Feasibility study of artifacts on the neutron logging curve near the boundary of layers

The neutron borehole probe equipped with a neutron source and a system of neutron detectors is commonly used to determine porosity of geological formations. Transport of neutrons through media relies on their interaction with atomic nuclei, mainly with the process of scattering and absorption. Curves recorded by detectors for a given geological layer bordered by a medium with other neutron properties form the so-called anomalies. The course of the anomaly mainly depends on neutron properties of the bordering media and also on the measurement geometry, borehole properties, etc. The paper focuses mainly on an influence of neutron properties of bordered rock media on the course of the anomaly curve, especially near the media boundaries. In some cases of bordering media, characterized by specific neutron properties, the course of anomalies can be significantly disturbed, forming specific artifacts. The paper explains that the presence of artifacts is closely related to the properties of bordered media for both fast and thermal neutrons, i.e., the slowing down length of fast neutrons and the absorption cross section of thermal neutrons. Artifacts can be the cause of misinterpretation of the neutron rock porosity probe measurement. All considerations presented in this paper are based on results obtained from MC calculations performed using MCNP5 code (MCNP Team in X-5 Monte Carlo Team, MCNP-a general Monte Carlo N-particle transport code version 5. Los Alamos National Laboratory LA-UR-03-1987, 2008).