ELECTRICAL METHODS 



513 



material of the tank to a distance greater than one per cent of the mini- 

 mum electrode separation employed in the investigations. 



Tank or small scale experiments may supply considerable information 

 which will be indicative of the results to be expected in field work. How- 

 ever, the tank experiments usually do not yield the same curve character- 

 istics obtained in field work. This may be due to the absence of polariza- 

 tion and related phenomena at the interface of strata in the small scale 

 tests. It is evident, therefore, that considerable care must be taken 

 in using tank results as an aid in interpretation. 



FACTORS TO BE CONSIDERED IN INTERPRETATION 



1. Depth of Measurement and "Detectability." — In all geophysical 

 investigations in which it is desired to map a certain formation or struc- 

 tural marker with reference to a horizontal datum plane, the most accurate 



Fig. 310. — -Sketch illustrating concepts of depth of penetration and 

 detectability. 



results will be obtained when the variation of the diagnostic quantity 

 measured at the surface is proportional to the depth of the marker. 

 In the electrical methods, however, the magnitude of the diagnostic 

 variable does not depend solely on the vertical depth of the marker. This 

 will be evident from a consideration of Figure 310. Part A of the figure 

 illustrates a small, relatively steep, dipping structure. To produce a detect- 

 able variation in the resistivity value, it is necessary that the structure 

 comprise a certain portion of the subsurface included in the measure- 

 ments. The effect of the structure is a weighted or mass effect, depending 



