Sonic data, ultrasonic data, density data, cased-hole neutron data, and open-hole neutron data of the wellbore are obtained. The sonic and ultrasonic data provides the amplitude, frequency, and phase of the altered sonic and ultrasonic waves. The far counts, near counts, and energy spectrum are obtained from density data, cased-hole (CH) neutron data, and open-hole (OH) neutron data. The amplitude, frequency, and phase provide the interface densities of the first, second, and third interfaces. The hydrogen index (HI) of the formation and the cased wellbore are obtained from the CH and OH neutron data. The widths of the second and third interfaces are obtained from the HI's and the densities of the second and third interfaces.

A first set of radiation detectors may be disposable on the a drill string, wherein the first set of radiation detectors are capable of detecting gamma radiation and neutron radiation; and a combined chemical source spaced from the first set of radiation detectors, wherein the combined chemical source comprises a gamma radiation emitting material and a neutron radiation emitting material. The first set of radiation detectors and combined chemical source may be used in methods for logging a wellbore.

An apparatus for estimating a property of an earth formation and a borehole fluid includes a carrier configured to be disposed in a borehole, and a pulsed neutron measurement assembly including a pulsed neutron source configured to emit neutrons into the borehole and the earth formation, and a gamma ray detector. The apparatus also includes a fluid density measurement assembly including the gamma ray detector and a gamma ray source configured to irradiate a borehole fluid with gamma rays. The gamma ray detector is positioned relative to the gamma ray source to detect both of: gamma rays resulting from neutron interactions and gamma rays emitted from the borehole fluid in response to irradiation from the gamma ray source. The apparatus further includes a processor configured to differentiate a pulsed neutron gamma ray spectrum associated with the interactions from a density gamma ray spectrum.

Disclosed herein are methods and system for determining an elemental composition of a downhole formation using neutron-induced gamma ray spectroscopy and doping the borehole fluid with a neutron absorber to remove at least part of the borehole thermal neutron flux before it can capture borehole nuclei and emit gamma rays within a spectral range of interest. For example, a method for determining the elemental composition includes adding a dopant to a borehole fluid, wherein the dopant absorbs thermal neutrons in the borehole, lowering a logging tool comprising a gamma ray spectrometer, emitting neutrons into a downhole environment, generating neutron-induced gamma rays comprising borehole gamma rays and formation gamma rays, detecting gamma rays from the downhole formation, and extracting the elemental composition of the downhole formation. The dopant may not emit gamma rays above 0.5 MeV or their peaks background should be predictable and/or consistent to be removed during data processing.

Disclosed herein are methods and system for determining an elemental composition of a downhole formation using neutron-induced gamma ray spectroscopy and doping the borehole fluid with a neutron absorber to remove at least part of the borehole thermal neutron flux before it can capture borehole nuclei and emit gamma rays within a spectral range of interest. For example, a method for determining the elemental composition includes adding a dopant to a borehole fluid, wherein the dopant absorbs thermal neutrons in the borehole, lowering a logging tool comprising a gamma ray spectrometer, emitting neutrons into a downhole environment, generating neutron-induced gamma rays comprising borehole gamma rays and formation gamma rays, detecting gamma rays from the downhole formation, and extracting the elemental composition of the downhole formation. The dopant may not emit gamma rays above 0.5 MeV or their peaks background should be predictable and/or consistent to be removed during data processing.