ROTATABLE ORIENTATION INDEPENDENT GRAVITY SENSOR AND METHODS FOR CORRECTING SYSTEMATIC ERRORS

• US 20100145620A1
• Filed: 12/04/2008
• Published: 06/10/2010
• Est. Priority Date: 12/04/2008
• Status: Active Grant

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: Estes, Robert, DiFoggio, Rocco, Fang, Sheng, Georgi, Daniel T., Edwards, Carl M.
• Granted Patent: US 8,131,494 B2
• Time in Patent Office: Days
• US Class Current: 702/6
• CPC Class Codes: G01P 15/093 by photoelectric pick-up
• CPC Class Codes: G01P 15/18 in two or more dimensions
• CPC Class Codes: G01P 2015/0814 for translational movement of the mass, e.g. shuttle type
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups
• CPC Class Codes: G01V 7/00 Measuring gravitational fields or waves; Gravimetric prospecting or detecting

Abstract

A method to correct for a systematic error of a sensor having a plurality of accelerometers configured to measure gravitational acceleration, the method including: rotating the plurality of accelerometers about a first axis; obtaining a first set of calibration measurements from the plurality of accelerometers from the rotation about the first axis; determining a first systematic error for each accelerometer in the plurality using the first set of calibration measurements; and removing the first systematic error from sensor measurements to correct for the systematic error.

Claims

1. A method to correct for a systematic error of a sensor comprising a plurality of accelerometers configured to measure gravitational acceleration, the method comprising:

rotating the plurality of accelerometers about a first axis;

obtaining a first set of calibration measurements from the plurality of accelerometers from the rotation about the first axis;

determining a first systematic error for each accelerometer in the plurality using the first set of calibration measurements; and

removing the first systematic error from sensor measurements to correct for the systematic error.

2. The method of claim 1, further comprising:

rotating the sensor about a second axis;

obtaining a second set of calibration measurements from the plurality of accelerometers from the rotation about the second axis;

determining a second systematic error for each accelerometer in the plurality using the first set of calibration measurements; and

removing the second systematic error from the future measurements.

3. The method of claim 2, wherein the first axis is substantially a longitudinal axis of a logging instrument comprising the sensor.

4. The method of claim 3, wherein the second axis is substantially orthogonal to the first axis.

5. The method of claim 1, wherein the first set of calibration measurements comprises a series of measurements performed by at least one sensing unit in the plurality of sensing units.

6. The method of claim 1, wherein the sensor is rotated 360 degrees about the first axis.

7. The method of claim 6, wherein the first set of calibration measurements correlates to a sine curve.

8. The method of claim 1, wherein each sensing unit has a direction of sensitivity and a first sensing unit in the plurality has a first direction of sensitivity and a second sensing unit in the plurality has a second direction of sensitivity that is different from the first direction of sensitivity.

9. The method of claim 1, wherein the systematic error is at least one of an offset bias and an error proportional to a value output by a sensing unit in the plurality.

10. The method of claim 1, wherein the systematic error is due to misalignment of an accelerometer in the plurality.

11. The method of claim 1, wherein removing comprises:

subtracting the associated first systematic error from a measurement performed by each accelerometer.

12. The method of claim 1, wherein removing comprises:

averaging the first systematic errors to determine an average systematic error; and

subtracting the average systematic error from a composite response that combines measurements performed by the accelerometers in the plurality.

13. The method of claim 1, wherein the method is implemented by a computer program product stored on machine-readable media comprising machine-executable instructions.

14. A sensor for measuring gravitational acceleration and correcting for systematic error, sensor comprising:

a plurality of collocated accelerometers providing output used for measuring the gravitational acceleration, each accelerometer in the plurality having a direction of sensitivity wherein at least two of the accelerometers in the plurality have different directions of sensitivity;

a rotational device configured to rotate the plurality of collocated accelerometers about a first axis; and

a processing system in communication with the plurality of collocated accelerometers, wherein the processing system is configured to;

obtain a first set of calibration measurements from the plurality of accelerometers from rotation of the plurality about the first axis;

determine a first systematic error for each accelerometer in the plurality using the first set of calibration measurements; and

remove the first systematic error from sensor measurements to correct for the systematic error.

15. The sensor of claim 14, wherein the first axis is substantially a longitudinal axis of a logging instrument comprising the sensor.

16. The sensor of claim 14, wherein the rotational device is further configured to rotate the plurality of accelerometers about a second axis.

17. The sensor of claim 16, wherein the processing system is further configured to:

obtain a second set of calibration measurements from the plurality of accelerometers from rotation of the plurality about the second axis;

determine a second systematic error for each accelerometer in the plurality using the second set of calibration measurements; and

remove the second systematic error from sensor measurements.

18. The sensor of claim 17, wherein the second axis is substantially orthogonal to the first axis.

19. The sensor of claim 14, wherein the rotational device comprises a gimbal.

20. An apparatus for measuring gravitational acceleration in a borehole and correcting for systematic error, the apparatus comprising:

a logging instrument;

a plurality of collocated accelerometers disposed at the logging instrument, the plurality providing output used for measuring the gravitational acceleration, each accelerometer in the plurality having a direction of sensitivity wherein at least two of the accelerometers in the plurality have different directions of sensitivity;

a rotational device configured to rotate the plurality of collocated accelerometers about a first axis; and

a processing system in communication with the plurality of collocated accelerometers, wherein the processing system is configured to;

obtain a set of calibration measurements from the plurality of accelerometers from rotation of the plurality about the first axis;

determine a systematic error for each accelerometer in the plurality using the set of calibration measurements; and

remove the systematic error from sensor measurements to correct for the systematic error.