The dual-neutron probe provides a calibrated borehole-compensated neutron porosity measurement in mud-filled open holes. It is the probe of choice for quantitative formation-fluid studies. A single-detector neutron probe is also available for qualitative porosity logging under most borehole conditions including through steel or plastic casing and drill-pipe.
Principle of Measurement:
The dual-neutron measurement uses two ³He proportional detectors and a detachable, sealed 241Am-Be neutron source. Fast neutrons emitted by the source are scattered and slowed to thermal levels principally by hydrogen in the formation. The ratio of the neutron flux reaching the near and far detectors depends on the hydrogen index and porosity. Use of dual detectors and a ratio method provides a porosity measurement which is independent of hole diameter.