Moisture monitoring in large diameter boreholes Page: 3 of 10
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The majority of borehole geometry difficulties can be overt-me if one
recalls that the purpose of the monitoring is leak detection and
monitoring. If boreholes drilled under less than optimm conditions,
results of logging following drilling may not reflect the conditions of
the tnsaturated zone outside the drilling radius. However, when results
of rozTine monthly or quarterly logging are compared from this same
borehole, relative change from the initial conditions may reflect changes
other than the drilling effects. Changes such as a wetting front moving
fro below an iapurdment will be easily abstracted from the logs even
when drilling effects are evident.
The second major difficulty arising in waste site monitoring lies in
the range of influence of the probe outside the borehole. Typical ranges
of influences for smaller neutron loggers is 5 - 80 cm from the borehole.
In the case of a large landfill or impoundment, peripheral monitoring
wells will not detect point source leaks (seam failures or punctures of
iimhrane liners). Many of these deficiencies may be overcome using slant
drilling techniques or installation of off-vertical monitoring wells prior
to landfill or impoundMent construction. In addition, field studies
(Johnson et al., 1981) have shrwn considerable lateral migration of fluids
from landfills located above the water table. The source of this lateral
migration is most likely cause by the natural inhomogeneities in the soils
underlying the landfills. This lateral spreading would increase the
detectability of a leak using a limited m.ber of neutron access holes.
In the case of land treatment facilities the difficulties of limited range
of influence are insignificant provided an adequate rn.ber of monitoring
wells are available to overcame the spatial variability of the soil and
application rate properties.
Objectives of the Study
The objective of this study was to evaluate the applicability of
neutron logging technology to large diameter boreholes similar to those
used at waste site investigations. To accomplish these goals, both field
and laboratory experim s were completed. Both of these studies were
aime at calibrating the neutron logger in larger (15 cm) dieamter
boreholes. The laboratory segment consisted of a calibration experiment
to determine the sensitivity of the tools (in this case a Traxler model
3222 with a 10 millicurie Am-241/Be source) in a 15 cm diameter steel-
cased borehole. The field experiment consisted of neutron logging two
recently drilled steel-cased holes from which core samples were available
for comparison. Large diameter PVC casing was not used in the calibration
procedures due to its increased neutron adsorption and the limited use of
large diameter PVC for monitoring wells. Smal1 diameter PVC, on the other
hand, is used extensively at waste disposal sites and has received
considerable review in the literature (Abeele, 1978).
Laboratory calibration procedure for neutron loggers have been well
docented (International Atomic Energy Agency, 1970). The standard
proceed consists of lagging a prepared material at several kmown water
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Tyler, S. Moisture monitoring in large diameter boreholes, article, November 19, 1985; United States. (digital.library.unt.edu/ark:/67531/metadc690968/m1/3/: accessed May 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.