1098 



EXPLORATION GEOPHYSICS 



detector into a form suitable for transmission to the surface, and a set of batteries 

 for supplying power required by the detector and electrometer. 



A hoisting cable, comprising an insulated copper conductor surrounded by two 

 layers of steel wires, wound reverse-concentric, furnishes the electrical circuit between 

 the bore-hole instrument and the surface measuring equipment. 



At the well head the cable passes over a calibrated measuring sheave, which is 

 supported by a spring-type weight indicator. (See Figures 652 and 666.) From the 

 sheave, the cable passes to the hoist truck which contains a hoisting drum, driven 

 through an auxilliary transmission from the truck engine, and the necessary instru- 

 ments and controls. Electrical connections from the well cable are brought out through 

 revolving slip-rings mounted on the hoist-drum shaft, and pass through an inter- 

 connecting cable to the instrument truck. 



i 



Fig. 686. — Radioactivity logging instrument truck. A, graphic recorder; 

 B, oscilloscope panel; C, amplifier panel; D, power supply panel; E, inter- 

 communicating loudspeaker to hoist operator; F, depth indicating odometer; 

 G, ventilator; H, recorder control panel. (Courtesy of Lane-Wells Company.) 



The instrument truck contains the surface amplifiers and the recorder as shown 

 in Figure 686. This equipment is operated from 115-volt, 60-cycle current generated 

 by a portable power unit carried on the hoist truck (Figure 663). 



Depth indicators in both trucks are electrically driven by synchros connected 

 to the measuring sheave. The recording chart is similarly driven in order to register 

 the subsurface radioactivity measurements as a function of depth. 



Subsurface instruments of several different types are used for gamma-ray logging 

 and neutron logging, depending upon the well diameters, bottom hole pressures and 

 other factors encountered in various areas. The radiation detectors employed in the 

 most frequently-used instruments are high pressure ionization chambers, containing 

 nitrogen, argon or other inert gases at pressures around 100 atmospheres. In the 

 presence of gamma radiation, a small current flows between a pair of electrodes in 

 the ionization chamber and the magnitude of this current is a measure of the intensity 

 of the incident radiation. 



