A Gyroscope is basically a balanced, spinning mass, which is free to rotate on one or more axis. The basic operation of a gyroscope can be compared to a spinning top. As long as the top spins fast enough, it attempts to hold its vertical orientation. If the top were propelled by a spin motor at a particular speed designated by its mass, it would stay vertical for as long as the motor ran, that is, if no external forces acted on it. This is the simple basis of all gyroscopes used in navigation, a spinning mass that through its momentum becomes resistant to external forces and attempts to maintain an orientation like the top in space. The term "resistant to external forces" is important, for a perfect gyro cannot be built, that will not be upon by external force and react by movement.
The classic example of a natural occurring gyroscope is the planet Earth - a spinning mass attempting to hold a particular orientation in space established long ago. Even the Earth is not a perfect gyro. It reacts to external forces with some movement, or drift, off its orientation. Fortunately, the drift is very small.
The next step in basic gyro understanding is the two-degree-of-freedom gyroscope, the same kind used in the oil. Free-gyros have been used in wellbore surveying since the 1930's.
The frames supporting the gyroscope, and allowing this freedom of rotation are referred to as Gimbals. Because gyroscopes can be extremely complicated, we will look at simplified gyroscopes initially, in order to understand the forces working upon them.
The gimbals isolate the gyro from the base so the spinning mass can attempt to maintain its original orientation no matter how the bass moves. As the probe moves downhole through different directions and inclinations, the gimballing allows the gyro to attempt to maintain a horizontal orientation in space.
In performing a wellbore survey, the gyro is pointed in a known direction prior to running in the well, so throughout the survey the spin axis attempts to hold its surface orientation. Note that a compass card is aligned with the horizontal spin axis of the gyro. Survey data is collected downhole by affixing a plumb-bob assembly over the compass.
At each survey station a picture is taken of the plumb-bob direction with respect to the compass card, resulting in readings of wellbore azimuth and inclination. The plumb-bob always, as a pendulum, points down toward the Earth's centre. When the tool is inclined off vertical, it points out the inclination of the well on the concentric rings and the azimuth by correlation with the known direction of the gyro spin axis established at surface. There are also electronic, surface read-out free-gyro systems which eliminate the plumb-bob.
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