A record of the fraction of gas present at different depths in the borehole. Although several techniques may be used for this purpose, the term usually refers to logs based on one of two principles. In the first, four or more optical probes are used to detect the passage of gas bubbles at different points across the borehole. As with other local probes, holdup is determined by the fraction of time the probe detects gas. In the second technique, a ⁵⁷Co (cobalt) source emits low-energy gamma rays that undergo backscattering and photoelectric absorption in the borehole fluid before being counted in a detector. The number of counts is related to the fluid density, and can be calibrated in terms of gas holdup.

The first technique produces an image of gas holdup along and around the borehole, while the second technique produces a log of the average holdup along the well.