Earth formation pulsed neutron porosity logging system utilizing epithermal neutron and inelastic scattering gamma ray detectors

A method for determining the porosity of earth formations in the vicinity of a cased well borehole, comprising the steps of:

• repetitively irradiating earth formations in the vicinity of a cased well borehole with relatively short duration bursts of fast neutrons from a pulsed neutron source;
• detecting essentially only during said repetitive neutron bursts at a spaced distance from said neutron source, inelastic gamma radiation produced by the scattering of fast neutrons, said inelastic gamma radiation being representative of the fast neutron population in the vicinity of the borehole and generating a signal representative thereof;
• detecting essentially only during the intervals between said neutron bursts at a spaced distance from said neutron source, the epithermal neutron population in the borehole and generating signals representative thereof;
• and
• combining said inelastic scattering gamma radiation and said epithermal neutron representative signals to derive a measurement signal functionally related to the porosity of earth formations in the vicinity of the borehole.