[en] A description is given of a method for measuring the porosity of earth formations surrounding a well borehole comprising the steps of: 1) repetitively irradiating the earth formations about the well borehole with high energy neutron pulses of selected duration; 2) detecting, during at least two intervals of selected duration following one neutron pulse and before the next, gamma radiations resulting from the capture of thermalized neutrons by the earth formations about the well borehole; 3) deriving, from the gamma ray detections effected in at least two intervals, a thermal neutron decay time characteristics of the earth formations about the borehole; 4) combining, according to a predetermined relationship, the thermal neutron decay time characteristic, the selected neutron pulse duration and the number of gamma ray counts occurring during one of the at least two selected duration detecting intervals, thus deriving a porosity related parameter of the earth formations. (author)