BORE-HOLE INVESTIGATIONS 



1099 



Under normal gamma-ray well-logging conditions the ionization current may be 

 of the order of 10"^'* amperes, and changes in this current of the order of 10% are 

 required to produce a recording pen movement of one or more inches. A pen de- 

 flection of one-tenth inch, which is about the smallest readable detail, thus may cor- 

 respond to a change in ionization current as small as lO"'"^ amperes. The accurate 

 measurement of these extremely minute direct current signals and the transmission 

 of this data to the surface require instrumentation of rather unusual characteristics. 



A continuous automatic null-balance system is employed to transmit and record 



FILTER 



WELL 

 CABLE 



^ 



SYNCHRONOUS 

 RECTIFIER 



D.C. FEEDBACK 

 CURRENT 



<d- 



FILTER 



¥- 



RECTIFIER AMPL. 



^ 



RECORDER 

 MOTOR 



.SLIDE 

 EWIRE 



DC 

 POWER 

 SUPPLY 



L^|N_£: 







DRIVE 

 COIL 



VIBRATING 

 CONDENSER 



|R2 



^ 



+- 



^ 



PSCILLATOR a 



IMPULSE 



GENERATOR 



c 



POWER 

 SUPPLY 

 BATTERIES 



Fig. 687." 



-(Above) Functional schematic diagram of radioactivity well-logging system surface 

 section. (Below) Subsurface section. 



the magnitude of the ionization current. This is accomplished by the use of a "capacita- 

 tive commutator" electrometerf in the subsurface instrument, together with appropriate 

 amplifying and recording apparatus at the surface. 



A block diagram of the system is shown in Figure 687. Operation of this system 

 may best be explained by tracing the actions of the successive elements, starting with 

 the ionization chamber. 



Assuming the system to be in balance, with the recording pen at rest and the 

 logging instrument hoisted into a region of greater radiation intensity. The minute 

 current in the ionization chamber will increase, thus raising the potential of the upper 



t See Figure 618, Chapter X. 



