DECONVOLUTION OF TEMPERATURE GRADIENT LOGS

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Abstract
A complete system for continuous logging of borehole temperature gradients has been developed and subjected to field tests with prototype equipment. To meet the requirements of this system, a time‐domain operator was derived consisting of a smoothing term, a deconvolution term (to compensate for lag due to the thermistor time constant and probe velocity), and a gradient term. When convolved with raw field data, this combined operator will yield directly a high‐precision, high‐resolution temperature gradient profile. The system was field tested in two partially cased, water‐filled boreholes. There was good correlation between gradient logs and thermal resistivity profiles from laboratory measurements on core material, indicating that even in cased wells the gradient log is a good approximation of the thermal resistivity profile. For a lowering rate of 18 m/minute, the gradient profile exhibits a repeatability better than ±0.5°C/km. Comparison of the gradient profile with a fine‐scale geologic log indicates a stratigraphic resolution threshold on the order of 2 m for a 10–20 percent thermal resisitivity contrast. For isolated resistivity contrasts of 50–100 percent, the resolution is better than 0.5 m.

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