By Nhan Nguyen

Photoelectric logging is named so because it uses the photo-electric absorption factor of formations to provide the required data to determine the lithology. It works similarly to the density log and requires a source that steadily emits gamma rays. However the detectors of the photoelectric log are able to determine gamma rays that have higher energies ("hard" gamma rays) and lower energies ("soft" gamma rays).

"Hard" gamma rays are used in measuring formation density while "soft" gamma rays undergo photo-electric absorption. The photo-electric absorption factor (Pe or PEF) is used to provide a parameter which is dependent upon the atomic number of the formation, which drastically helps to find the formations lithology.

Gamma rays are emitted from the source which travel through the rock and scatter and lose energy. As the gamma rays undergo random collisions more energy is loss. "Soft" gamma rays have low energies around the 0.2MeV. Any gamma ray with an energy below the 0.2MeV threshold is completely absorbed into the rock and can encounter an electron under correct conditions. This is photo-electric absorption. Different elements are capable of absorbing different amounts of "soft" gamma rays. The PEF shows the density of the material by how many electrons there are to take part in photo-absorption. Hence the electron density of the rock can be found and so the lithology is determined.

The formula for specific photo-electric absorption index.

From "Chapter 14 - The Litho-Density Log" - Dr. Paul Glover, Petrophysics MSc Course Notes.