Neutron well-logging profiles in sloped thin layerformations simulated by Monte Carlo method

Neutron well-logging is one of the basic methods of well logging for porosity and lithology determination. A neutron source, placed in a borehole tool, creates a neutron field in the nearest environment – around the borehole. A neutron detector placed in the same tool at some distance from the source registers neutrons coming from that field. It gives information on some of the petrophysical parameters of rocks surrounding the borehole. Descriptions of neutron fields and properties of neutron transport in matter have been strongly developed in reactor physics, both in the field of analytical and numerical methods. The same rules of neutron transport physics in various environments and the same mathematical methods can be used also for the description of neutron fields in geophysical applications. Analytical description of the neutron fields can be done only for simple geometries, e.g. for the neutron field created by a point neutron source located in an infinite cylindrical borehole – surrounded by infinite and homogeneous rock medium. However, many interesting analytical solutions that are useful in the interpretation of neutron well-logging have been designed and applied in practice [1, 2]. More complex geometries – as, for instance, dipped thin layer formations crosscut by the cylindrical borehole – are not suitable for the analytical considerations.