Gyro-magnetic wellbore surveying

1. A method for surveying a wellbore, comprising:

applying a statistical estimation process that combines magnetic survey measurement data and gyroscopic survey measurement data to form error estimates in a magnetic survey system;

using the error estimates to correct magnetic survey data provided by the magnetic survey system;

controlling a path of the wellbore in a drilling process based on one of:

the corrected magnetic survey data; and

magnetic-only survey data upon convergence of the error estimates; and

using the magnetic-only survey data to control the path of the wellbore for a remainder of the drilling process upon convergence of the error estimates.

2. The method of claim 1 , further comprising determining a gyroscopic survey from the gyroscopic survey measurement data.

3. The method of claim 2 , further comprising determining magnetometer measurement estimates from the gyroscopic survey and a plurality of parameters.

4. The method of claim 3 , further comprising comparing the magnetometer measurement estimates and the magnetic survey measurement data.

5. The method of claim 4 , wherein the error estimates are determined from the comparison, a sensor noise factor, and a magnetic sensor uncertainty factor.

6. The method of claim 3 , wherein the plurality of parameters comprise total Earth's field, dip angel, and declination.

7. The method of claim 3 , wherein the magnetometer measurement estimates are determined from the gyroscopic survey, the plurality of parameters, and Earth's field correction data.

8. The method of claim 1 , wherein controlling the path of the wellbore comprises determining a drilling trajectory.

9. A method for surveying a wellbore, comprising:

applying a statistical estimation process that combines magnetic survey measurement data and gyroscopic survey measurement data to form error estimates in a magnetic survey system;

using the error estimates to correct magnetic survey data provided by the magnetic survey system;

controlling a path of the wellbore in a drilling process based on one of:

the corrected magnetic survey data; and

magnetic-only survey data upon convergence of the error estimates;

determining a gyroscopic survey from the gyroscopic survey measurement data; and

determining magnetometer measurement estimates from the gyroscopic survey and a plurality of parameters;

wherein the magnetometer measurement estimates are determined from the gyroscopic survey, the plurality of parameters, and Earth's field correction data;

wherein the Earth's field correction data is determined from the error estimates and the plurality of parameters.

10. The method of claim 9 , wherein the plurality of parameters comprise total Earth's field, dip angel, and declination.

11. The method of claim 10 , wherein the plurality of parameters further comprise in field referencing (IFR) data.

12. A method for surveying a wellbore, comprising:

applying a statistical estimation process that combines magnetic survey measurement data and gyroscopic survey measurement data to form error estimates in a magnetic survey system;

using the error estimates to correct magnetic survey data provided by the magnetic survey system; and

controlling a path of the wellbore in a drilling process based on one of:

the corrected magnetic survey data; and

magnetic-only survey data upon convergence of the error estimates;

wherein the error estimates are used to provide corrections to a local geomagnetic model.

13. The method of claim 12 , wherein the local geomagnetic model comprises a British Geological Survey Global Geographic model.

14. The method of claim 13 , wherein the local geomagnetic model includes in field referencing (IFR) data.

15. A method for surveying a wellbore, comprising:

applying a statistical estimation process that combines magnetic survey measurement data and gyroscopic survey measurement data to form error estimates in a magnetic survey system;

using the error estimates to correct magnetic survey data provided by the magnetic survey system; and

controlling a path of the wellbore in a drilling process based on one of:

the corrected magnetic survey data; and

magnetic-only survey data upon convergence of the error estimates;

wherein the statistical estimation process is a least squares estimation process or a Kalman filter process.

16. A method for surveying a wellbore, comprising:

applying a statistical estimation process that combines magnetic survey measurement data and gyroscopic survey measurement data to form error estimates in a magnetic survey system;

using the error estimates to correct magnetic survey data provided by the magnetic survey system;

controlling a path of the wellbore in a drilling process based on one of:

the corrected magnetic survey data; and

magnetic-only survey data upon convergence of the error estimates;

using the corrected magnetic survey data to control the path of the wellbore prior to convergence of the error estimates; and

using the magnetic-only survey data to control the path of the wellbore upon convergence of the error estimates.

17. A method for surveying a wellbore, comprising:

applying a statistical estimation process that combines magnetic survey measurement data and gyroscopic survey measurement data to form error estimates in a magnetic survey system;

using the error estimates to correct magnetic survey data provided by the magnetic survey system; and

controlling a path of the wellbore in a drilling process based on one of:

the corrected magnetic survey data; and

magnetic-only survey data upon convergence of the error estimates;

wherein the convergence of the error estimates is determined by monitoring a covariance of the error estimates and a magnitude of a difference between magnetometer measurements and magnetometer measurement estimates until the covariance and the magnitude reduce to predefined levels.

18. A method for surveying a wellbore, comprising:

applying a statistical estimation process that combines magnetic survey measurement data and gyroscopic survey measurement data to form error estimates in a magnetic survey system;

using the error estimates to correct magnetic survey data provided by the magnetic survey system;

automatically modifying a drilling trajectory based on one of:

the corrected magnetic survey data; and

magnetic-only survey data upon convergence of the error estimates; and

using the magnetic-only survey data to modify the drilling trajectory upon convergence of the error estimates.

19. A method for surveying a wellbore, comprising:

applying a statistical estimation process that combines magnetic survey measurement data and gyroscopic survey measurement data to form error estimates in a magnetic survey system;

using the error estimates to correct magnetic survey data provided by the magnetic survey system;

controlling a path of the wellbore in a drilling process based on one of:

the corrected magnetic survey data; and

magnetic-only survey data upon convergence of the error estimates;

deactivating a processing of the gyroscopic measurement data to form the error estimates upon convergence of the error estimates;

periodically reactivating the processing of the gyroscopic measurement data to determine an accuracy of the magnetic-only survey data; and

determining, based on the accuracy, whether to continue using the magnetic-only survey data to control the path of the wellbore or provide new corrected magnetic survey data until convergence of the error estimates re-occurs.