Calibration of radioactive well logging system

Claim 1

IN RADIOACTIVE WELL LOGGING WHEREIN THE PRESENCE OF CHLORINE IN SUBSURFACE FORMATIONS ADJACENT A BOREHOLE IS DETECTED BY IRRADIATING SAID SUBSURFACE FORMATIONS WITH A SOURCE OF NEUTRONS AND DETECTING THE RESULTING NEUTRONCAPTURE GAMMA RAYS OF CHLORINE ONLY WITHIN THE ENERGY BAND OF FROM ABOUT 4.6 TO ABOUT 6.3 MEV. BY MEANS OF A LOGGING SYSTEM INCLUDING A GAMMA RAY DETECTOR AND AN ENERGY DISCRIMINATOR, THE METHOD COMPRISING CALIBRATING SAID LOGGING SYSTEM BY: POSITIONING SAID SOURCE OF NEUTRONS AND SAID GAMMA RAY DETECTOR WITHIN AN ENVIRONMENT INCLUDING A SUBSTANTIAL PROPORTION OF HYDROGEN AND IRON, DETECTING THE RESULTING HYDROGEN-CAPTURE AND IRON-CAPTURE GAMMA RAYS, PRODUCING FROM SAID DETECTED HYDROGEN-CAPTURE AND IRON-CAPTURE GAMMA RAYS PULSES HAVING MAGNITUDES PROPORTIONAL TO THE ENERGIES OF SAID GAMMA RAYS, CONVERTING SAID PULSES INTO AN OBSERVABLE ENERGY SPECTRUM INCLUDING SPECTRAL PEAKS OCCURING AT ENERGY LEVELS KNOWN TO BE REPRESENTATIVE OF SAID HYDROGENCAPTURE AND IRON-CAPTURE GAMMA RAYS, AND ADJUSTING SAID DISCRIMINATOR WITH SAID SPECTRAL PEAKS AT SAID KNOWN ENERGY LEVELS AS A REFERENCE, FOR RESPONSE BY SAID DISCRIMINATOR TO SAID NEUTRON-CAPTURE GAMMA RAYS ONLY WITHIN SAID ENERGY BAND OF FROM ABOUT 4.6 TO ABOUT 6.3 MEV.