Calibration of a neutron log in partially saturated media IV: effects of sonde-wall gap

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A gap between a neutron sonde and the wall of a borehole can have a significant effect on the observed count rate. This effect was determined experimentally to be linear with gaps as large as 2.5 cm. The count rate is given by N/sub N/ = K/sub 0/ + K/sub 1/g where K/sub 0/ is the count rate that would be observed at zero gap, and g is the gap. The parameters K/sub 0/ and K/sub 1/ are dependent on both water (ie. hydrogen) content and bulk density. In many situations failure to correct the count rate for this gap … continued below

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16 pages

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Axelrod, M. C. & Hearst, J. R. March 8, 1984.

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A gap between a neutron sonde and the wall of a borehole can have a significant effect on the observed count rate. This effect was determined experimentally to be linear with gaps as large as 2.5 cm. The count rate is given by N/sub N/ = K/sub 0/ + K/sub 1/g where K/sub 0/ is the count rate that would be observed at zero gap, and g is the gap. The parameters K/sub 0/ and K/sub 1/ are dependent on both water (ie. hydrogen) content and bulk density. In many situations failure to correct the count rate for this gap effect can result in a significant degradation in the accuracy of the water content calculated from the count rate. In a dry borehole, K/sub 1/ is small at zero formation water content, and increases with formation water content. In a water-filled borehole, K/sub 1/ is large at zero formation water content, and tends to decrease with increasing formation water content, becoming zero, as of course it must, if the formation is pure water. The absolute value of K/sub 1/ increases with increasing density. A representation was determined for K/sub 0/ and K/sub 1/ from experimental data. This representation can be used to adjust the count rate at a given gap to equal its zero-gap value. The accuracy of the zero gap equation can then be recovered.

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16 pages

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NTIS, PC A02/MF A01; 1.

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  • 25. logging symposium, New Orleans, LA, USA, 12 Jun 1984

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  • Other: DE84008570
  • Report No.: UCRL-90040
  • Report No.: CONF-8406110-1
  • Grant Number: W-7405-ENG-48
  • Office of Scientific & Technical Information Report Number: 5124272
  • Archival Resource Key: ark:/67531/metadc1059953

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  • March 8, 1984

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  • Jan. 22, 2018, 7:23 a.m.

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Axelrod, M. C. & Hearst, J. R. Calibration of a neutron log in partially saturated media IV: effects of sonde-wall gap, article, March 8, 1984; California. (https://digital.library.unt.edu/ark:/67531/metadc1059953/: accessed June 8, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.

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