Front tangential antenna for nuclear magnetic resonance (NMR) well logging
The magnet and antenna assemblies of a logging tool can be arranged according to geometries optimal for concentric or eccentric well logging operations. In an example configuration, a logging tool can include a magnet, a magnetic core made of a magnetically permeable material, and an antenna. The outward path of the antenna can be positioned along a surface of the magnetically permeable magnetic core that faces away from the magnet, and the return path of the antenna can also be positioned along a surface that faces away from the magnet.
1. A nuclear magnetic resonance system for well logging using an axially-extending tool, the system comprising:
a magnet;
a core having a first surface facing the magnet and one or more other surfaces facing away from the magnet; and
an antenna positioned along at least one of the one or more other surfaces, the antenna comprising a first loop of electrically conductive material and a second loop of electrically conductive material, the first and second loops comprising a common segment extending along an axis of the axially-extending tool.
2. The system of claim 1 , wherein the common segment is positioned along a surface of the core opposite the magnet.
3. The system of claim 2 , wherein the core is between the magnet and the antenna.
4. The system of claim 1 , wherein the common segment is positioned along a surface of the core that extends along the axis of the axially-extending tool.
5. The system of claim 4 , wherein tool further comprises an additional magnet;
wherein the core is between the magnet and the additional magnet.
6. The system of claim 1 , wherein the first loop and the second loop are between the core and a point beyond the radial periphery of the tool.
7. The system of claim 6 , comprising an electronic controller in communication with the magnet and the antenna assembly, the electronic controller being programmed to cause the magnet to induce a first magnetic field at the point and to cause the antenna assembly to induce a radio frequency magnetic field at the point, wherein the radio frequency magnetic field is orthogonal to the first magnetic field at the point.
8. The system of claim 1 , wherein the first loop is symmetric to the second loop.
9. The system of claim 1 , wherein
the first loop further comprises a first segment;
the second loop further comprises a second segment;
the common segment extends from a first trace; and
the first segment and the second segment converge into a second trace.
10. The system of claim 9 , wherein the common segment diverges to form the first segment and the second segment.
11. The system of claim 9 , wherein the common segment, the first segment, and the second segment extend axially from a notional arc, and the common segment extends axially from a center of the arc.
12. The system of claim 11 , wherein the first segment and the second segment extend axially from positions on the arc a distance from the center of the arc.
13. The system of claim 12 , wherein an angular distance between the common segment and the first segment is in a range from about 80 degrees to about 85 degrees relative to a focus of the arc.
14. The system of claim 1 , wherein the one or more other surfaces comprise a planar surface.
15. The system of claim 1 , wherein the one or more other surfaces comprise a curved surface.
16. The system of claim 1 , wherein the core is composed of a material having a relative magnetic permeability in a range from about 9 to 20.
17. The system of claim 1 , wherein the common segment comprises a plurality of conductors that extend in a common direction.
18. The system of claim 7 , wherein the first magnetic field is a static magnetic field.
19. The system of claim 1 , wherein the common segment is disposed near a surface of the core opposite the magnet.
20. The system of claim 1 , wherein the first loop and the second loop are disposed between the core and a volume beyond the radial periphery of the tool.
21. The system of claim 20 , comprising an electronic controller in communication with the magnet and the antenna assembly, the electronic controller being programmed to cause the magnet to induce a first magnetic field at the volume and to cause the antenna assembly to induce a radio frequency magnetic field at the volume, wherein the radio frequency magnetic field is orthogonal to the first magnetic field at the volume.
22. A method comprising:
obtaining an axially extending magnetic resonance well logging tool comprising:
a magnet; and
a core having a first surface facing the magnet and one or more other surfaces facing away from the magnet;
positioning an antenna along at least one of the one or more other surfaces, wherein the antenna comprises a first loop of electrically conductive material and a second loop of electrically conductive material, and the first and second loops comprise a common segment extending along an axis of the axially-extending tool.
23. The method of claim 22 , wherein positioning the antenna comprises positioning the common segment along a surface of the core opposite the magnet.
24. The method of claim 23 , wherein positioning the antenna comprises positioning the antenna such that the core is between the magnet and the antenna.
25. The method of claim 22 , wherein positioning the antenna comprises positioning the common segment along a surface of the core that extends along the axis of the axially-extending tool.
26. The method of claim 22 , wherein positioning the antenna comprises positioning the first loop and the second loop between the core and a point beyond the radial periphery of the tool.
27. The method of claim 26 , further comprising:
using the magnet to induce a first magnetic field at the point; and
using the antenna assembly to induce a radio frequency magnetic field at the point;
wherein the radio frequency magnetic field is orthogonal to the first magnetic field at the point.
28. The method of claim 22 , wherein the first loop is symmetric to the second loop.
29. The method of claim 22 , wherein the first loop further comprises a first segment;
the second loop further comprises a second segment;
the common segment extends from a first trace; and
the first segment and the second segment converge into a second trace.
30. The method of claim 29 , wherein the common segment diverges to form the first segment and the second segment.
31. The method of claim 29 , wherein positioning the antenna comprises positioning the antenna such that the common segment, the first segment, and the second segment extend axially from a notional arc, and the common segment extends axially from a center of the arc.
32. The method of claim 31 , wherein positioning the antenna comprises positioning the antenna such that the first segment and the second segment extend axially from positions on the arc a distance from the center of the arc.
33. The method of claim 32 , wherein positioning the antenna comprises positioning the antenna such that an angular distance between the common segment and the first segment is in a range from about 80 degrees to about 85 degrees relative to a focus of the arc.
34. The method of claim 22 , wherein positioning the antenna comprises positioning the common segment near a surface of the core opposite the magnet.
35. The method of claim 22 , wherein positioning the antenna comprises positioning the first loop and the second loop between the core and a volume beyond the radial periphery of the tool.
36. The method of claim 35 , further comprising:
using the magnet to induce a first magnetic field at the volume; and
using the antenna assembly to induce a radio frequency magnetic field at the volume;
wherein the radio frequency magnetic field is orthogonal to the first magnetic field at the volume.