The primary standard unit for Compensated Neutron Log (CNL).
It uses the API Neutron Calibration Pit at the University of Houston.
This pit contains three blocks of carbonate with accurately known porosities that are filled with fresh water that are drilled to accept a tool.
The tool is calibrated in 19% porosity Indiana limestone, for which an API value of 1,000 NAPI neutron units is defined.
See Also
Petroleum Industry / Upstream / Data Acquisition / Well & Reservoir Surveillance / Well Logging / Open-Hole Formation Logging (OHFL) / Compensated Neutron Log (CNL)