BORE-HOLE INVESTIGATIONS 



1101 



The radioactivity-logging instrument is often combined with a magnetic casing- 

 collar locator which detects the magnetic discontinuities in the steel well-casing and 

 transmits this intelligence to the surface equipment. There a second pen, working in 

 the margin of the recorder chart, places a mark corresponding to the depth of each 

 casing collar. By this means the radioactivity curves, and thus the location of the 

 strata, are tied directly to permanent depth reference markers which can be located 

 precisely on subsequent runs of these or other well tools. Much use is made of these 

 collar determinations in the subsequent positioning of gun perforators in order that 

 the shots may be placed in the desired formations with accuracy. 



The field techniques employed in obtaining a radioactivity log are very similar 

 to those used in other forms of well-logging and therefore need not be detailed here. 

 However, certain phases of the procedure which are peculiar to the measurement of 

 radioactivity will be discussed. 



Just before the subsurface instrument is lowered into the well, its sensitivity to 

 radiation is checked and standardized by placing a standard source of radiation adjacent 

 to the center of the detector. 



The response of the system to this standard radiation is recorded in terms of inches 

 of pen travel. After traversing the well on the downward trip, and observing the 

 relative activity obtained, the scale sensitivity may be changed at the operator's 

 discretion. The initial response to the standard radiation is then multiplied by the 

 factor by which the scale has been changed. This latter figure is then a direct measure 

 of the overall sensitivity employed in making the record, and is indicated on the log, 

 at the beginning and end of any section logged at this value of sensitivity. 



Fig. 688. 

 (Courtesy of Lane-Wells Company.) 



Logging is always conducted during the upward trip in order to secure the most 

 accurate depth registration, since the upward travel of the instrument with the hoisting 

 cable in tension is generally more uniform. Logging speeds from 25 to 50 feet per 

 minute are employed customarily, although higher speeds may be used at some sacrifice 

 in accuracy. This is brought about by the extremely low levels of radiation intensity 

 being observed. The total number of radiation quanta entering the detector per 

 second is generally quite small, and a sufficient number of quanta must be observed 

 in order to determine the intensity, or average rate, with the desired accuracy. By 



