Electromagnetic survey systems and methods with rotation-corrected motion compensation

• US 20110279120A1
• Filed: 05/12/2010
• Published: 11/17/2011
• Est. Priority Date: 05/12/2010
• Status: Active Grant

• Current Assignee: PGS Geophysical As (PGS ASA)
• Original Assignee: PGS Geophysical As (PGS ASA)
• Inventors: Skogman, Carl Joel Gustav, Südow, Gustav Göran Mattias, Lindqvist, Ulf Peter, Juhasz, Andras Robert
• Granted Patent: US 8,754,649 B2
• Time in Patent Office: Days
• US Class Current: 324/334
• CPC Class Codes: G01R 33/025 Compensating stray fields G01R33/0017 takes precedence
• CPC Class Codes: G01V 1/3835 measuring position, e.g. by GPS or acoustically

Abstract

A disclosed electromagnetic survey system includes one or more streamer(s) having multiple electromagnetic sensors and motion sensing units. Each motion sensing unit has one or more accelerometer(s) to measure motion perpendicular to an axis of the streamer, and a rotation sensor to measure rotation about the axis. The measurements of the accelerometer are adjusted based on measurements from the rotation sensor. The survey system also includes one or more processor(s) that determine, for each electromagnetic sensor, a motion signal based on the adjusted measurements. A described electromagnetic survey method includes processing acceleration and rotational motion measurements to obtain an orientation of motion sensing units as a function of time. The measured acceleration is manipulated based on the orientation to obtain one or more velocity signal(s) for each motion sensing unit. Interpolation is performed on the velocity signals to determine at least one velocity signal for each electromagnetic sensor.

Claims

1. An electromagnetic survey system comprising:

at least one streamer that includes;

a plurality of electromagnetic sensor pods, each comprising at least one electromagnetic sensor; and

a plurality of motion sensing units each having;

at least one accelerometer to measure motion perpendicular to an axis of said streamer; and

a rotation sensor to measure rotation about said axis;

wherein measurements of said accelerometer are adjusted based at least in part on measurements from said rotation sensor; and

at least one processor that determines for each electromagnetic sensor a motion signal based at least in part on said adjusted measurements.

2. The system of claim 1, wherein the motion signal represents velocity of that electromagnetic sensor perpendicular to said axis.

3. The system of claim 1, wherein each electromagnetic sensor pod includes one of said motion sensing units.

4. The system of claim 1, wherein said at least one processor determines at least some of the motion signals by interpolating between motion sensing units.

5. The system of claim 1, further comprising an information storage medium that stores measurements of said electromagnetic sensors and information sufficient to determine said motion signals.

6. The system of claim 1, wherein the at least one processor further determines an error signal for each electromagnetic sensor based at least in part on said motion signal and uses that error signal to correct the measurements of that electromagnetic sensor.

7. The system of claim 6, further comprising an information storage medium that stores measurements of said electromagnetic sensors that have been corrected for motion-induced voltages.

8. The system of claim 1, wherein the rotation sensor comprises a rotating coil magnetometer.

9. The system of claim 8, wherein the rotating coil magnetometer has multiple windings to provide multiple signals that are at different phase offsets from one another.

10. The system of claim 8, wherein the rotating coil magnetometer further provides a magnetic field strength measurement that is used to determine motion-induced voltages for each electromagnetic sensor.

11. The system of claim 1, wherein the rotation sensor comprises a sensor from the group consisting of:

an inclinometer, a gyroscope, and a magnetic compass.

12. The system of claim 1, wherein each motion sensing unit determines a base orientation from measurements of said accelerometer, and further determines a current orientation by combining the base orientation with a processed angular velocity signal from said rotation sensor.

13. The system of claim 12, wherein each motion sensing unit rotates the accelerometer measurements to an external reference frame based on the current orientation, and wherein the motion sensing unit further corrects the rotated measurements for gravity.

14. An electromagnetic survey method comprising:

measuring acceleration at multiple positions along a towed marine streamer, said streamer having multiple electromagnetic sensors;

using a rotating coil magnetometer to determine rotational motion of each motion sensing unit used to measure said acceleration;

processing acceleration and rotational motion measurements to obtain an orientation of each motion sensing unit as a function of time;

manipulating the measured acceleration based on the orientation to obtain at least one velocity signal for each motion sensing unit;

performing interpolation of the velocity signals from the motion sensing units to determine at least one velocity signal for each electromagnetic sensor;

correcting measurements of the electromagnetic sensors based at least in part on the respective at least one velocity signal; and

recording corrected electromagnetic sensor measurements on an information storage medium.

15. The method of claim 14, wherein said correcting measurements is further based on a magnetic field strength measured by the rotating coil magnetometer.

16. The method of claim 15, wherein said correcting measurements includes determining a voltage signal induced in each of the electromagnetic sensors by their motion through a magnetic field.

17. The method of claim 14, wherein said manipulating includes:

rotating multi-axial accelerometer measurements from a coordinate system of the motion sensing unit to an external coordinate system, and

subtracting gravity from a vertical component of the rotated measurements to obtain estimated acceleration components.

18. The method of claim 14, wherein said manipulating further includes processing the estimated acceleration components to obtain velocity component signals.

19. A motion sensing system for a towed marine streamer, the system comprising:

a tri-axial accelerometer positioned on a streamer axis, with one component of the accelerometer aligned along said axis;

a magnetometer having at least one coil that rotates about an axis of symmetry contained in a plane that includes said coil, said axis being aligned along the streamer axis; and

a signal processor that demodulates a signal from the magnetometer to determine a magnetic field strength and an angular velocity signal representing rotational motion of the motion sensing unit about the streamer axis,

wherein the signal processor combines accelerometer measurements with the magnetometer measurements to produce output signals representing components of the unit'"'"'s motion in an external frame of reference.

20. The unit of claim 19, wherein the external frame of reference is defined by a three-axis coordinate system having one axis aligned with the streamer axis, one axis aligned with gravity, and the third axis perpendicular to the first two.

21. The unit of claim 19, wherein the output signals represent acceleration components.

22. The unit of claim 19, wherein the output signals represent velocity components.