Geothermal heat flow through the soil at Wairakei

Abstract

The variation of ground temperature with depth was measured between 1954 and 1961 at numerous locations in the Wairakei thermal area of New Zealand. Analysis of these data clearly shows that conduction is the dominant mechanism of heat transfer until the difference between the temperatures at the surface and at a depth of 1 metre reaches a value of about 25 deg c. For higher ground temperatures, heat transport by convection of a mixture of air and water vapour becomes progr,essively more important and the conductive effect becomes insignificant. These conclusions are used to present a simplified picture of the nature of the mechanisms in the transfer of heat to the surface in areas of steaming ground at Wairakei. The temperature data are also used to study the geothermal microclimate, to determine the thermal conductivity of the soil, and to derive relations between the temperatures at various

Vegetation on heated soils at Karapiti, central North Island, New Zealand, and its relation to ground

“…Temperatures of C)7>c are usually reached within 5 cm of ground surface. This zone includes grades Super, A, B, and C of Dawson (1964) in Table I.…”

Fluctuations in near-surface ground temperature, Taupo, New Zealand

“…1961), and the I-m probe was developed (Thompson 1960) so that boundaries could be located and changes monitored as exploitation proceeded. In particular, Robertson & Dawson (1964) concluded that until the difference in temperature between surface and a depth of 1 m reaches 25°c, conduction is the dominant mechanism of heat transfer. Thereafter convection becomes increasingly important.…”

Diurnal and seasonal ground temperature variations at Wairakei

“…HEAT FLOW Robertson and Dawson (1964) have shown that in the near-surface material at Wairakei, the geothermal heat is transported dominantly by conduction until the mean annual temperature at a depth of 1 m becomes 23 deg c greater than the mean annual surface temperature; that the heat transport by convection becomes increasingly important for temperature differences greater than 26 deg c; and that a transition region exists between 23 deg c (T lon = 38°c) and 26 deg c (1\00 = 41°C). They also derived a value of 2 X 10-4 caljsec cm deg C for the thermal conductivity at the transition temperature.…”

“…The heat discharged through the soil by conduction or as steam forms a significant part of the natural heat escape in most geothermal areas. Although the calorimeter developed by Benseman (1959a) and the Venturi meter developed by Dawson (1964) have been used in New Zealand for measuring the heat flow directly, they are extremely slow and are unsuitable for small conductive heat flows. On this account, they have chiefly been used for calibrating the rapid indirect methods which were introduced by Benseman (1959b) and are based on ground temperature measurements.…”