Dual antenna for circular polarization

Patent metadata

Field	Value
Publication number	US-2016266272-A1
Application number	US-201315029534-A
Country	US
Kind code	A1
Filing date	Nov 13, 2013
Priority date	Nov 13, 2013
Publication date	Sep 15, 2016
Grant date	—

Abstract

In an example embodiment, a nuclear magnetic resonance (NMR) logging tool comprises an arrangement of one or more magnets that generates a static magnetic field (B 0 ) in a material body. A pair of first and second antenna coils is attached to the tool around an antenna core, wherein the turns of the first and second antenna coils are unaligned with either a longitudinal axis of the tool, or an orthogonal axis substantially transverse thereto. Circuitry drives the pair of first and second antenna coils in concert to produce a radio frequency field (B 1 ) in the material body. In one embodiment, the turns in the first antenna coil are separated from corresponding turns in the second oppositely wound antenna coil by an angle (α). In an example embodiment, the angle (α) is selected to minimize a power draw of the tool, or maximize one or more of SNR/echo, (I), SNR/time, or (II) of the tool, for a predetermined configuration of the magnet arrangement and antenna core of the tool.

First claim

What is claimed is:

1 . A nuclear magnetic resonance (NMR) logging tool comprising:
an arrangement of one or more magnets to generate a static magnetic field (B 0 ) in a material body; a pair of first and second antenna coils attached to the tool around an antenna core, wherein turns of the first and second antenna coils are unaligned with either a longitudinal axis of the tool, or an orthogonal axis substantially transverse thereto; and circuitry that drives the pair of first and second antenna coils in concert to produce a radio frequency field (B 1 ) in the material body.

2 . The tool of claim 1 , wherein the first and second antenna coils of the tool have a common longitudinal axis and are wound around a common antenna core.

3 . The tool of claim 2 , wherein the first antenna coil includes a spirally wound coil configuration around the antenna core, and wherein the second antenna coil includes an oppositely wound spiral configuration around the common antenna core.

4 . The tool of claim 3 , wherein the turns in the first antenna coil are separated from corresponding turns in the second oppositely wound antenna coil by an angle (α).

5 . The tool of claim 4 , wherein the angle (α) is in the range of 20 to 160 degrees or 70 to 110 degrees.

6 . The tool of claim 4 , wherein the angle (α) exceeds 90 degrees.

7 . The tool of claim 4 , wherein the angle (α) is selected to minimize a power draw of the logging tool, or maximize one or more of SNR/echo, SNR*√{square root over (Gradient)}, SNR/time, or SNR/√{square root over (power)} of the tool, for a predetermined configuration of a magnet arrangement and common antenna core of the tool.

8 . The tool of claim 1 , wherein the first and second antenna coils are configured to provide substantially orthogonal radio frequency fields in the material body.

9 . The tool of claim 1 , wherein the first antenna coil is energized by radio frequency power having a 90 degree phase difference from radio frequency power energizing the second antenna coil, whereby a circularly polarized RF magnetic field is generated by the tool.

10 . A nuclear magnetic resonance (NMR) logging method comprising:
lowering an NMR tool down a borehole to produce a static magnetic field (B 0 ) in a material body, the tool comprising a pair of first and second antenna coils attached to the tool around an antenna core, turns of the first and second antenna coils being unaligned with either a longitudinal axis of the tool or an orthogonal axis substantially transverse thereto, the tool further comprising circuitry that drives the pair of first and second antenna coils in concert to produce a radio frequency field (B 1 ) in the material body; concurrently driving the first and second antenna coils with the circuitry to produce the radio frequency field (B 1 ) and polarize nuclei in the material body; and receiving nuclear magnetic resonance signals from the polarized nuclei.

11 . The method of claim 10 , wherein the first and second antenna coils of the tool have a common longitudinal axis and are wound around a common antenna core.

12 . The method of claim 11 , wherein the first antenna coil includes a spirally wound coil configuration around the antenna core, and wherein the second antenna coil includes an oppositely wound spiral configuration around the common antenna core.

13 . The method of claim 12 , wherein the turns in the first antenna coil are separated from corresponding turns in the second oppositely wound antenna coil by an angle (α).

14 . The method of claim 13 , wherein the angle (α) is in the range of 20 to 160 degrees or 70 to 110 degrees.

15 . The method of claim 13 , wherein the angle (α) exceeds 90 degrees.

16 . The method of claim 13 , wherein the angle (α) is selected to minimize a power draw of a tool, or maximize one or more of SNR/echo, SNR*√{square root over (Gradient)}, SNR/time, or SNR/√{square root over (power)} of the tool, for a predetermined configuration of a magnet arrangement and common antenna core of the tool.

17 . The method of claim 10 , wherein the first and second antenna coils are configured to provide substantially orthogonal radio frequency fields in the material body.

18 . The method of claim 17 , wherein the first antenna coil is energized by radio frequency power having a 90 degree phase difference from a radio frequency power energizing the second antenna coil, whereby a circularly polarized RF magnetic field is generated by the tool.

19 . The method of claim 10 , further comprising decoupling the first antenna coil from the second antenna coil.

20 . A method of making a nuclear magnetic resonance (NMR) logging tool, the method comprising:
providing, for the tool, an arrangement of one or more magnets for generating a static magnetic field (B 0 ) in a material body; providing, for the tool, a pair of first and second oppositely-wound antenna coils attached to the tool around an antenna core, wherein turns of the first and second antenna coils are unaligned with either a longitudinal axis of the tool or an orthogonal axis substantially transverse thereto, and wherein the turns in the first antenna coil are separated from corresponding turns in the second oppositely wound antenna coil by an angle (α); selecting the angle (α) to minimize a power draw of the tool, or maximize one or more of SNR/echo, SNR*√{square root over (Gradient)}, SNR/time, or SNR/√{square root over (power)} of the tool, for a predetermined configuration of the magnet arrangement and common antenna core of the tool; and providing, for the tool, circuitry to drive the pair of first and second antenna coils in concert to produce a radio frequency field (B 1 ) in the material body.

Assignees

• Halliburton Energy Services Inc

Inventors

• Jachmann Rebecca Corina
• Yang Jie

Classifications

• G01V3/32 Primary

  operating with electron or nuclear magnetic resonance · CPC title

• G01R33/38

  Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field · CPC title

• G01R33/36

  Electrical details, e.g. matching or coupling of the coil to the receiver · CPC title

• G01R33/3678

  involving quadrature drive or detection, e.g. a circularly polarized RF magnetic field · CPC title

• G01R33/34

  Constructional details, e.g. resonators {, specially adapted to MR} · CPC title