A new approach to the characterisation of rock formations for use in logging tool calibration facilities

A novel approach to the characterization of rock formations for the calibration of neutron porosity tools was developed using the SPARTAN sandstone test pits. The responses measured with different tools and their sensitivities to formation characteristics were calculated with the McBend Monte Carlo code. Consistent values of the porosity, grain density, and macroscopic thermal neutron absorption cross-section of the sandstone were derived from physical measurements together with their uncertainty covariance matrix. For this purpose it was necessary to supplement the chemical composition with the trace element concentrations obtained from prompt gamma analysis. Maximum likelihood adjustments were made to all parameters using the new code DATAK. Very close agreement between measurement and calculation had been obtained before making these adjustments, indicating that the uncertainties had originally been overestimated. While the magnitude of the adjustments was correspondingly small, the uncertainties on all the parameters were reduced, leading to a refined description of the formation characteristics