Computed departure curves for the thermal neutron decay time log

The Thermal Neutron Decay Time (TDT) Log has become an important tool for formation evaluation in cased holes. For accurate interpretation of this log, it is necessary to know how to correct for borehole effects and for the effect of spatial diffusion of thermal neutrons. Some experimental measurements of tool response have been made in laboratory formations. However, these are limited as to the cases that can be handled, and they are very time-consuming. Furthermore, for the many situations that arise in logging practice, results are needed for a wide range of formation porosities, formation salinities, hole sizes, casing sizes, and borehole salinities. For this reason, a computer program has been developed to calculate tool response under various conditions. The nature of the program and its uses and limitations are described. Some indications of its use are also given. Illustrative computed departure curves are shown for the TDT-K tool, which is in wide use at the present time. A log example is shown