566 BUSCH. SUBSURFACE TECHNIQUES [Ch. 31 



The ionization chamber is suspended on a cable which transmits the 

 electrical impulses to the surface, where they are further amplified and 

 their magnitude is graphically recorded. A gamma-ray log is illus- 

 trated along the left side of the radioactivity log of Fig. 2. 



Neutron logging is still another method of recording the formations 

 exposed in a well bore. Such a log is illustrated along the right side 

 of the radioactivity log of Fig. 2. In this method a stream of neutron 

 particles bombards the formations in the well and causes the emission 

 of gamma rays by the hydrogen atoms in the pore liquids. The mag- 

 nitude of the emitted gamma rays is measured with the aid of an 

 ionization chamber as described above and is dependent, to a large ex- 

 tent, on the fluid content of the formation being bombarded. A mixture 

 of radium and beryllium is the radioactive material responsible for 

 the atomic dissociation and the generation of a stream of neutron 

 particles. 



A unique feature of radioactivity logs is that they may be taken 

 through several strings of casing surrounded by cement. Thus they are 

 of considerable value in determining the positions of cased-off sands in 

 old wells. Many old wells were drilled without accurate logs having 

 been kept, and this method of logging is especially adaptable in such 

 cases. 



DRILLING-TIME LOGS 



A drilling-time log is a graphic picture of the variable amount of 

 time (minutes per foot of new hole) required to drill through the dif- 

 ferent types of rock encountered. Commercial instruments have been 

 devised with which this type of information is automatically recorded 

 as the drilling of the hole progresses. A drilling-time graph is shown 

 on the left side of the mud log, illustrated in Fig. 1. A short bar in- 

 dicates a rapid rate of drilling, and a long bar a slow rate. Such a log, 

 when checked against sample cuttings or an electric log, may be of 

 considerable help in the determination of top and bottom of a stratum 

 or in detecting the most porous and soft portions of a sandstone. The 

 rate of drilling is dependent on numerous factors, chief of which are 

 the lithologic character of the rock, weight of the drill stem at the 

 bottom of the hole, rate of rotation of the bit, design and extent of wear 

 on the bit, rate of circulation of the drilling fluid, and the ability of the 

 driller to know the optimum rate of "feeding" the drill. The factors 

 that affect the hardness of a formation are mineral composition, degree 

 of cementation, and nature of cement, porosity, and so on. 



