[en] A method of investigating an earth formation traversed by a well bore is claimed. It comprises: cyclically irradiating the formation with a number of time-spaced pulses of fast neutrons; obtaining representations of the energies of gamma rays produced by thermal neutron capture interaction with nuclei of the formation during a first detection period following each of the neutron pulses; obtaining representations of the number of gamma rays produced during a second detection period; obtaining representations of the number of background gamma rays detected during a period following the second detection period; deriving a measurement of a neutron characteristic of the formations from accumulated counts of the gamma rays measured during the second detection periods and the tau decay time background period, and controlling the timing of the pulses of fast neutrons and the detection periods as a function of the measurement of the neutron characteristics of the formation derived from the preceding cycle of pulse irradiations, measurements and derivations. Apparatus is also claimed