Subsurface Logging Methods 377 



zones. The true resistivity required for this latter work may be obtained 

 from the departure curves or sometimes directly from the long normal or 

 lateral curve. 



Experience and research have proved the general over-all utility of 

 the multi-electrode method for making resistivity measurements, as it 

 minimizes the effect of the drilling fluid and the well bore, and it makes 

 possible a direct comparison of the several recorded resistivity curves. 

 Multi-electrode recording, as distinguished from single- or point-electrode 

 measurements, is made with a system of four electrodes; two of these are 

 current emitting and two are for potential measurement. The curves re- 

 corded are termed "normal" or "lateral," depending upon the electrode 

 arrangement. 



Terminology 



Electrodes — The current electrodes are designated "A" and "B," the 

 measuring electrodes "M" and "N." Common practice is to have the two 

 current electrodes, A and B, and one potential-measuring electrode, M, 

 in the hole with the other potential electrode, N, at the surface. Some of 

 the electrodes in the hole are mounted on a mandrel, called a "sonde," 

 which serves as a guide and weight for the cable. 



Normal Curve — A normal curve is a resistivity log recorded with the 

 four-electrode system where the distance between one current and one 

 potential-measuring electrode, AM, is of primary importance. The posi- 

 tion of the other current electrode, B, is relatively unimportant as long as 

 the distance, AM, is small as compared to AB (fig. 161). 



Amplified Normal Curve — A resistivity log recorded with the normal- 

 curve-electrode arrangement but using an amplified or exaggerated scale is 

 an amplified normal curve. For example, the normal curve might be re- 

 corded on a 20-ohm scale, the amplified normal with a 4-ohm scale. 



Long Normal Curve — A long normal curve is a resistivity log re- 

 corded with the same electrode arrangement as the normal but with the 

 distance, AM, several times as great as the normal. 



Lateral Curve — A resistivity log recorded with the four-electrode sys- 

 tem, where the distance between one potential-measuring electrode and a 

 point midway between the two current electrodes, AB, is of primary im- 

 portance, is a lateral curve. The distance AB is small as compared with 

 the distance AM (fig. 162). 



Long Lateral Curve — A resistivity log recorded with the same elec- 

 trode arrangement as the lateral but with the distance between M and the 

 midpoint of AB longer than that of the regular lateral is a long lateral 

 curve. 



Electrode Spacing 



Normal Curves — The spacing is considered as the distance, AM, be- 

 tween the current electrode, A, and the potential-measuring electrode, M. 

 Depending upon the geologic province, this spacing varies between eight 



