Patent ID: 8476584

Claim:
A method for estimating a density of an earth formation penetrated by a borehole, the method comprising: conveying a carrier through the borehole; emitting a pulse of fast neutrons into the formation during a neutron-pulse time interval using a pulsed-neutron source disposed at the carrier; detecting gamma-rays due to inelastic scattering and thermal capture of the emitted neutrons in the formation with a gamma-ray detector to provide a gamma-ray energy spectrum due to the inelastic scattering and the thermal capture during a first time interval within the neutron-pulse time interval and to provide a time spectrum of counts or count rates due to thermal capture during a second time interval occurring after the neutron-pulse time interval; determining a macroscopic capture cross section Σ F of the formation from a decay in the time spectrum of counts or count rates; determining a weight fraction for one or more elements in the formation from the gamma-ray energy spectrum and estimating the density of the formation using the macroscopic capture cross section and the weight fraction for the one or more elements; wherein the second time interval begins after a cut-off time t 0 beyond which contribution to count rate due to borehole elements is negligible with respect to count rate due to formation elements.