EVALUATING THE DIFFERENCE IN HEAT FLUX MEASUREMENTS FROM UNCONSOLIDATED SOILS IN A VERTICAL SEQUENCE THROUGH THERMOCOUPLE SENSORS

Heat flux estimations at the land surface in geothermal areas can be complicated. This is partly because the thermal conductivity of the soils is spatially variable. Another, more important factor is the time-varying atmospheric boundary condition, which propagates a quasi-periodic signal into the subsurface, causing the near surface flux to vary temporally such that it generally reverses on a diurnal time-scale. Under these conditions, the most accurate way to estimate the annual average heat flux is to install a vertical sequence of thermocouple sensors in the shallow subsurface, usually consisting of four to six sensors distributed over about 1 m of depth or less. With accurate data on the subsurface temperature profile and the soil thermal conductivity, it is possible to correct for the time-varying surface boundary condition. Our experiments suggest that many materials that are commonly employed as a framework for mounting thermocouple sensors impact measured temperatures, and may result in inaccurate estimates of subsurface heat flux. Here, we evaluate the feasibility of using an array of thermocouple sensors mounted on a low thermal conductivity framework, constructed of low-cost materials, and evaluate the array’s suitability for measuring ground temperature gradients and estimating shallow heat flux. On the basis of our experiments, we provide practical guidelines for the collection and interpretation of shallow temperature profile time-series data and the estimation of shallow heat flux.