1010 EXPLORATION GEOPHYSICS 



since a 2-inch layer of 2 per cent ore will produce essentially the same effect as a 4-inch 

 bed of 1 per cent ore. In either case, the peak meter deflection will be substantially 

 smaller than it would be for a 2-foot layer of the same grade. The relationship of 

 deflection versus thickness for a given grade is shown graphically in Figure 628. The 

 curve levels off at about 2 feet. This distance will be somewhat less for shorter detec- 

 tors and somewhat greater for longer ones, but it is usually impractical to make small 

 tubes shorter than about 2 inches or longer than a foot or so. Nevertheless, approxi- 

 mate information on the thickness of thin radioactive beds can be obtained by logging 

 a drill hole first with a long G-M tube and then relogging the radioactive zones with a 

 very short tube. The relative widths of the deflection peaks will serve as an indication 

 of the thickness of the ore beds. 



Neutron Logging'\ 



The gamma-ray logging instrument can be used for neutron logging 

 when a source of neutrons is attached about one or two feet below the y 

 detector. The usual source of neutrons is a mixture of about 3 parts beryl- 

 lium and one part radium bromide, intimately mixed and compressed in a 

 small capsule. Such a source gives off about IC^ neutrons/sec and about 

 10^^ gammas/sec per gram of radium. A lead plug occupies the space 

 between the source and the detector, to absorb the heavy y background of 

 the neutron source. 



Most of the neutrons from the source are fast (mean energy very 

 roughly 5 Mev). They fly out into the formation and collide with the nuclei 

 of the atoms that make up the rock. They lose most of their energy in 

 collisions with hydrogen and very little in elastic collisions with other 

 nuclei. After about 25 collisions with hydrogen the neutrons are slowed 

 down to thermal energies (about .025 ev) and diffuse through the rock 

 until they are absorbed, A y is emitted in most neutron capture reactions, 

 and some of the y's find their way into the detector. 



Now if the formation contains little or no hydrogen, the neutrons will 

 travel far before they are slowed down and absorbed. A good many of 

 them will be absorbed near the y detector and we shall observe a high 

 reading on the log. When the instrument passes through formations with 

 much hydrogen (in the form of water or hydrocarbons) the y indication 

 will be small, for most of the neutrons will be slowed down and absorbed in 

 the immediate vicinity of the source and few y's will reach the recorder. 



The neutron curve is therefore useful as an indicator of the amount of 

 hydrogen in the formation. The electrical resistivity of sedimentary rocks 

 usually is an inverse function of their water content, and the correlation 

 between resistivity logs and neutron curves is frequently excellent (see 

 page 1107). 



Unfortunately, the neutron log is subject to a number of errors which must be 

 carefully considered before we attempt any interpretation. The neutron source emits 

 about 10,000 times as many y's as neutrons. Some of the quanta will pass into the 

 formation and reach the detector after one or two Compton collisions. The actual 

 number will be determined by the 7 scattering and absorption properties of the rock, 

 so that this parameter will be reflected in the neutron log. 



t B. Pontecorvo, "Neutron Well Logging," Oil and Gas Jour. 40, No. 18, 32-3 (Sept. 11, 1941). 

 R. E. Fearon, "Neutron Well Logging," Nucleonics 4, No. 6, 30-42 (June, 1949). 



