The principal behind density logging is the detection of Compton-scattered gamma rays that originate from a small radioactive source housed in the logging probe. The intensity of the radiation scattered back to the detectors is primarily a function of the bulk density of the media in which the gamma rays are introduced and scattered.
A compensated dual-detector density data set provides a qualitative measurement of the formation density that is compensated for the borehole rigosity or imperfections in the borehole size and shape. These tools utilize a Cesium137 radioactive source with two detectors at 8 and 14 inches from the source and the tool is decentralized with a 1-arm caliper and decentralizing band (s). The measurement is calibrated normally with two know materials, quantified and is typically one of the most accurate log measurements recorded. These gamma density measurement tools have a shallow depth of investigation from 1-2″ is all. They can be collected in wet or dry holes and some through casing. Due to their shallow depth of investigation their response in casing may be dominated by the well construction. These data on gamma rays are collected while logging up the hole only.