Notes On The Accuracy Of The Neutron Lifetime Measurement

During the past several years a number of papers have been published which point out that the most well-known commercial logging devices for the measurement of the thermal neutron capture cross section do not always agree in the absolute values measured. There are a number of reasons for these discrepancies, including different philosophies in tool design, gating systems and types of detectors. This paper reviews the results of a series of tests in a variety of controlled environments and also actual logged results for one of these devices, the Dresser Atlas Neutron Lifetime Log.* The controlled environments include the API test pits and several test tanks in which measurements were made with boric acid in concentrations such that the capture cross section of the solutions varied up to a maximum of almost 100 sigma units (0.lcm-1); The field results include multiple runs in the same borehole under a wide spectrum of different possible borehole conditions, including holes that are air or gas and fluid filled, cased and uncased, and high to low salinity fluid filled. The results of instruments of varying diameter are also compared. The inescapable conclusion of this collage of test measurements and field results is the fact that the Neutron Lifetime Log measures the value of thermal neutron capture cross section with an exceptional accuracy.