Climate Change and Subsurface Temperature

While the majority of climatologists are looking up to the sky or plunge themselves in the oceans to understand why and how the climate has changed, an adroit "borehole cli-matologist" is looking inside deep holes drilled in the ground. The most important research implement of the "Borehole Climatology" is a borehole. Typical borehole site looks like as it is shown in Figure 18. A borehole, usually a small-diameter hole drilled from the land surface to the depth of several tens or hundreds meters, presents a deep narrow shaft in the ground which enables to lower a temperature-measuring device (thermometer) down the hole. The temperature measurements are repeated to progressively greater depths until a long temperature-depth profile is obtained. The thermometer measures the temperature of the borehole filling fluid (usually water), not the surrounding rock, so as to obtain meaningful values of the ambient temperature of the surrounding subsurface strata, the borehole fluid must be in thermal equilibrium with its surroundings. If the hole has been only recently drilled, the fluid may not have time enough to attain thermal equilibrium. Also, any event that subsequently disturbs the bore fluid may cause certain thermal disturbance. The disruption of the thermal equilibriun caused by the drilling process is slowly dissipating; to obtain a reliable precise temperature-depth record a long recovery time (up to several months) is indispensable. Production, i.e. removal of fluid from the borehole, also causes thermal disturbances, so in many cases the oil wells or water-pumping holes are hardly suitable for the borehole climate reconstruction.

Temperature logging is actually a part of borehole geophysics, the science that records and analyzes measurements of various physical properties in boreholes. Probes that measure different properties are lowered into the borehole to collect continuous or point-by-point data, so-called geophysical log. These records may be used for various environmental investigations and help better understand the subsurface conditions. Geothermics or geothermal research, the sub-branch of geophysics, is the study of the thermal state of the interior of the solid and of the thermal properties of the Earth material. Knowledge of the subsurface temperature field is central for interpreting and understanding practically all geophysical processes. Temperature log is the temperature record in the borehole and

Fig. 18. The Torun IG-1 (Poland) deep borehole site. Standard measurement technique, when the temperature probe is lowered to the hole with the help of cable on the winch. Temperature data are taken and stored with the pre-selected time interval (usually each 5 s), and depth is simultaneously recorded by computer from the number of revolutions of the pulley.

Fig. 18. The Torun IG-1 (Poland) deep borehole site. Standard measurement technique, when the temperature probe is lowered to the hole with the help of cable on the winch. Temperature data are taken and stored with the pre-selected time interval (usually each 5 s), and depth is simultaneously recorded by computer from the number of revolutions of the pulley.

represents a general tool of geothermics. Examples of the temperature logs measured in boreholes are presented in Figures 16 and 17 (Chapter 1). At the constant surface (temperature) conditions the underground temperature is governed by the outflow of heat from the Earth's interior. For the homogeneous stratum (constant thermal conductivity of the subsurface rocks) temperature increases steadily with depth, i.e. the geothermal gradient is constant. Temperature changes at the Earth's surface (as the response to the climate changes) slowly propagate downward into the subsurface and appear as tiny temperature deviations superimposed on the background geotherm. While the part of underground temperature field governed by the heat flow from the Earth's interior is generally steady state, the response to the surface conditions is a transient perturbation. Present precise borehole temperature measurements at depth of up to several hundred meters thus provide an archive of temperature changes that have occurred on the surface in the past. They can be analyzed to yield a ground surface temperature (GST) history over the past few centuries.

2.1 Methods and Technique to Carry Out Borehole Temperature Measurements

Temperature data measured in boreholes serve as an input to many fields of the scientific research as well as engineering and exploration, and the techniques and equipment for such measurements are well developed. Among others, Lee and Uyeda (1965) and Jessop (1990) presented detailed review of a history of the geothermal measurement and the interpretation of the heat flow data in terms of the basic geophysical studies. The earliest measurement of the subsurface temperatures started at about 1700s, soon after thermometers had been developed, in mine shafts, tunnels, and/or water wells. Some of the early systematic measurements in boreholes were conducted between 1868 and 1883 under the aegis of the Committee of the British Association for the Advancement of Science (see Bullard, 1965). Initially, these measurements were simply individual readings obtained by the maximum-reading thermometers at shallow depths. The development of the petroleum industry during the second half of the nineteenth century made deep boreholes available for subsurface temperature loggings and, together with the development of electrical-resistance thermometers, significantly improved the accuracy of the measurements. Schlumberger services first introduced the temperature survey, using continuous-recording logging tools, in the late 1930s. Guyod (1946) had presented a series of papers, which discussed the theory and the various current and potential uses of the underground temperature data in the petroleum industry and inspired a widespread application of the temperature logging technique.

Haenel et al. (1988) and Jessop (1990) have presented reviews of the methodology and technology of the scientific borehole temperature measurements for the heat flow determination. The most accurate temperature and heat flow data are obtained with high-resolution thermistors sunk into small-diameter, thermally stable boreholes at logging speeds of 10-15 m/min. These data are generally recorded as continuous temperature or temperature gradient logs. The different kinds of the logging tools have a resolution of 1-3 mK with typical accuracy of several hundreds of degree.

The borehole GST reconstruction methods deal with very small disturbances to the subsurface temperatures, where even tiny variations of some hundreds of degree are considered significant and accuracy of the measured data is crucial. As mentioned before, the drilling operations disturb the temperature field in the vicinity of the boreholes, while good-quality steady-state data reflecting "formation temperature" are indispensable for proper evaluation of the past climate. The undisturbed borehole temperature can be measured only in the equilibrium conditions after the long period the hole was shut in and drilling mud circulation ceased. The "thermal recovery" time for a borehole may range from a few days for a shallow (100-150m) well to several months for deeper holes.

The main assumptions for the mathematical approximation of the temperature disturbances due to drilling are: (1) drilling was continuous and regular, (2) there is no fluid loss, and (3) thermal diffusivity equals that of the surrounding rocks. In this case, the temperature disturbance due to drilling (heat exchange with drilling fluid and frictional heating) Td at a distance r can be approximated by a constant line source (Carslaw and Jaeger, 1962)

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