Ground surface temperature reconstructions: Using in situ estimates for thermal conductivity acquired with a fiber-optic distributed thermal perturbation sensor Page: 5 of 12
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shown in Figure 3. In addition, the inverse problem is mildly regularized by penalizing large amplitudes in GSTH fluctuations
around an unknown mean surface temperature.
The numerical inversion code iTOUGH2 (Finsterle, 2004) was used to estimate GSTH up to year 2000 by automatically
matching the calculated to the observed baseline temperature profile (Figure 1). Surface temperature was parameterized by
specifying the rate of temperature increase or decline at every 100 years. In addition, the geothermal heat flux at the bottom of
the model is an adjustable parameter. Finally, the mean temperature over the 2000-year simulation period is added as a
parameter to be estimated. Deviations of calculated surface temperatures from this mean are mildly penalized by specifying the
mean temperature as a reference data with a prior uncertainty of 50C. Assuming that we have some prior knowledge about the
mean surface temperature-even with large uncertainty-provides the regularization needed to avoid large fluctuations in the
estimated, time-dependent surface heat fluxes. A total of 100 temperature data measured in borehole HL03-28 between a depth
of 30 m and 400 m are specified as calibration points with an uncertainty of 0.10C. A weighted least-squares objective function
was used to asses the deviations between the measured and calculated profile temperature, and between the estimated and
calculated mean surface temperature. The minimum of the objective function was identified using the Levenberg-Marquardt
algorithm.
The match obtained (Figure 1) is consistent with the expected measurement error, with a mean temperature residual of 0.10C.
The inferred GSTH is shown in Figure 4. The average surface temperature at High Lake over the past 1000 years is estimated
to be -9.3 0.20C. This estimation uncertainty is sufficiently small so that the recent temperature increase to -6.3 0.70C can
be identified as statistically significant. The ground surface temperature is seen to decline 0.60C between years 1000 to a
minimum at 1860, similar to the trend noted by Marjorwicz et al. (2004) for their inversion of temperature data from 61 wells
in Northern Canada (north of 600N). The ground surface temperature increase at High Lake from the Little Ice Age minimum
at year 1860 to present is estimated to be 3.40C.
The vertical thermal flux at borehole HL03-28 is estimated to be 67 1 mW/m2, which is greater than the 54 4 mW/m2
estimated at the Muskox Intrusion, 200 km west of High Lake, by Beck and Sass (1966) and the value of 46 6 mW/m2
reported for two wells 320 km south of High Lake by Mareschal et al. (2004) at Lac du Gras, Nunuvut, Canada. It is also
greater than the value of 54.1 mW/m2 estimated by inversion of thermobarometric data by Russell & Kopylova (1999) at the
Jericho Kimberlite Pipes located 230 km southwest of High Lake. The higher heat flow at High Lake can possibly be attributed
to the effect of the massive volcaniclastic sulfide deposit, which can act as a conduit for conduction of heat to the surface.
The reduced gradient in the thermal profile at depths shallower than 150 m at HL03-28 is consistent with other studies
investigating GSTH in Canada. Similar changes are apparent in Northern Canadian temperature logs compiled by Marjorwicz
et al. (2004), Superior Province temperature logs (Shen & Beck, 1992), temperature logs from three boreholes in northern
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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/m1/5/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.