Compensated formation density log a well log that measures the density of rocks adjacent to the wellbore and is adjusted for irregularities in the well bore. The sonde bombards the rocks with gamma rays from a cesium source. Rock atoms cause Compton scattering and the returning gamma rays are counted on two detectors at different distances from the source on a skid pressed against the side of the well. The rugosity of the well is compensated by using the twodetectors.
The log determines the bulk-volume density of each formation in the well. By knowing or estimating the matrix and fluid densities, the porosities of each formation can be calculated. Newer density logs also measure the photoelectric adsorption cross section of the formation which is used to identify the lithology and correctly estimate the matrix density.