Geophysics lecture 08: Electric (wireline) logs - Neutron and density porosity logs
Neutron logs have a radioactive source (tritium?) that generate neutrons. As they pass into the rock, they lose energy, especially if they hit nuclei of similar weight (e.g. hydrogen). It distinguishes hydrogen concentration, the more hydrogen, the more absorption, and the more porosity.
Density logs use a gamma ray source attached to the skid plate. It measures the bulk formation density, which is related to porosity.