19.4.1 Gyro Tool Types and Running Modes

Whereas we considered that all MWD tools were essentially the same and comprised three orthogonal accelerometers and three orthogonal accelerometers, there are many different types of gyro survey tool and
many different ways of running them in a well.

The running modes can broadly be divided into tools which take measurements whilst stationary at regular survey stations and those which run into hole, whilst constantly moving and recording data. Furthermore, a gyro tool may include three orthogonal accelerometers and three orthogonal gyros or might only use a subset of sensors. Tools with additional redundant sensors can be reflected in the error magnitudes assigned rather in than in the model mathematics.

So the gyro model covers tools which have:
1. x-y-z accelerometers – run in stationary or continuous mode
2. x-y only accelerometers – run in stationary or continuous mode
3. x-y-z gyros – run in stationary mode
4. x-y gyros only – run in stationary mode
5. x-y-z gyros – run in continuous mode
6. x-y gyros only – run in continuous mode
7. z gyro only – run in continuous mode

In addition, the model considers these other running conditions:
8. initialisation to an external sighting reference
9. indexing in hole to remove gyro drifts
10. rotation in hole
11. the cant angle of the sensors in x-y accelerometers systems.

Furthermore, the survey mode may vary within a survey run, as a function of inclination.