Effects of Controlling Parameters and Their Spatial Variability on Heat Flow in the Vadose Zone
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While the use of geothermal energy is becoming more common, a key factor holding back the growth of geothermal energy production is the cost of exploration. Rapid shallow (two meter) temperature measurement is a relatively inexpensive exploration method that can be used to locate blind geothermal systems. This method has been successfully used to locate geothermal anomalies at two meters depth that are representative of a similar geothermal pattern at a greater depth (Coolbaugh et al., 2007). However, studies have shown that increased temperatures at two meters depth are not always analogous to increased temperature at a deeper depth (Olmsted and Ingebritsen, 1986). Therefore, a better understanding of the parameters that control heat flow to shallow depths is needed.This study uses numerical modeling to assess the effect of material heterogeneities on heat transport in the vadose zone. The TOUGH2 computer program is used to numerically simulate heat, water, and air transport through two-dimensional porous media. A 100 by 100 meter grid was created with a water table at the lower boundary held constant at 95°C and the average surface temperature at the upper boundary held constant at 20°C. Intrinsic permeability, porosity, residual liquid saturation and thermal conductivity of the porous media were systematically altered in different spatial patterns to determine the controlling factors affecting heat flow in the vadose zone. Results of this study indicate that individual changes in the parameters tested do not result in significant horizontal changes in temperature at shallow depths. Although these individual changes in the parameters tested are not enough to significantly change temperature horizontally at two meters depth, the results reveal conditions under which heat flows more readily to the surface.