Ground surface temperature reconstructions: Using in situ estimates for thermal conductivity acquired with a fiber-optic distributed thermal perturbation sensor Page: 2 of 12
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In studies for which drill core has not been recovered, thermal conductivity is assigned based on published values for similar
facies. Chouinard et al. (2007) estimated GSTH based on temperatures in three closely spaced boreholes using one of three
thermal conductivity values to represent the local lithology: argillite (2.4 W in-' K1), peridotite/gabbro (2.8 W n' K-') and
basalt (3.1 W in-' K'1). However, when detailed thermal conductivity profiles have been estimated based on borehole
temperature profiles, thermal conductivity is usually highly variable (Henninges et al., 2005; Mwenifumbo, 1993).
We developed a methodology to determine thermal conductivity with depth with a spatial resolution of 1 meter by
combining a fiber-optic distributed temperature sensor (DTS) with a borehole length electrical resistance heater. We refer to
the coupled instruments as a distributed thermal perturbation sensor (DTPS). The DTPS methodology is similar to the
Temperature Recovery Method proposed by Gtinzel and Wilhelm (1999), although our use of an electrical heater provides a
more precisely controlled thermal disturbance than circulation of fluid within the wellbore.
In the summer of 2007 we installed a multifunctional borehole observatory at the High Lake Project Site (67022'N,
110050'W), in Nunavut, Canada, with the aim of acquiring a broad spectrum of environmental data. In addition to installing a
DTPS, a pneumatic packer was set above the base of the permafrost. A U-tube geochemical sampling system acquired fluid
samples for delineating gas concentration, pH, pe, microbial abundance and community structure and activity, as well as
isolating pristine subpermafrost brine for future studies (Freifeld et al., 2005). A pressure-temperature sensor collocated with
the U-tube sampling inlet facilitated estimation of hydraulic conductivity. This paper focuses on the data collected using the
DTPS, the in situ estimates for thermal conductivity, and our predicted GSTH.
High Lake Site and Observatory Description
Site description
The High Lake Project Site is located in an Archaen mafic volcanic belt, with permafrost extending down to 460 m on a
mining exploratory lease originally purchased by Wolfden Resources and currently operated by Zinifex Canada Inc. All of the
work was conducted within a 75 mm diameter borehole (designated HL03-28), which was initially drilled in 2003 to a length
of 335 m (depth 304 m) as part of Wolfden Resource's characterization of base metals for potential economic extraction from
the High Lake volcanogenic massive sulfide deposit. In July 2006, our project team cored continuously to lengthen HL03-28 to
535 m. After removing an ice blockage that formed in the borehole during the previous season, we installed a permanent
borehole observatory.Borehole observatory
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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/m1/2/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.