Patent ID: 8868346

Claim:
A downhole tool comprising: a neutron generator configured to emit neutrons into a formation at an energy superior to 2 MeV, sufficient to cause some of the neutrons to inelastically scatter off elements of the formation, creating inelastic gamma rays; a neutron detector configured to detect a count rate of neutrons that return to the downhole tool; a first gamma ray detector configured to detect a first count rate of inelastic gamma rays that Compton scatter through the formation to reach the downhole tool, wherein the first gamma ray detector is spaced a first distance from the neutron generator; a second gamma ray detector configured to detect a second count rate of gamma rays that Compton scatter through the formation to reach the downhole tool, wherein the second gamma ray detector is spaced a second distance from the neutron generator; and data processing circuitry configured to: receive the count rate of neutrons, the first count rate of inelastic gamma rays, and the second count rate of inelastic gamma rays; determine an apparent porosity of the formation or an estimate of a fast neutron signal that would be detected by a fast neutron detector were the fast neutron detector present in the downhole tool, wherein the estimate of the fast neutron signal is determined based at least in part on: the count rate of neutrons, or the first count rate of inelastic gamma rays, or the second count rate of inelastic gamma rays, or a combination thereof; when the apparent porosity of the formation is less than a porosity limit or when the estimate of the fast neutron transport of the formation is outside a fast neutron transport limit, apply a respective correction to: the count rate of neutrons, or the first count rate of inelastic gamma rays, or the second count rate of inelastic gamma rays, or a neutron transport correction function relating the count rate of neutrons to the fast neutron transport of the formation, or a combination thereof; and determine a density of the formation based at least in part on the corrected count rate of neutrons, the corrected first count rate of inelastic gamma rays, the corrected second count rate of inelastic gamma rays, or the corrected neutron transport correction function, or a combination thereof.