ELECTRICAL METHODS 



503 



Double - Depth Investigations. — The dififerentiation of the near- 

 surface effects from the deeper structural effects can oftentimes be accom- 

 plished by comparing measurements at two different depths of penetration. 



Double-depth investigations may be divided into two general groups : 

 (a) lateral exploration and (b) vertical exploration. Illustrations of both 

 types will be given to indicate the general principles involved in the field 

 technique. Various other modifications will occur to the reader. 



TRAVERSE DISTANCE OF LEADING ELECTRODE. NS I 



Fig. 301. — Illustration of surface corrections in the lateral exploration 

 method. 



Lateral Exploration 



The operating principle of the lateral exploration method will be 

 evident from a consideration of Figure 301. An electrode system com- 

 prising electrodes 1, 2, 3, and 4 is moved as a unit along a traverse line. 

 The separation between power electrodes 1 and 4 is only sufficient to give 

 an effective penetration which includes the upper near-surface beds, A 

 third energizing electrode 5 trails the electrode system (consisting of 1, 

 2, 3, and 4) at a constant distance which depends on the desired depth 

 of effective penetration into the deeper lying beds. Two sets of resistivity 

 measurements are made for each position of the electrodes : In one, the 

 energizing current flows between electrodes 1 and 4 ; in the other, the 

 energizing current flows between electrodes 1 and 5. 



The apparent resistivity data are plotted with the resistivity values 

 as ordinates and the traverse positions of the leading power electrode as 

 abscissas. (Lower portion of Figure 301.) Curve A corresponds to the 

 case that the energizing current flows between electrodes 1 and 4, and 

 curve B to the case that 1 and 5 are the energizing electrodes. In this 

 particular area, the curves show that the shallow material affecting the 



