Method and apparatus of using soft non-ferritic magnetic material in a nuclear magnetic resonance probe

• US 6,452,388 B1
• Filed: 06/28/2000
• Issued: 09/17/2002
• Est. Priority Date: 06/28/2000
• Status: Expired due to Term

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: Beard, David R., Reiderman, Arcady
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Fetzner, Tiffany A.
• Application Number:
  US09/605,463
• Time in Patent Office: 811 Days
• Field of Search: 324/318, 324/319, 324/321, 324/322, 324/303
• US Class Current: 324/303
• CPC Class Codes: G01N 24/081 Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity
• CPC Class Codes: G01R 33/34 Constructional details, e.g. resonators , specially adapted to MR
• CPC Class Codes: G01V 3/32 operating with electron or nuclear magnetic resonance

Abstract

The present invention provides a novel use of a powdered high saturation flux density soft magnetic material as a NMR probe core material. The probe structural geometry facilitates the use of powdered material, which has a relatively low magnetic permeability.

Claims

1. A nuclear magnetic resonance sensing apparatus, comprising:

a magnet for inducing a static magnetic field in materials to be analyzed; and

an antenna assembly for inducing a radio frequency magnetic field within said materials and for detecting nuclear magnetic resonance signals from said materials, the antenna assembly comprising a coil and at least one magnetic core formed from a non-ferritic powdered soft magnetic material having high saturation flux density and a non-conductive bonding agent, said magnetic core having a magnetic permeability μ

_m less than 500 and wherein said saturation flux density is greater than about 0.4 T.

2. The apparatus of claim 1, wherein, the magnetic core further comprising dimensions which are related to the direction of the RF magnetic field and to the magnetic permeability of the powdered soft magnetic material.

3. The apparatus of claim 1 wherein the powdered soft magnetic material is conductive and has a maximum grain size to substantially prevent intragranular power loss of said radio frequency magnetic field.

4. The apparatus of claim 1 wherein an effective demagnetizing factor of the magnetic core in a direction of the radio frequency magnetic field substantially exceeds the inverse magnetic permeability of the powdered soft magnetic material.

5. The apparatus of claim 4, wherein the core has an effective permeability, μ

, less than 5, as defined by a first equation,

6. The apparatus as defined in claim 1 wherein the magnet is made of a conductive permanent magnet material.

7. The apparatus as defined in claim 1 wherein the powdered soft magnetic material possesses a maximum magnetic permeability given a predetermined maximum RF antenna power loss.

8. The apparatus as defined in claim 1 wherein the magnet and the antenna possess an elongation direction, the radio frequency magnetic field and the static magnetic field being perpendicular to the elongation direction.

9. The apparatus of claim 1 wherein said flux density is greater than that of a magnetic consisting primarily of ferrite.

10. The apparatus of claim 1 wherein the magnetic core further comprises relative dimensions that are related to the direction of the RF magnetic field and to the magnetic permeability of the powdered soft magnetic material.

11. A probe, moveable through a bore hole for measuring nuclear magnetic resonance (NMR) properties in a volume of investigation in a formation surrounding a bore hole, comprising;

an elongated permanent magnet having an axis of elongation and a magnetic field perpendicular to the axis of elongation, the axis of elongation collinear with the bore hole; and

an elongated antenna assembly having an axis of elongation generally parallel to the axis of the magnet, the permanent magnet and the antenna assembly disposed adjacent one another, the antenna assembly comprising at least one core comprising a non-ferritic powdered soft magnetic material having a magnetic permeability μ

_m less than 500, said core having a saturation flux density greater than about 0.4 T, the antenna assembly generating a RF magnetic field substantially perpendicular to the magnetic field of the permanent magnet in the volume of investigation.

12. The probe of claim 11 wherein the permanent magnet has a magnetic dipole moment in a plane perpendicular to the axes of elongation, the antenna assembly comprising an elongated core placed in an antenna coil, the antenna coil having a coil dipole moment in a plane perpendicular to the axes of elongation, the coil dipole moment perpendicular to a line between an effective center of the magnet dipole moment and an effective center of the coil dipole moment.

13. The probe of claim 11, wherein the permanent magnet has a magnetic dipole moment in a plane perpendicular to the axes of elongation, the antenna assembly comprising two or more elongated cores placed in an antenna coil, the antenna coil having a coil dipole moment in a plane perpendicular to the axes of elongation, the coil dipole moment parallel to a line between an effective center of the magnet dipole moment and an effective center of the coil dipole moment.

14. The probe of claim 11, wherein the permanent magnetic has a magnetic dipole moment in a plane perpendicular to the axes of elongation, the antenna assembly comprising two or more elongated cores placed in an antenna coil, the antenna coil having a coil dipole moment in a plane perpendicular to the axes of elongation, the coil dipole moment and the magnet dipole moment having coincident effective centers.

15. A method of making measurements of a parameter of interest of materials to be analyzed comprising:

using a magnet for inducing a static magnetic field in said materials; and

using an antenna assembly for inducing a radio frequency magnetic field within said materials and for detecting nuclear magnetic resonance signals from said materials, the antenna assembly comprising a coil and at least one magnetic core formed from a non-ferritic powdered soft magnetic material having high saturation flux density and a non-conductive bonding agent, said magnetic core having a magnetic permeability μ

_m less than 500 and a saturation flux density greater than about 0.4 T.

16. The method of claim 15 further comprising selecting dimensions for the magnetic core which are related to the direction of the direction of the RF magnetic field and to the magnetic permeability of the powdered soft magnetic material.

17. The method of claim 15 further comprising selecting relative dimensions for the magnetic core which are related to the direction of the direction of the RF magnetic field and to the magnetic permeability of the powdered soft magnetic material.

18. The method of claim 15 wherein the powdered soft magnetic material is conductive, the method further comprising selecting a maximum grain size for the soft magnetic material to substantially prevent intragranular power loss of said radio frequency magnetic field.

19. The method of claim 15 wherein an effective demagnetizing factor of the magnetic core in the direction of the direction of the radio frequency magnetic field substantially exceeds the inverse magnetic permeability of the powdered soft magnetic material.

20. The method of claim 19, wherein the core has an effective permeability, μ

, less than 5, as defined by a first equation,

21. The method of claim 15, wherein the magnet is made of a conductive permanent magnet material.

22. The method of claim 15, wherein the powdered soft magnetic material possesses a maximum magnetic permeability given a predetermined maximum RF antenna power loss.

23. The method of claim 15, wherein the magnet and the antenna possess an elongation direction, the radio frequency magnetic field and the static magnetic field being perpendicular to the elongation direction.

24. A method of using a probe moveable through a bore hole for measuring nuclear magnetic resonance (NMR) properties of a volume of investigation in a formation surrounding the bore hole comprising:

using an elongated permanent magnet having an axis of elongation for providing a static magnetic field in the volume of investigation having a direction perpendicular to the axis of elongation, the axis of elongation collinear with the bore hole; and

using an elongated antenna assembly on the probe for providing a radio frequency magnetic field in the volume of investigation having a direction substantially orthogonal to the static magnetic field, the antenna assembly having an axis of elongation generally parallel to the axis of the magnet, the permanent magnet and the antenna assembly disposed adjacent one another, the antenna assembly comprising at least one core comprising a non-ferritic powdered soft magnetic material said core having a magnetic permeability μ

_m less than 500 and a saturation flux density greater than about 0.4 T.

25. The method of claim 24, the permanent magnet having a magnetic dipole moment in a plane perpendicular to the axes of elongation, the antenna assembly comprising an elongated core placed in an antenna coil, the antenna coil having a coil dipole moment in a plane perpendicular to the axes of elongation, the coil dipole moment perpendicular to a line between an effective center of the magnet dipole moment and an effective center of the coil dipole moment.

26. The method of claim 24, wherein the permanent magnet has a magnetic dipole moment in a plane perpendicular to the axes of elongation, the antenna assembly comprising two or more elongated cores placed in an antenna coil, the antenna coil having a coil dipole moment in a plane perpendicular to the axes of elongation, the coil dipole moment parallel to a line between an effective center of the magnet dipole moment and an effective center of the coil dipole moment.

27. The method of claim 24, wherein the permanent magnetic has a magnetic dipole moment in a plane perpendicular to the axes of elongation, the antenna assembly comprising two or more elongated cores placed in an antenna coil, the antenna coil having a coil dipole moment in a plane perpendicular to the axes of elongation, the coil dipole moment and the magnet dipole moment having coincident effective centers.