Statistical uncertainties of elemental concentrations extracted from neutron-induced gamma-ray measurements

The uncertainties in elemental concentrations obtained from borehole thermal neutron capture measurements have been determined, when the normalization to absolute concentrations is by an associated element closure model that accounts for unmeasured elements in the rock. Typical uncertainties under normal logging conditions are between 2 and 3 wt.% for Ca and Si, an little over 1 wt.% for S, a few tenths of a weight percent for Fe and Ti, and about 0.2 p.p.m. for Gd. Adverse geophysical effects, large effective tool standoff, and large borehole thermal neutron absorption cross section increase the elemental concentration uncertainties because of an increase in the number of borehole gamma rays detected. Acceptable uncertainties can be obtained by slower logging speed or multiple passes. Regardless of geophysical parameter values, correct elemental concentrations are obtained within the measurement statistics.