Subsurface Logging Methods 



431 



which is a record of the exact location of each casing collar. It is re- 

 corded electrically by means of a collar locator which is a component 

 part of the gamma-ray instrument. Thus, the collar log is recorded simul- 

 taneously and on the same chart with the gamma-ray curve. This provides 

 a permanent record of the fixed relationship of collars to formations. 



Figure 199. Radioactivity curves in Salt Flat field, Texas. Radioactivity increases 

 from left to right. Note maximum point on gamma curve opposite Eagle Ford 

 shale. (Lane-Wells.) 



Formation tops and bottoms are accurately established in relation to 

 the nearest casing collar. This combination eliminates the many measur- 

 ing discrepancies that may occur in various means of well measuring. 

 The second advantage of radioactivity well logging is its ability to 

 log in contaminated well fluids. In areas such as central Kansas where 

 salt beds are encountered in drilling, the resistivity of the drilling fluid 



