Statistical Precision Of Neutron-Induced Gamma Ray Spectroscopy Measurements

With the increased use of neutron-induced gamma ray spectroscopy in the borehole environment, it is becoming more important for the user to understand the statistics of the measurement. The statistics of both inelastic and capture induced radiation are affected by parameters such as borehole size and salinity, formation sigma, neutron burst timing, and casing. By understanding how these parameters affect the statistical uncertainty, logging conditions can sometimes be optimized to produce better results. For example, the statistical uncertainty of the inelastic carbon-to-oxygen ratio is affected by the salinity in the borehole because of the increased capture background that is associated with the chlorine. Even though the saltwater does not affect the value of the carbon-to-oxygen ratio, it does affect the precision to which it is determined. For the capture measurement, various parameters affect the partitioning of the signal coming from the borehole versus the formation; thus, the statistical uncertainty of the measurements is affected by the magnitude of the borehole correction required. This paper shows how the statistics in the borehole spectroscopic measurement are propagated and how these statistics affect the interpretation models.