Method and apparatus for measuring thermal neutron characteristics

The thermal neutron decay characteristics of an earth formation are measured by detecting indications of the thermal neutron concentration in the formation during measurement intervals G1, G2.....following irradiation of the formation with a burst of fast neutrons. These measurement intervals consist of a sequence of discrete time gates which begins following a predetermined time delay after the preceding neutron burst and is divided into a number of groups, e.g. G1-G4, G5-G8, and so on. The durations of the individual discrete time gates within the groups are equal, but the duration of the time gates progressively increases from group to group in the sequence, e.g. geometrically, preferably doubling in consecutive groups. The groups, and the gates within them, may be contiguous as shown. The durations of the gates may extend from one which is shorter than the neutron burst, to one which is at least as long. The number of groups may equal the number of gates in each group, preferably four. (author)