It is generally assumed that, because of heat exchange with the drilling fluid, a bore hole must be left for some considerable time after drilling has ceased before temperature measurements can be made in it for the purpose of determining the geothermal flux. To test this point, a series of measurements of water temperature and flow were made during diamond drilling of a hole, and occasional temperature logs were taken. In this particular case it was found unnecessary at any time to wait more than a day after drilling ceased to determine the geothermal gradient to within 5 per cent. The reason for this is shown to lie in the action of the drilling system as a heat exchanger. For the relatively small circulation of water used in diamond drilling, heat exchange takes place in restricted regions at the top and bottom of the hole. In diamond drilling, therefore, unless very large quantities of water are circulated, the temperature gradient near the center of the hole is little affected. On the other hand, in rotary drilling, in which very high fluid velocities are used, the temperature in the hole is determined almost entirely by the input temperature of the fluids. Methods of correcting observed temperatures for the effects of drilling fluid are discussed.
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J. C. Jaeger (1961) studied this question.