of current flow. The ratio of the effective depth of penetration to the 

 distance between the two power electrodes is termed the "penetration 

 factor." This factor usually is not constant in any one area or even for 

 any one station but varies with changes in the ratio of the conductivities 

 of the near surface and the deeper strata. 



Fig. 311. — Sketch illustrating current paths for a structure comprising a layer of high resistivity 

 underlain by a good conductor. 



The reason for the changes in the penetration factor will be evident 

 from the simplified diagram given in Figure 311. The diagram shows a 

 layer of high resistivity underlain by an excellent conductor, t At elec- 

 trode separations less than d, the effects of the good conducting layer is 

 small and the penetration factor is relatively small. The highest pene- 

 tration factor will be obtained when the distance L is slightly more than 

 2d, due to the bending down of the current lines toward the good con- 

 ductor. As the electrode separation is made greater than the distance 

 2d, the current lines tend to be confined within the highly conducting 

 layer and the penetration factor decreases again, 



3. Anisotropic Media. — In many cases, the formations investigated 

 by the resistivity methods possess a marked electrical anisotropy which 

 influences the data obtained in surface measurements. Theoretical inves- 



*The other potential methods, gravitational and magnetic, also depend on mass 

 or weighted effects. 



t See also, Warren Weaver, "Certain Applications of the Surface Potential Method," A.I.M.E. 

 Geophysical Prospecting, 1929, p. 68. 



