Ground surface temperature reconstructions: Using in situ estimates for thermal conductivity acquired with a fiber-optic distributed thermal perturbation sensor Page: 3 of 12
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The High Lake borehole observatory targeted both the permafrost region for geothermal investigation and the subpermafrost
formation for estimation of hydrologic properties and collection of fluid samples. The instrumentation at the bottom of the
borehole, consisting of a pneumatic packer, a U-tube sampling system with a sample fluid reservoir, and a pressure-
temperature sensor, are collectively referred to as the bottom hole assembly (BHA). The fluid, electrical, and fiber-optic lines
running between the BHA and the surface are referred to as the deployment string. The downhole pressure/temperature
transducers serve as a calibrated reference point for the DTS.
The DTPS deployed at High Lake consisted of an HDPE jacketed multimode fiber-optic cable that runs from the top of the
packer fluid reservoir up to a DTS (DTS; Agilent Technologies Manufacturing GmbH & Co. KG, Model N4385A, Bdblingen,
Germany) located at the surface. The DTS uses a laser backscattering technique to measure temperature with a 1 m spatial
resolution along the fiber. A description of DTS technology as applied to wellbore temperature monitoring can be found in
Hurtig et al. (1994). Parallel to the fiber-optic cable is a two-conductor 14 AWG direct burial (outdoor) cable shorted at the
bottom, which provides uniform heating along the length of the well when current is applied. Following the wellbore
completion process, the temperature was allowed to equilibrate for 1 month before we acquired a baseline thermal profile and
started acquiring data with the DTPS.
Results
Permafrost thickness and hydrostatic head
Our baseline thermal profile provides the most accurate measurement of permafrost thickness in this region, as other nearby
estimates by the Geological Survey of Canada are extrapolated from much shallower boreholes (e.g., Taylor et al., 1998). A
linear extrapolation through the lower 120 m of DTS data through the pressure-temperature sensor (Figure 1), indicates that the
base of the permafrost (00C isotherm) is at 458 5 m. The depth uncertainty is based upon propagating a temperature error of
0.10C into the depth estimate. A small correction (+0.0250C) has been applied to account for the thermal perturbation created
near the base of the well during the wellbore completion process, following the method suggested by Lachenbruch and Brewer
(1959). By plotting the temperatures measured after wellbore completion as a function of Loge(t/(t-s)), where t is the time
elapsed since wellbore completion and s is the duration of the thermal perturbation (assumed to be 1.25 days), we correct for
the effect of cooler water being introduced deeper in the borehole during the completion process.
The steady-state subpermafrost hydrostatic pressure can also be estimated by plotting pressure as a function of Loge(t/(t-
s))-referred to as a Horner Plot in well test literature. Using the pressure data from the Level Troll pressure-temperature
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Freifeld, B. M.; Finsterle, S.; Onstott, T. C.; Toole, P. & Pratt, L. M. Ground surface temperature reconstructions: Using in situ estimates for thermal conductivity acquired with a fiber-optic distributed thermal perturbation sensor, article, October 10, 2008; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc934383/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.