1. n. [Formation Evaluation]
A measurement of the bulk density of the formation, based on the reduction in gamma ray flux between a source and a detector due to Compton scattering. The gamma ray source, usually 137Cs (cesium), is chosen so that gamma ray energies are high enough to interact by Compton scattering but not by pair production. The detectors discriminate against low gamma ray energies that may have been influenced by photoelectric absorption. Although Compton scattering depends on electron density and not bulk density, density logs are calibrated to give the correct bulk density in the majority of sedimentary rocks. Due to the Z/A effect there are small differences in some formations. The measurement responds to the average density of the material between source and detector. In the wireline measurement, care is taken to minimize the mud between the sensors and the formation by pressing a pad against the borehole wall, with source and detector focused into the formation. In the logging-while-drilling measurement, a sleeve may be mounted on the collar around the sensors to exclude the mud. The detectors measure the gamma rays scattered from the formation. Even then, mudcake or borehole rugosity can affect the measurement. It is common practice to compensate for the mudcake by using two or more detectors at different spacings.
See: azimuthal density, compensated density log, Compton scattering, cross section, delta rho, logging-while-drilling, pair production, PEF, scintillation detector, spine and ribs plot, Z/A effect
2. n. [Formation Evaluation]
A measurement of the bulk density of the formation based on borehole-gravity measurements. As the gravitational attraction between two bodies is dependent upon their masses and their separation, it follows that its measurement also can be used to make a direct determination of density. The density thus measured is highly accurate but averaged over a large volume.
See: borehole gravity
