A record of the velocity and direction of water flowing in and around a borehole based on oxygen activation. The log may also include estimates of the flow volume and the distance from tool to flowing water. Water, and occasionally carbon dioxide, is the only source of moving oxygen in and around the borehole. Hence, water flow can be detected by oxygen activation, which, being a nuclear technique, is sensitive to flow inside and outside the casing. The measurement is sensitive to small flows, and can be configured to measure upward or downward flow. It is particularly useful as a leak and channel detector, to identify locations of water entry or exit and as a measurement of water velocity in multiphase flow. Logs may be continuous, but the most accurate measurements are made with the tool stationary.

Although first tried in the 1960s, the log was not fully studied and implemented until the late 1970s with a purpose-built experimental tool. Standard pulsed-neutron spectroscopy tools were modified to record the log in the 1980s.