422 Subsurface Geologic Methods 



small that one thousand metric tons would be required to extract a few 

 milligrams of the radium element. The well-logging instrument employs 

 an ionization chamber to observe the radiations. The thick wall of the 

 ionization chamber serves two purposes, which are (1) to resist the 

 pressure of the fluid head in the well, and (2) to select the most pene- 

 trating part of the radiation in the bore hole. The ionization chamber 

 contains an inert gas under pressure, in which are immersed two insulated 

 electrodes. One of these insulated electrodes is connected to a battery, 

 which keeps it at a positive potential with respect to ground. Gamma 

 rays passing through this inert gas partly ionize the gas, permitting a 

 current flow between the two electrodes. This current is amplified by an 

 amplifier in the subsurface detector and is transmitted through a conduc- 

 tor cable to the surface, where it is further amplified by surface equip- 

 ment and recorded on a pen-and-ink-type recorder. 



The neutron curve is recorded in a similar manner with the excep- 

 tion that the strata are bombarded by a very strong source of neutrons, 

 which are contained in a neutron source immediately below the ionization 

 chamber. The ionization chamber of a neutron well-logging instrument 

 is so designed that the instrument will not respond to natural radioactive 

 emanations but is sensitive only to secondary-gamma-ray radiations ex- 

 cited by the neutron bombardment. Current amplification, transmission . 

 to the surface, and surface amplification are similar to the gamma-ray- 

 recording process. 



Figure 192 demonstrates a typical field setup. Service equipment 

 consists of one truck, called the "hoist truck," which carries a reverse 

 concentric cable, the hoist, the power-supply unit, and other equipment 

 necessary to the mechanical part of the operations. The second truck, 

 lighter in weight and called the "instrument truck," carries the automatic 

 recorder, amplifiers, and other electronic equipment used in recording the 

 logs. Constant communication between the two trucks on the job is pro- 

 vided by an electric intercommunication system. The measuring sheave 

 and weight indicator suspended and centered over the hole keep the hoist 

 operator informed as to well conditions and the position of the subsurface 

 instrument. Depth of the instrument as measured by the calibrated sheave 

 is indicated electrically on the odometer dial in the hoist operator's control 

 panel, on a similar odometer in the instrument truck, and on the record- 

 ing paper. 



Figure 193 is an illustration of the interior of an instrument truck. 



Interpretation 



Any well log must be interpreted in terms of geology and strati- 

 graphy before its utility can be realized. Because the proper interpretation 

 of a radioactivity well log will identify the various formations represented 

 on the log and determine their characteristics and extent, such interpreta- 

 tion necessarily involves a wide variety of both geology and bore-hole 



