Heat Transfer and Thermal Properties of the Subsoil In Belém

Abstract

Temperature profiles with depth were monitored in two neighboring boreholes at a site in Belém, Parâ, 4OO m from a river, at five-day intervals during a one year cycle of observations. The first borehole was cased with galvanized steel pipe closed at its lower end and filled with water. Three thermistor thermometers were fixed to a thin aluminium rod at depths of 1, 3 and 6 meters within this hole. The second borehole was cased with a plastic (PVC) pipe open at the bottom and consequentþ filled with water to the local water table level. Temperature profiles in the second hole were determined with a thermistor sonde to 16 meters depth. These observations were complemented with measurements of temperature at O.O2, O.'1.,, O.2, O.3 and 0.5 meters, using mercury soil thermometers and with solar radiation flux incident at the surface, determined by an actinograph. The temperatufe profiles for months of the same season show a remarkable resemblance, while between the rainy and dry season one notices very different features. The daily amplitudes of the temperature at O.5 m reached O.loC in the rainy season and O.34oC during the dry season. The seasonal temperature ranges at these depths were found to be approximately 5.OoC at0.Oz m and 4.OoC at O.5 m. The temperature profiles in both seasons show changes of sign of the gradient at different depths. As expected, the amplitude of temperature variations decreases rapidly with depth but it is appreciable down to 16 meters and certainly at lower depths. A high correlation was observed between the variations of the temlleratur€ measured at I m and the solar radiation flux incident at the surface. The rare cases in which this correlation was not observed could be explained by an increase of the pluviometric precipitation at the site. Continuous sediment sampling during drilling allowed the laboratory determination of the profiles of specific heat, thermal conductivity and density. These parameters were used to determine the thermal diffusivity profiles. The heat flow near the surfâce was calculated from 0.1 to 8.0 meters. The estimates for 9:OO AM and 6:O0 PM, local time, showed an almost symmetrical behaviour down to 0.5 m, with relation to the zero flux line. The estimated heat flow presents a rapid decay of amplitude with depth, and sudden changes of sign, due to the storcd heat from previous periods of surface exposure to insolation.