Method and apparatus for enhanced formation porosity measurement using pulsed neutron source and dual-function detectors

A method of evaluating downhole formation porosity, comprising:

• emitting neutron pulses from a pulsed neutron tool deployed in a wellbore to irradiate earth formation surrounding the wellbore;
• detecting neutrons and gamma rays using a plurality of detectors disposed in the pulsed neutron tool, wherein each detector is operable to detect neutrons and gamma rays from the formation;
• and
• estimating a plurality of neutron porosities and a plurality of gamma-ray porosities based on data from the plurality of detectors,
• wherein the estimating step comprises:
• obtaining a neutron count rate and a capture gamma-ray count rate from each of the plurality of detectors;
• calculating a ratio of neutron count rate and a ratio of capture gamma-ray count rate between every two of the plurality of detectors so as to obtain a plurality of ratios of neutron count rate and a plurality of ratios of capture gamma-ray count rate;
• and
• estimating a plurality of neutron porosities using the plurality of ratios of neutron count rate and/or estimating a plurality of gamma-ray porosities using the plurality of ratios of gamma-ray count rate.