Simultaneous oxygen and silicon neutron activation well log using pulsed neutron source

A method for determining simultaneously the oxygen and silicon content of earth formations in the vicinity of a well borehole, comprising the steps of:

• passing a pulsed source of fast neutrons through a well borehole and repetitively irradiating earth formations in the vicinity of the borehole with bursts of fast neutrons of energy sufficient to cause the nuclear reaction 0.sup.16 (N,P)N.sup.16 ;
• detecting radiation occurring from the decay of unstable radioactive isotopes of elements in the vicinity of the borehole created by the capture of neutrons from said repetitive bursts of fast neutrons and generating voltage signals representative of the energy of said detected radiation;
• separating, according to a predetermined energy relationship, said detected radiations into at least two energy components corresponding to the decay of radioactive isotopes produced by the capture of neutrons by the elements silicon and oxygen and making counts representative thereof;
• correcting said representative counts of the radiation produced by the decay of unstable isotopes produced by the capture of neutrons by silicon for background radiation due to the degradation of gamma radiation produced by the decay of radioactive isotopes produced by the capture of neutrons by oxygen by Compton scattering of said gamma radiation;
• and
• recording said representative counts of radioactive isotopes produced by the capture of neutrons by silicon and oxygen as a function of borehole depth.