Log interpretation

Prompt Fission Neutron (PFN) uranium logging data are interpreted using calculational models which relate observed data and independently-determined parameters to standard uranium grade values. The observed data are epithermal neutron detector response, which is proportional to uranium grade, and thermal neutron detector response. Both detector responses depend on some of the properties of the earth formation (such as the borehole diameter, the formation water content, and formation bulk density) and on the neutron generator output. The procedure by which grade estimates are made is to postulate model and to calibrate the model against standards, such as the US DOE Fission Neutron Model test pits at Grand Junction, CO, and other locations. The calibration gives a factor which relates the field data logs to the uranium grades reported for the test pits. Different calibration factors may be needed for water-filled boreholes and for air-filled boreholes, but the grade model is designed to accommodate different earth formations or borehole diameters