Analysis of the spatial-energy distribution of neutrons in the vicinity of formation boundaries: Neutron log examples

Highlights: • Anomalies - the curves recorded by the neutron probes for a given geological layer surrounded by a medium with different neutron properties. • Artifacts - the significantly disrupted course of an anomaly in the vicinity of formation boundaries. • The presence of artifacts is closely related to differences in the neutron transport properties of the adjacent media. • Monte Carlo calculations – a tool used to explain the presence of artifacts. We present cases of so-called artifacts observed in the response of neutron detectors in the vicinity of formation boundaries (e.g., in neutron logging for geophysical prospecting). The appearance of artifacts is related mainly to differences in the neutron transport properties of adjacent media: the fast neutron slowing-down length and the absorption cross section of thermal neutrons. The basis for examining the causes of the appearance of artifacts on the profiling curve was a numerical experiment consisting of our measuring the counts of the thermal neutron detector of the neutron probe under given measurement conditions in the well. The detector response was determined with a 1 cm step in the vicinity of the border of two geological formations, significantly different in terms of neutron parameters, which allowed accurate registration of artifacts on the logging curve. The numerical code MCNP6 with the ENDF B-VII.1 data library was used to solve neutron transport problems in complex systems and to explain the presence of artifacts on the basis of the physics of neutron transport and the spatial-energy distribution of neutron fields in the vicinity of formation boundaries.