The gravel pack logging tool is a [[neutron]]-type device that evaluates the condition of the [[gravel pack]]. Count rates from two [[detector|detectors]] are used to compute porosity. The count rates from both detectors are presented with the [[cased hole|cased-hole]] porosity and compared to the porosity measured in the [[open hole]] if open-hole measurements are available. The log provides a quantitative analysis of the areas in the pack that need to be replaced before the well is placed on production.