1. n. [Formation Evaluation]

A log of photoelectric absorption properties. The log measures the photoelectric absorption factor, P_e, which is defined as (Z/10) ^3.6 where Z is the average atomic number of the formation. P_e is unitless, but since it is proportional to the photoelectric cross section per electron, it is sometimes quoted in barns/electron. Since fluids have very low atomic numbers, they have very little influence, so that P_e is a measure of the rock matrix properties. Sandstones have low P_e, while dolomites and limestones have high P_e. Clays, heavy minerals and iron-bearing minerals have high P_e. Thus, the log is very useful for determining mineralogy. In interpretation, PEF is normally converted to the simpler volumetric cross section, U in barns/cm³, by taking the product of PEF and density. The log is recorded as part of the density measurement. The depth of investigation is of the order of one inch, which is normally in the flushed zone. PEF can be affected by heavy minerals such as barite in the mudcake or mud filtrate. PEF logs were introduced in the late 1970s.

See related terms: azimuthal density,  density measurement,  flushed zone,  photoelectric effect