Some insights on neutron measurements

The basic physics of commercial compensated neutron porosity logging tools is described. Both thermal and epithermal devices are largely controlled by the slowing-down length, which provides an understanding of lithology effects and porosity sensitivity of static measurements. Several codes for computing the slowing-down length from the formation constituents are available. A simple physical model suggests an approximation that may be adequate to determine slowing-down length in many cases. For dynamic measurements the two-detector compensation scheme presents depth matching problems at bed boundaries. Methods exist for treating these problems