Three frequency modulated thermal neutron lifetime logging

The techniques used in the present invention include the generation as a function of time of a phase coherent intensity modulated cloud of fast neutrons in a well bore which results in the creation of a phase coherent thermal neutron cloud being produced as the fast neutrons are slowed to thermal energy by the materials in the vicinity of the well bore. Measurements of the relative phase of the thermal neutron cloud population density with respect to the fast neutron cloud generated lead to the determination of parameters relating to the thermal neutron decay time or neutron lifetime of the formations in the vicinity of the well bore and the borehole fluid itself. (author)