Pulsed neutron measurements in boreholes

The thermal neutron decay times of earth formations traversed by a well borehole are measured by … … a) generating, in the well borehole, a relatively short duration discrete burst of fast neutrons which are rapidly moderated by interaction with nuclei of materials in the borehole and surrounding earth formations and slowed down to thermal energy, creating a thermal neutron population in the borehole and surrounding earth formations; … b) detecting, in the borehole, radiations representative of the thermal neutron population in the borehole and surrounding earth formations, in at least two time intervals subsequent to said burst of fast neutrons, said at least two time intervals being chosen to maximise the borehole component radiations relative to the formation component, and generating at least two count signals representative of said thermal neutron population during said at least two time intervals; and … c) combining said count signals from said at least two borehole count maximising time intervals according to a predetermined relationship to provide a measurement indicator representative of the thermal neutron decay time of the borehole medium. …