19.2.3 Error Magnitude

For a particular tool model, each error source has an error magnitude. The error magnitude is essentially the standard deviation of the range of values which the error source might be expected to take over a statistical sample of survey data under normal field operating procedures. For example, if you consider a large number of MWD runs, what are the statistics for the range of tool misalignments to the borehole?

The values may change depending on what survey techniques or corrections are being applied to the job in hand. So as previously mentioned, using an IFR survey to measure the declination value in the field reduces the magnitude of the declination error term. There is still a declination error source, and the weighting functions remain the same, but compared to the MWD tool model, the MWD+IFR tool model will have a smaller magnitude for the declination term.

The MWD paper gives values for these for ‘normal’ MWD surveying. However, beyond that initial set error magnitudes are not set by the ISCWSA.

For the gyro error model, it is expected that error magnitudes will be supplied by the gyro contractor. Test cases are defined in the gyro paper but these are not considered to model any particular survey tools.

The error magnitudes defined in the MWD paper are all at one standard deviation (1-sigma). If the user requires final error ellipses at two standard deviations (or three) then the 1-sigma ellipses can simply be multiplied up and typically drilling software has a user control to define at what level error ellipses are to be output. There are two provisos to this – some drilling software packages allow the user to enter error magnitudes at different defined confidence levels (e.g. 1-sigma or 2-sigma etc.) Also, changes to the MWD at revision 4 may require the user to define the required output confidence level first so that certain magnetic field reference terms can be correctly calculated (for further details see section 19.3.4).