System and method using magnetic anomaly field magnitudes for detection, localization, classification and tracking of magnetic objects

• US 7,932,718 B1
• Filed: 03/12/2009
• Issued: 04/26/2011
• Est. Priority Date: 03/12/2009
• Status: Expired due to Fees

• Current Assignee: Navy The USA As Represented By The Secretary of The
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Wiegert, Roy
• Primary Examiner(s): LeDynh; Bot L
• Application Number:
  US12/383,083
• Time in Patent Office: 775 Days
• Field of Search: 324/207.11, 324244-245, 324/247, 324/260, 324/326, 333/55.R
• US Class Current: 324/247
• CPC Class Codes: G01R 33/022 Measuring gradient
• CPC Class Codes: G01V 3/081 the magnetic field is produced by the objects or geological structures characterised by the method of magnetic field measurement G01R33/00

Abstract

A magnetic anomaly sensing system and method uses at least four triaxial magnetometer (TM) sensors with each of the TM sensors having X,Y,Z magnetic sensing axes. The TM sensors are arranged in a three-dimensional array with respective ones of the X,Y,Z magnetic sensing axes being mutually parallel to one another. The three-dimensional array defines a geometry that forms at least one single-axis gradiometer along each of the X,Y,Z magnetic sensing axes. Information sensed by the TM sensors is to generate scalar magnitudes of a magnetic anomaly field measured at each of the TM sensors, comparisons of the scalar magnitudes to at least one threshold value, distance to a source of the magnetic anomaly field using the scalar magnitudes when the threshold value(s) is exceeded, and a magnetic dipole moment of the source using the distance.

Claims

1. A magnetic anomaly sensing system, comprising:

at least four triaxial magnetometer (TM) sensors with each of said TM sensors having X,Y,Z magnetic sensing axes, said TM sensors arranged in a three-dimensional array with respective ones of said X,Y,Z magnetic sensing axes being mutually parallel to one another in said three-dimensional array;

said three-dimensional array defining a geometry that forms at least one single-axis gradiometer along each of said X,Y,Z magnetic sensing axes; and

processing means coupled to each of said TM sensors for generating

scalar magnitudes of a magnetic anomaly field measured at each of said TM sensors,

comparisons of said scalar magnitudes to at least one threshold value,

distance to a source of said magnetic anomaly field using said scalar magnitudes when said at least one threshold value is exceeded, and

a magnetic dipole moment of said source using said distance.

2. A magnetic anomaly sensing system as in claim 1 wherein said TM sensors comprise four TM sensors arranged at vertices of a right-angle tetrahedron.

3. A magnetic anomaly sensing system as in claim 2 wherein said tetrahedron is an isosceles tetrahedron.

4. A magnetic anomaly sensing system as in claim 1 further comprising at least one output device coupled to said processing means for processing data indicative of said distance and said magnetic dipole moment.

5. A magnetic anomaly sensing system as in claim 4 wherein said output device processes said data for presentation in a human-discernable format.

6. A magnetic anomaly sensing system as in claim 4 wherein said output device processes said data for presentation in a computer-discernable format.

7. A magnetic anomaly sensing system as in claim 1, said processing means further

generating a total gradient contraction for said three-dimensional array, and

comparing said total gradient contraction to a contraction threshold value, wherein said distance and said magnetic dipole moment are generated only when said contraction threshold is exceeded.

8. A magnetic anomaly sensing system as in claim 2 wherein said processing means generates said distance and said magnetic dipole moment by

processing, in accordance with a magnetic scalar triangulation and ranging (STAR) processing scheme, said scalar magnitudes and baseline distances defined by each of said single-axis gradiometers to determine a directional derivative associated with said scalar magnitudes,

calculating a unit vector using said directional derivative,

determining a projection distance from each of said vertices to said source wherein each said projection distance is a function of said unit vector and one of said baseline distances, and

processing each said projection distance in accordance with said STAR processing scheme to determine said position and said magnetic dipole moment.

9. A magnetic anomaly sensing system as in claim 2 wherein said processing means generates said distance and said magnetic dipole moment by

processing said scalar magnitudes to determine a least squares fit to the relationship

(μ

/4π

)kMr_I<sup>−

3</sup>

where

μ

is the magnetic permeability of an environment surrounding said source,

k is an asphericity constant characterizing variance of said scalar magnitudes from true spherical symmetry,

M is said magnetic dipole moment of said source, and

r_I is said projection distance from an I-th one of said vertices to said source,

wherein least squares fit values of said projection distances r_I along a baseline defined by each said single-axis gradiometer are defined, and

processing said least squares fit values in accordance with a magnetic scalar triangulation and ranging (STAR) processing scheme to determine said position and said magnetic dipole moment.

10. A magnetic anomaly sensing system as in claim 1 wherein, while generating said scalar magnitudes, said processing means applies a compensation routine to zero-out effects of the Earth'"'"'s magnetic field on said scalar magnitudes.

11. A magnetic anomaly sensing system as in claim 1 wherein, while generating said scalar magnitudes, said processing means applies a compensation routine to zero-out effects of variances in magnetic anomaly field-sensing capabilities of said TM sensors.

12. A magnetic anomaly sensing system as in claim 1, wherein said processing means further applies an asphericity compensation routine to said distance and said magnetic dipole moment, said asphericity compensation routine iteratively adjusting said position and said magnetic dipole moment using said scalar magnitudes until a position so-adjusted and a magnetic dipole moment so-adjusted are defined that form a best fit to a magnetic field dipole approximation relationship.

13. A method of detecting, localizing and classifying magnetic objects, comprising the steps of

providing at least four triaxial magnetometer (TM) sensors with each of said TM sensors having X,Y,Z magnetic sensing axes, said TM sensors arranged in a three-dimensional array with respective ones of said X,Y,Z magnetic sensing axes being mutually parallel to one another in said three-dimensional array, said three-dimensional array defining a geometry that forms at least one single-axis gradiometer along each of said X,Y,Z magnetic sensing axes;

generating scalar magnitudes of a magnetic anomaly field measured at each of said TM sensors;

comparing said scalar magnitudes to at least one threshold value;

determining distance to a source of said magnetic anomaly field using said scalar magnitudes when said at least one threshold value is exceeded; and

determining a magnetic dipole moment of said source using said distance.

14. A method according to claim 13, wherein said TM sensors comprise four TM sensors arranged at vertices of a right-angle tetrahedron.

15. A method according to claim 14, wherein said tetrahedron is an isosceles tetrahedron.

16. A method according to claim 13, further comprising the step of processing data indicative of said distance and said magnetic dipole moment for presentation in a human-discernable format.

17. A method according to claim 13, further comprising the step of processing data indicative of said distance and said magnetic dipole moment for presentation in a computer-discernable format.

18. A method according to claim 13, further comprising the steps of:

generating a total gradient contraction for said three-dimensional array; and

comparing said total gradient contraction to a contraction threshold value, wherein said distance and said magnetic dipole moment are generated only when said contraction threshold is exceeded.

19. A method according to claim 14, wherein said steps of determining said distance and said magnetic dipole moment comprise the steps of:

processing, in accordance with a magnetic scalar triangulation and ranging (STAR) processing scheme, said scalar magnitudes and baseline distances defined by each of said single-axis gradiometers to determine a directional derivative associated with said scalar magnitudes;

calculating a unit vector using said directional derivative;

determining a projection distance from each of said vertices to said source wherein each said projection distance is a function of said unit vector and one of said baseline distances; and

processing each said projection distance in accordance with said STAR processing scheme to determine said position and said magnetic dipole moment.

20. A method according to claim 14, wherein said steps of determining said distance and said magnetic dipole moment comprise the steps of:

processing said scalar magnitudes to determine a least squares fit to the relationship

(μ

/4π

)kMr_I<sup>−

3</sup>

where

μ

is the magnetic permeability of an environment surrounding said source,

k is an asphericity constant characterizing variance of said scalar magnitudes from true spherical symmetry,

M is said magnetic dipole moment of said source, and

r_I is said projection distance from an I-th one of said vertices to said source,

wherein least squares fit values of said projection distances r_I along baseline defined by each said single-axis gradiometer are defined; and

processing said least squares fit values in accordance with a magnetic scalar triangulation and ranging (STAR) processing scheme to determine said position and said magnetic dipole moment.

21. A method according to claim 13, wherein said step of generating said scalar magnitudes includes the step of applying a compensation routine to zero-out effects of the Earth'"'"'s magnetic field on said scalar magnitudes.

22. A method according to claim 13, wherein said step of generating said scalar magnitudes includes the step of applying a compensation routine to zero-out effects of variances in magnetic anomaly field-sensing capabilities of said TM sensors.

23. A method according to claim 13, further comprising the step of applying an asphericity compensation routine to said distance and said magnetic dipole moment, said asphericity compensation routine iteratively adjusting said position and said magnetic dipole moment using said scalar magnitudes until a position so-adjusted and a magnetic dipole moment so-adjusted are defined that form a best fit to a magnetic field dipole approximation relationship.