By Nhan Nguyen

The temperature log is used to measure the temperature of the borehole. Temperature sensors are attached to any tools that is used in the well to measure the maximum temperature and the minimum temperature of the well.
The maximum temperature of the borehole is assumed to be at the bottom of the well. The bottom hole temperature (BHT). The BHT and the temperature at the top of the borehole is used to find the geothermal gradient, also known as the geotherm. Borehole temperature is a vital parameter for resistivity logs but is also used to detect any fluid movement in the well, the analysis of fluid pressure, geochemical modeling of formations and the maturity of hydrocarbons.

Each tool is attached with a temperature sensor. The measurements are always made at the bottom of the well and sometimes at intervals up the well. The general case to measure the temperature was to attach multiple temperature monitors to the outside of a tool string, which would only allow the temperature at the bottom of the borehole to be measure. However, special new temperature logging tools have been developed that allow for continuous reading of temperature up the well.
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An Example of a Temperature Log <http://hillpetrophysics.com/wp-content/uploads/2010/06/Buildup.gif>