A comparison of the moisture gauge and the neutron log in air-filled holes at NTS Page: 10 of 13
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2 I- a----- ... 4 .6
0.5
0 -
0 10 20 30 40 60
Water content (vol%)
Figure 5a: Normalized count rate for an 8-cm-spacing moisture
gauge in a 2.4 m hole in the NTS calibrator at different gaps
between the tool and the wall of the hole. The legend shows the
gap in cm. Normalization extrema 6 to 49%
does not significantly affect the sensitivity, doubling the
thickness does. We have no data on the effect of cadmium
thickness on reduction of the effect of high absorption
cross-section elements. Figure 4b is a similar plot of a
neutron log, with a built-in 0.079 cm cadmium shield,
again in the 2.4-m-diameter central hole. The sensitivity of
this tool agrees with those seen in Figure 3b. Each tool was
forced against the wall of the hole.
The simplest way to compare the effects of other
perturbations among tools is to calculate the water content
that would be obtained using a standard calibration curve,
such as those in Figure 4, with and without the
perturbation, and then compare the derivative of the
calculated water content with respect to the size of the
perturbation.
Effect of density
The effect of bulk density on a moisture gauge in a
small-diameter hole is discussed by Couchat (no date), cited
in IAEA (1970). No hole size or gauge type is stated.
Couchat found that a change in bulk density of 0.4 g/cm3
changed the apparent water content by about 4 vol% at
water content values of both 20 vol% and about 35 vol%.
This change gives a derivative of 10 vol% water per g/cm3.
The effect of density on an epithermal neutron log in a 30-
cm hole is similar to the effect seen in the 2.4-m hole,
discussed below (Axelrod and Hearst, 1984.).
In our tests in the 2.4-m hole, we used two cells with
densities higher than those used to obtain the curves in
Figure 4. One had a bulk density of 1.88 g/cm3 and water0
1
11.6
20
10 20 30 40 50
Water content (vol%)
Figure 5b. Normalized count rate for a collimated epithermal
neutron log in the NTS calibrator at different gaps between the
tool and the wall of the hole. The legend shows the gap in cm.
Normalization extrema 6 to 49%.
content of 20.3% to compare with the cell of bulk density of
1.34 g/cm3 and water content of 19.3%. The other had a
bulk density of 2.44 g/cm3 and water content of 329% to
compare with the cell of bulk density of 1.34 g/cm3 and
water content of 33.4%. We found that the density effect
was a function of water content and was approximately the
same for the moisture gauge and the neutron log. The
derivative is about 5 vol% per g/cm3 at 20 vol% water and
15 vol% per g/cm3 at 33 vol% water. It is, of course,
feasible to correct the water content for the effect of density
using data from a density log (Axelrod and Hearst, 1984).
In fact, some moisture gauges include density logs, and
automatic, in-field, corrections could easily be developed.
Effect of borehole size
Abeele (1979) observed the effect of hole size from
5.1 to 10.2 cm on a centralized moisture gauge and found it
to vary from 0.07 vol% per cm at 5 vol% water to 1.2 vol%
per cm at 27 vol% water. Tittman et al. (1966) obtained
similar results for a decentralized neutron log over a larger
range of hole sizes: 0.08 vol% per cm at 5 vol% water to
1.1 vol% per cm at 30 vol% water.
Effect of tool-wall gap
In small air-filled holes, data ar, sparse for both
tools. Abeele (1979) states that the count rate of a 5-em-dia
moisture gauge does not depend on whether a tool is
centered or eccentered in an air-filled hole up to 10.2 cm in
diameter. Tyler (1988) suggests that an effect is observed
in a 14-cm hole. The effect of gap on an epithermal
neutron log in a 30-cm hole is similar to that in the 2.4-m
hole discussed below (Axelrod and Hearst, 1984).6
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Hearst, J. R. & Carlson, R. C. A comparison of the moisture gauge and the neutron log in air-filled holes at NTS, article, August 1, 1993; California. (https://digital.library.unt.edu/ark:/67531/metadc1276050/m1/10/: accessed May 5, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.