Estimation of soil properties for the West Dock site

wlcable - Mon, 2010-03-01 15:51

Temperature sensors were placed at 0.30, 0.37, 0.52, 0.68 and 0.83 meter depths at the West Dock site. At these depths, in this analysis, hourly temperature measurements were used during the following time intervals: at West Dock between August 7, 1997 and August 18, 2002. Drilling records were used to determine lithology, see Table 2. Results of previous investigations (Lachenbruch et al., 1982; Osterkamp and Romanovsky, 1996; Lachenbruch and Marshall, 1986; Zhang, 1993) were used to determine the initial approximation $\mathscr{C}_0$. After applying the variational approach, we list the estimated soil properties in the active layer and upper permafrost in Table 2 and compare them to the corresponding values in $\mathscr{C}_0$. We note that the first layer, at the West Dock site, we obtained a good correlation with previous results reported by Romanovsky and Osterkamp (1997). Also we note that for the West Dock site, we obtain relatively high thermal conductivities $\lambda_f$ for the deeper soil layers, see Table 1. These values of $\lambda_f$ are 10% higher than previous estimates in (Osterkamp and Romanovsky, 1996).

Using the estimated properties, soil temperature dynamics are calculated for the entire period of measurements involved in this analysis. Results are shown in the left plot of Figure 1. In this plot, we compare the calculated and measured permafrost temperatures at depths of 0.52 and 0.68 meters. In the right plot of Figure 1, we display a histogram of differences between the measured and computed optimal temperature dynamics at 0.52 and 0.68 meters. The mean value of the differences is about $0.01^\circ{C}$, and their standard deviations are less than $0.08^\circ{C}$. In the left plot of Figure 2, we show measured and calculated optimal permafrost temperature profiles at the West Dock site on July 3, 1998, June 16, 1999, June 14, 2000, and on June 18, 2001. The largest deviation of the computed temperature from the measured one is a few tenths of a degree in the upper 10.0 meters below the soil surface, due to coarsely discretized soil lithology. In the right plot of Figure 2, we show computed and measured permafrost temperature profiles at the Franklin Bluffs. At this site the thermal conductivity of the deep soil layer is smaller, so the permafrost warming is at much slower rate than at Deadhorse and West Dock, see Figure 2.

Table 2: Thermal properties of the ground material estimated from the best fit of the numerical model to the data
Depth Soil type $\lambda_f$ $\eta$ $b$ $T_*$
0.3-1.0 silt $1.86{\pm}0.01$ $0.34{\pm}0.01$ $0.75{\pm}0.01$ $-0.045{\pm}0.001$
1.0-8.5 silt/sand/gravel $3.21{\pm}0.03$ $0.14{\pm}\infty$    
8.5-50.0 gravel/sand $3.57{\pm}0.05$    

 

Figure 1: On the left plot, measured and optimally computed soil temperature dynamics at 0.52 and 0.67 meter depth at the West Dock site. On the right plot, the histogram of differences between the measured and computed temperature dynamics shown on the left plot.
\includegraphics[width=0.45\textwidth]{f05a.eps} \includegraphics[width=0.45\textwidth]{f05b.eps}

 

Figure 2: Measured and computed soil temperature profiles at bore holes at the West Dock (left) and Franklin Bluffs site (right) during several consecutive years.
\includegraphics[width=0.45\textwidth]{f06a.eps} \includegraphics[width=0.45\textwidth]{f06b.eps}