Neutron-neutron logging

In accordance with the present invention, a steady-state source of primary radiation is located within a borehole for irradiating the formations surrounding the borehole with fast neutrons. A first pair of detectors located at spaced-apart positions from the source within the borehole measure that secondary radiation affected predominantly by the epithermal neutron parameters of the formation. A second pair of detectors located at spaced-apart positions from the source within the borehole measure that secondary radiation affected by both the epithermal and the thermal neutron parameters of the formation