506 



EXPLORATION GEOPHYSICS 



The dotted curves of Figure 303 are the diagnostic curves utihzed 

 in the interpretation. These curves are obtained by "smoothing-out" the 

 observed data so as to minimize the near-surface variations. (Experience 

 is required in drawing the best-fitting "smoothed-out" curve.) 



Alternative Configuration. — A useful modification of the electrode 

 system described above is obtained by placing the stationary current elec- 

 trode midway between the stationary potential electrodes. With this 

 arrangement, the electrical fields produced at the potential electrodes by 

 the fixed current electrode are the same, provided the earth between the 

 two power electrodes is homogeneous. Hence, variations in the measured 

 potential difference depend only on the relative position of the moving 

 power electrode. This arrangement also has a greater sensitivity than the 

 arrangement previously described wherein both power electrodes are 

 located outside the potential electrodes. 



Dip Determination at a Single Station. — Determination of the dip 

 at single stations may often be accomplished by comparing the curves 

 obtained from measurements made in three or more directions from the 

 station hub.t The measurements are usually made by a tri-directional 

 system, as illustrated in Figure 304. The three lines of measurement are 

 laid out at 120° angles, with the corresponding points of measurement on 

 each line being equidistant from the station hub. Current is passed into 

 the ground between electrodes Ia. and Is, then between /^ and Ic, and 

 and then between Ib and Ic, the points of measurement moving progres- 

 sively outward as the depth of penetration is increased. It will be recog- 

 nized that this procedure is a com- 

 bination of lateral and vertical in- 

 vestigation. 



The subsurface distribution of 

 current can be predicted by one 

 of two types of measurement: (a) 

 potential measurements at a given 

 spacing from the energizing elec- 

 trodes or (b) electromagnetom- 

 eter measurements wherein the 

 strength of the magnetic field as- 

 sociated with the subsurface cur- 

 rent fiow is measured at the hub. 



Analysis of the multi-directional measurements is usually done by 

 some form of curve correlation procedure. The curves for each line of 



t J. J. Jakosky, "Methods of Determining Underground Structure," U. S. Patent No. 2,138,818, 

 Dec. 6, 1938. 



J. J. Jakosky and C. H. Wilson, "Electrical Mapping of Oil Structures," Mining and Metal- 

 lurgy, May 1936, pp. 231-237. 



J. J. Jakosky and C. H. Wilson, "Prospecting for Oil Structure by Electrical Methods," 

 The Petroleum Engineer, Feb. 1937, pp. 143-149. 



RCSfSTtVITY 



Fig. 304. — Tri-directional system of measurement 

 for determining dip at a single station. 



