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area is measured at high resolution, the field strength data can be used to derive the effects on the compass and good estimates of the declination and dip angle can then be mapped. At sea, specialist non-magnetic equipment can be towed behind a vessel or carried on board a non-magnetic survey vessel with very accurate attitude sensors and magnetometers that output their data at high frequency and the motion effects are taken out in the processing. 6.1.1 IFR Survey Maps Once the measurements have been taken, contoured maps are produced to allow the MWD contractor to interpolate suitable magnetic field values for the well.
Figure 39: Examples of IFR survey results.
The IFR survey results are usually provided as digital data files which can be viewed with the supplied computer program. This allows the contractor to view the data and determine magnetic field values at any point within the oilfield.
Figure 40: Typical software display of an IFR survey.
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