BORE-HOLE INVESTIGATIONS 



1057 



AC. Supplij 



An alternate type of equipment is schematically illustrated in Figure 653. An alter- 

 nating current, which is generated by a vacuum tube oscillator and has a frequency of 

 approximately 700 cycles per second, flows in the cable to the main electrode and back 

 through the formations to the return electrode at the surface. Impedance measure- 

 ments are made between the single electrode in the well and the surface electrode. At 

 the same time over the same cable, it is possible to measure the D.C. self potential 

 existing between the two electrodes. The single conductor cable therefore serves both 

 for the D.C. and the A.C. measurements. 



Measurements are made between the 

 moving electrode A in the hole and the sta- 

 tionary surface electrode B. Changes in the 

 effective resistance or impedance are recorded 

 on the meter 1. The direct current recording 

 meter 3 is shunted by the large condenser 2, 

 which by-passes the alternating current. The 

 winch and commutator are shown diagram- 

 matically at C. 



Numerous telemetering arrangements 

 have been proposed for transmitting multi- 

 channel information over a single conductor.f 

 To date these devices have not met with favor 

 in well-logging applications because of instru- 

 mental complications. Usually the multi- 

 conductor cable is cheaper and easier to 

 maintain than the single-conductor cable and 

 more complicated carrier system. 



Induction Logging 



Electrical logging, using electrodes, 

 requires electrical contact with the for- 

 mations. Ordinarily this contact is 

 made through the drilling mud in the 

 bore-hole which usually has a water 

 base and is therefore conductive. 



However when the drill hole is 

 empty, as may be the case when the 

 drilling is done with cable tools, or when oil-base mud is used, electrical 

 contact between electrodes and formation is no longer attained by conduc- 

 tion through the drilling fluid. Scratcher electrodes, which are forced by 

 spring action against the wall of the bore-hole to make direct contact with 

 the formations, are often employed. In some cases fairly satisfactory 

 resistivity curves are recorded in this manner, but oftentimes when the 



t H. W. Lensner and J. B. Singel, "A V'ersatile Power Line Carrier System," A.I.E.E. Tech. 

 Pub. 44-34, Dec, 1943. 



R. C. Cheek, "A Comparison of the Amplitude-Modulation, Frequency-Modulation, and Single- 

 Side-Band Carrier Systems for Power-Line Carrier Transmission," A.I.E.E. Transactions, May, 

 1945, pp. 215-220. 



S. Krasnow, "Method and Apparatus for Taking Physical Measurements in Bore-holes, U. S. 

 Patent 2,421,423, June 3, 1947. 



W. D. Mounce, "Logging Boreholes," U. S. Patent 2,415,364, Feb. 4, 1947. 



A. Frosch, "Well Logging," U. S. Patent 2,436,563, Feb. 24, 1948, 



L. Dillon, "Method and Apparatus for Logging Drill Holes," U. S. Patents 2,425,868 and 

 2,425,869, Aug. 19, 1947. ^ ,^ 



P. Subkow & L. Dillon, "Method and Apparatus for Logging Drill Holes, U. S. Patent 

 2,225,668, Dec. 24, 1940. 



Fig. 653. — Method for simultaneous resis- 

 tivity and potential logging with single 

 moving electrode. A, moving electrode; B, 

 stationary surface electrode; C, winch and 

 commutator assembly; 1, A.C. recording 

 meter; 2, by-pass condenser; 3, D.C. record- 

 ing meter; 4, coupling transformer; 5, sheave 

 wheel. (Halliburton Oil Well Cementing 

 Company.) 



