| Abstrakt: |
One of the methods of processing temperature measurements in boreholes is the extrapolation procedure, developed and applied by J. Polášek as a part of the geothermal evaluation of the Všebořice area near Ústí nad Labem. This procedure is based on a divison of the measured temperature profile into sections (regions) based on its course and geological profile of the borehole. Using a linear regression, the best-fit lines through the measured temperature depth profiles are calculated for each of the regions. Subsequently, pair combinations of the best-fit lines of the individual regions of the measured temperature curve are generated, depending on the temperature depth profile of the particular borehole, and parameters of their extrapolation function are calculated. Based on the assessment and evaluation of the results of the calculation for each pair of regions of the measured temperature curves and the assessment of the geological structure at the borehole area, the appropriate pair combinations of the measured temperature curve regions are selected, for which the average of the extrapolation function values is calculated. This procedure is repeated for several other suitable pair combinations of the measured temperature curve regions. Values that show the lowest root mean square deviation from the measured temperatures, while respecting the geological structure in the borehole area, are considered the most suitable for extrapolating temperature to a greater depth. Some temperature curves in boreholes processed using this procedure show a close temperature gradient in all selected regions of the curve. In such boreholes, the total extrapolation function is a straight line. Common case is a presence of several successive regions of the curve with a linear pattern whose gradient gradually increases. The extrapolation function of such borehole temperature curve is represented by a second degree polynomial. In some cases, however, several successive regions of the curve, each with a linear pattern, showed a gradually decreasing gradient. Such extrapolation gives meaningless outputs and the causes of such a result need to be searched for. This extrapolation can be in some cases and to a certain extent explained by the geological structure of the area. Due to the very limited knowledge of the deeper geological structure in the Czech Republic and also as the change of the temperature curve does not always have obvious geological causes, the risk of considerable inaccuracies in temperature determination at depth is high and definitely increases with extrapolation depth (length). Results of the extrapolation of temperature curves in boreholes are therefore only approximate. Significant refinement of extrapolation of temperature measurements in boreholes deeper than 1 km can be achieved, for example, by electromagnetic geophysical measurements with a long reach, and by use of special methods of their geothermal interpretation. [ABSTRACT FROM AUTHOR] |
