A neutron porosity log in which the effects of the borehole environment are minimized by using two detectors. In the most common technique, the two source-detector spacings are chosen so that the ratio of the two count rates is relatively independent of the borehole environment. This ratio is then calibrated in terms of porosity in a known formation and borehole environment typically with the tool placed against the side of an 8-in. (20-cm) borehole in a limestone block, both filled with fresh water at surface temperature and pressure. The response is also determined at different porosities and in sandstones, dolomites and other borehole environments. Correction factors are developed to convert the measured log to the standard conditions.

The source and detectors are not azimuthally focused. Wireline tools are run eccentralized against the borehole wall. Since the neutrons emitted into the mud are strongly attenuated, the resulting log is effectively focused into the formation. Measurements-while-drilling tools will normally be unfocused since they are centralized unless the borehole is overgauge.

The vertical resolution is about 2 ft (0. 6 m), but can be improved by alpha processing.