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https://hdl.handle.net/2142/21852
Description
Title
Interpretation of shallow geothermal surveys
Author(s)
Larson, Timothy Howe
Issue Date
1990
Doctoral Committee Chair(s)
Hsui, Albert T.
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Geophysics
Hydrology
Language
eng
Abstract
Over the last several decades, the shallow temperature survey has been developed as a surface geophysical method in ground water and geothermal energy exploration. Advances in numerical modeling techniques have made it possible to build a realistic model of the shallow subsurface. In particular, variable hydraulic and thermal properties can be included in models which couple moisture and heat processes in multi-dimensions.
Coupled thermal and moisture transport equations were solved using an integrated finite difference method to simulate both one-dimensional profiles and two-dimensional cross sections. The two-dimensional model simulated a cross-section through a portion of the Sheffield low-level radioactive waste disposal site, Bureau Co., Illinois. Material properties are from laboratory data and generic values. Factors investigated include surface moisture boundary conditions, material thermal properties, layering, variable moisture content and topographic changes.
Of those studied, the most important factor affecting the temperature at 1.0 m depth is the thermal conductivity of the material within the first meter of the subsurface. Advective flux will be measurable when the soils are moist and have a hydraulic conductivity of about 10$\sp{-5}$ cm/s or greater. Vapor flux will be measurable only when the soil is relatively dry, having a soil pressure gradient of at least 10$\sp3$ cm/cm. For example, in the first 2 weeks following a summer storm, advective flux will result in a slight warming of the soil. Following the passage of the moisture front, evapotranspiration will cause a relative cooling of the soil.
Shallow thermal structures are also sensitive to local, topographically driven ground water flows. Advective flux can be expected to increase when the topographic gradient increases. However, in soils with high hydraulic conductivity, the effect can be more accurately predicted based on the slope of the water table.
Soil temperatures at 1.0 or 2.0 m depth should be measured with a precision of 0.01$\sp\circ$C. If possible, thermal conductivities of the shallow materials should also be obtained. Using these data, and available weather information, quantitative interpretation based on numerical models is feasible.
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