An error source is any physical phenomenon which contributes to the overall positional measurement error.
Examples of error sources include residual sensor errors after calibration, misalignment of the survey tool with the borehole axis, uncertainties in the Earth’s magnetic field, which is the reference against which MWD
measurements are made, deflection of the survey tool under gravity, stretch of drillpipe or wireline under the weight of the BHA or survey tool etc.
Between all versions of the MWD and gyro models, some ninety-eight individual error sources are identified. These are not all applicable to each survey tool, but are dependent on the type of tool in operation and how it works. A typical tool model (or IPM file) defines which of these error sources are invoked to model that tool.
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