Simultaneous thermal neutron decay time and porosity logging system

A method for simultaneously determining the porosity and thermal neutron capture cross-section of earth formations in the vicinity of a well borehole is claimed. It comprises the steps of: passing a well tool with a pulsed source of fast neutrons, a combination fast neutron and gamma ray detector and an epithermal neutron detector, into a cased well borehole; repetitively irradiating the earth formations with bursts of fast neutrons; detecting the fast and epithermal neutron populations in the borehole (during the neutron bursts) and generating first and second measurement signals; detecting for second and third time intervals during the time between the neutron bursts, the gamma radiation present in the borehole due to the capture of thermalized neutrons by the nuclei of elements comprising the earth formations and generating third and fourth measurement signals; and combining the first and second measurement signals to derive an indication of the porosity of the earth formations and the third and fourth measurement signals to derive an indication of the thermal neutron capture cross-section of the earth formations