A well log that records formation density. The logging tool consists of a gamma-ray source (e.g., Cs137) and a detector so shielded that it records backscattered gamma rays from the formation. This secondary radiation depends on the density of electrons, which is roughly proportional to the bulk density. The compensated density-logging tool (FDC) includes a second detector that responds more to the mud cake and small borehole irregularities; its response is used to correct the readings of the main detector. See Figure D-5. Sometimes called gamma-gamma log. Compare nuclear cement log and photon log.