Method for determining thermal neutron decay times of earth formations

A method for determining the thermal neutron decay time of materials in the vicinity of a well borehole comprising the steps of:

• continuously irradiating the earth formations in the vicinity of a well borehole with a phase coherent harmonically intensity modulated cloud of fast neutrons at first, second and third different modulations frequencies;
• detecting as a function of time the intensity modulated thermal neutron population resulting from said irradiation at said at least three different modulation frequencies;
• determining from said detected thermal neutron populations measured as a function of time, the tangents of the relative phase shift angles between said fast neutron cloud and said thermal neutron population at each of said first, second and third modulation frequencies;
• combining said first and second phase angle tangents to derive a first approximation to the thermal neutron decay time of earth formations in the vicinity of a well borehole;
• combining said first and third phase angle tangents to derive a second approximation to the thermal neutron decay time of earth formations in the vicinity of a well borehole;
• and
• combining said first and second approximations of the thermal neutron decay time according to a predetermined relationship to derive a value for the true thermal neutron decay time of the earth formations in the vicinity of a well borehole.