- The process of adjusting a measurement to a standard, so that copies of the same type of logging tool or laboratory instrument will read the same. The tool or instrument is placed in the presence of a calibrator or calibrating environment, for example, a source of gamma rays for a gamma ray tool, or the air, far from the ground, for an induction tool. Calibration coefficients, typically a gain and an offset, are calculated so that the tool or instrument reads correctly in the calibrator. The coefficients are then applied during subsequent measurements.The term master calibration is used for the regular, as for example quarterly, calibration of a logging tool in the workshop. For most wireline tools, a secondary calibrator is adjusted during the master calibration and taken to the wellsite so that a wellsite calibration can be done just prior to the logging job. Some tools, such as the gamma ray, are calibrated only at the wellsite. For most measurements-while-drilling tools, the environment requires that the calibration be performed at the workshop and only a verification made at the wellsite.For some measurements, there is a primary worldwide standard against which calibrators are calibrated, as for example, the radioactive formations at the University of Houston used to define gamma ray API units.
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