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Density logs first appeared in 1957, based on the principle of gama ray absorption by Compton scattering. Early tools were called gamma - gamma density logs because they emitted and recorded gamma rays. The log displayed counts per second, which was transformed to density by a semi-logarithmic transform. Modern tools have two detectors, which allows borehole compensation to be applied. They are scaled in units of density (grams / cc or Kilograms / cubic meter). Some density logs also record photoelectric capture cross section which is useful in lithology analysis. Some density logs are displayed in porosity units (percent or decimal fraction).

The tool can be used in air or mud filled open boreholes. Experimental tools approaching commercialization are being developed for cased hole applications.

The density logging tool emits gamma rays from a chemical source at the bottom of the tool The gamma rays enter the surrounding rocks where some are absorbed. Some gamma rays survive to reach scintillation counters mounted about 18 and 24 inches above the source. The number of gamma rays arriving at the far detector is inversely proportional to the electron density of the rock, which in turn is proportional to the actual rock density. Data from the closer detector is used to correct for borehole effects.

Porosity can be derived from density and can be presented as a percent or as a decimal fraction on the log. This porosity may still contain artifacts from shale and minerals not accounted for by the logging computer, so this porosity is NOT a final answer.

The energy of the returning gamma rays is a function of the photoelectric capture cross section of the rock, which is indicative of mineralogy. A caliper and gamma ray curve are also presented, along with the density correction curve. Note that the correction has already been applied to the recorded density data by the computer.

Early density logs had only one detector and were recorded in counts per second. Density was derived with a semi-logarithmic transform.

Borehole gravity meters measure the pull of gravity in a station by station survey. Results are translated into formation density. Depth of investigation is large compared to normal logs so anomalies some distance from the borehole may be detected.

A typical density logging tool is shown at the right. The tool is pressed against one side of the borehole by a back-up arm that also serves to measure a diameter of the borehole. Two detectors at fixed spacings from the source are shown. The source is well-shielded from the two detectors and only scattered gamma radiation is detected. The intensity of the scattered radiation will be dominated by the density variations along the path from source to detector.

If there is no stand-off (of mud or mudcake) between the tool face and the formation, and if the tool is properly calibrated, then the apparent density from both detectors will be the same and equal to the true formation density. If they are different, there must be mud between the tool face and the rock.

If there is some standoff, a correction to the density from the long spaced detector can be generated from the difference between the apparent density seen by the far and the near detectors. The actual correction function can be determined empirically by placing the density device in a number of formations to measure the apparent long-spaced and short-spaced densities for various thicknesses of mudcake of a variety of densities. Computer modeling has augmented these laboratory studies.

Most modern two-detector density devices use multiple energy windows to derive the density, the photoelectric factor, and the correction curve as described above. In one three-detector wireline version, the combination of multiple detectors and multiple energy windows produce on the order of a dozen counting rate measurements at each depth. Each counting rate can be described by a forward model relating the rate to the five important parameters of density logging: formation density, formation photoelectric factor, mudcake density, mudcake photoelectric factor, and the thickness of the mudcake.