1002 



EXPLORATION GEOPHYSICS 



and quick response. Such instruments are usually provided with strip 

 chart recording meters and are generally mounted in light trucks (Figure 

 617), together with the accessory equipment, storage batteries, charger, etc. 

 Heavy-duty trucks carry the large winches required to hold the long cables 

 necessary for oil-well logging. The winches have internal slip-rings to 

 connect the cable to the surface instruments (Figures 664 and 665). The 

 diamond-drill holes commonly drilled for mineral exploration are relatively 

 shallow, and instruments for logging them are generally much lighter than 

 their oil-well counterparts. 



Both high-pressure D.C. ioni- 

 zation chambers and special high- 

 efficiency G-AI counters are used 

 for T-ray logging of oil wells. The 

 chambers are filled with argon or a 

 mixture of argon and CO2 at pres- 

 sures up to several hundred atmos- 

 pheres, and electrometers of the 

 vibrating-reed or capacitative com- 

 mutator typef are used to measure 

 the ionization current. The combi- 

 nation has a very high sensitivity 

 for Vs. 

 The vibrating-reed electrometer (Figure 618) is based on the principle that a small 

 alternating current is easier to amplify than a small direct current. The A.C. is pro- 

 duced by impressing the small D.C. voltage on a condenser, one plate of which is made 

 to vibrate at a frequency of several hundred cycles per second. The resulting A.C. 

 passes through a condenser into a linear audio amplifier. After sufficient amplification 

 it is synchronously rectified and measured. 

 Simple Geiger counters are not 



AMPLIFIER 



Fig. 618. — Principle of vibrating-reed electrometer. 



METAL PLATES 



4 



ft 



Fig. 619. — High-efficiency gamma counter with cellu- 

 lar cathode for increased sensitivity and faster rate of 

 logging. (After S. C. Brown.) 



sensitive enough for the fast gamma- 

 ray logging of oil wells (up to 100 

 feet/min.) but their sensitivity may 

 be increased by increasing the effec- 

 tive area of their cathode, either by 

 fabricating it from wire screenj or 

 by cellular construction (Figure 

 619). A wire-screen cathode has a 

 greater effective area than a solid 

 wall and therefore emits more pho- 



toelectrons in a given gamma-ray flux. The comparable efficiency of a cellular cathode 

 may be made several times greater than that of a simple cylinder of equivalent size. 



The diamond-drill holes are usually too small for the ionization instruments and 

 cellular counters. On the other hand, the holes are frequently shallow so that great 

 logging speeds and correspondingly high sensitivity are not required. Simple G-M 

 counters are amply sufficient for most diamond-drill-hole logging, and counters with 

 very low sensitivity are even essential for the exploration of drill holes in uranium- 

 bearing ore bodies. 



The smallest conventional diamond-drill holes are 1.45 inches in diameter, and it 

 may be necessary to case them before the logging probe can be lowered safely. There- 



t H. Palewsky, R. K. Swank and R. Grenchik, "Design of Dynamic Condenser Electrometers," 

 Rev. Sci. Inst. 18, 298-314 (1947). 



S. A. Scherbatskoy, T. H. Gilmartin and G. Swift, "The Capacitative Commutator," Rev. Sci. 

 Inst. 18, 415-421 (1947). 



t W. Good, A. Kip and S. Brown, "Design of Beta-Ray and Gamma-Ray Geiger-Miiller Count- 

 ers," Rev. Sci. Inst. 17, 262-5 (1946). 



