19.4.5 Differences Between the Gyro and MWD Models

The gyro error model is essentially a superset of the MWD model. It uses the same framework for prorogation error sources, but naturally since the physics behind the tools are different, the error sources and weighting functions for the gyro error model are quite different to the MWD model.

The gyro error model uses the method of handling borehole misalignment, introduced into the MWD model as revision 1 and described in the MWD model section above.

But there are three main changes which introduced with the gyro model. Firstly, the tool running mode and hence the error sources in use, may change along the wellbore as a function of inclination. So the tool model must include inclination bounds, either for the possible running modes into which the error sources are grouped in [2] or simply for each individual error source and the software must accommodate these.

Secondly, in the MWD model the weighting functions remain fixed from survey station to station. However, for the continuous gyro operating modes there are time dependent terms in the weighting functions, which are modelled as depth dependant and which will increment as the tool moves along the wellbore. So the software must initialise these and then implement the changes as it propagates the model. All six continuous gyro azimuth error sources have this property (gyro drift and random walk terms for the three continuous gyro sensor combinations, xyz, xy and z: GXYZ-GD, GXYZ-GRW, GXY-GD, GXY-GRW, GZ-GD, GZ-GRW).

Finally, for MWD the tool model need only define which error sources are used to model a tool, the magnitudes of these error sources and their propagation modes. In the gyro model, some additional parameter values may be needed to define details of the running conditions;

a) a cant angle for x-y accelerometer systems.
b) a logical operator indicating whether or not accelerometer switching is implemented.
c) a logical operator indicating whether or not the tool is indexed (z-rotated) at a survey station.
d) a logical operator defining whether or not a stationary tool is rotated between stations, which will change the propagation mode for some bias terms.

The latter two terms can also be accommodated either by correctly defining the error magnitude for the appropriate term to zero or by excluding these terms from the tool model, unless the values change with
inclination.