1100 EXPLORATION GEOPHYSICS 



plate of the continuously vibrating condenser to a higher value than that of the 

 opposite plate. The action of the vibrating condenser, which can be likened to that 

 of a condenser microphone, is such as to generate a small alternating current signal, 

 which is amplified in the subsurface amplifier and coupled to the hoisting cable through 

 a transformer. 



Another signal, having the form of a damped oscillation, is derived from the 

 action of a pair of contacts driven mechanically with the vibrating condenser. This 

 phase reference signal is transmitted continuously over the same circuit to the surface. 



At the surface, these signals are further amplified in frequency-selective ampli- 

 fiers and combined in a synchronous rectifier to produce a direct current output. The 

 polarity and magnitude of this direct current are representative of the magnitude 

 and polarity of the unbalance between the potentials at the vibrating condenser. This 

 output current is used to control the speed and direction of motion of the pen of a 

 commercial type high-speed motor-driven recorder. (Figure 686.) 



Under the conditions described above, the polarity of the current will be such 

 as to cause the motor to drive the recording pen to the right, and simultaneously to 

 move the contact arm on the slide-wire so that more direct current flows from the 

 surface power supply, through the well cable to the subsurface instrument. There 

 this increased current, flowing through R2, raises the potential of the lower plate of 

 the vibrating condenser. It will now be seen that the recorder motor continues to 

 run until it has caused the potential of the lower plate of the vibrating condenser 

 to equal that of the upper plate. When this condition has been achieved, the motor 

 stops, since the vibrating condenser produces no alternating output signal when its 

 plates are equipotential. The pen has now been moved to the right by an amount 

 corresponding to the increase in radiation intensity and the system is again in balance. 

 A decrease in radiation will cause the system to operate similarly, but in the opposite 

 direction, since the polarity of the signals will be reversed. 



The direct current transmitted down the cable to achieve and maintain balance 

 is, in practice, many thousands of times greater than the ionization chamber current, 

 the exact ratio being determined solely by the relationship between the resistors Ri, 

 through which the ionization current flows, and R2, through which the feed-back 

 current flows. Usually, the value of Ri is chosen about 10^° times larger than R2 so 

 that, at balance, the current transmitted down the cable to the subsurface instrument 

 is related to the ionization current by this ratio. Since the system continuously and 

 automatically maintains the condition of balance, the value of this feed-back current 

 accurately represents the value of the ionization current, and, therefore, the value of 

 the radiation intensity encountered by the ionization chamber. By virtue of operating 

 continuously to maintain a null-balance condition, the system is essentially independent 

 of variations of gain in the A.C. amplifiers, and of the transmission characteristics 

 of the cable. This enables the subsurface equipment to be powered by self-contained 

 dry batteries, even though such batteries undergo large changes when subjected to 

 the elevated temperatures encountered in many wells. 



The scale-factor of the recorder, or the amount of pen travel for a given change 

 in radiation intensity, may be varied by adjustment of the value of the resistor R3, 

 through which the feed-back current flows. It will be seen that a greater travel of 

 the slide-wire arm, and hence of the recording pen, is required to produce the same 

 current change when this resistor is made larger. 



In secondary ray logging, one example of which is the neutron curve, the instru- 

 mentation remains basically unchanged. The subsurface instruments used for this 

 type of logging carry an emitter of radiation, consisting usually of a small capsule 

 of radioactive material, and generally employ a smaller, less sensitive detector in 

 order to minimize the eff^ects of the naturally occurring variations in radioactivity. 

 By suitable choice of the strength of the radiation source, the secondary radiation 

 is made many times greater than the normal gamma-ray activity of the earth's strata, 

 so that the curve produced by the neutron-logging instrument is essentially free from 

 this effect. 



