Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1994-10-12
1996-01-30
Phillips, Michael W.
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
335301, 335304, 324319, 324320, 324303, G01V 300, G01R 3320, H01F 702
Patent
active
054883423
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a magnet assembly for generating a magnetic field of sufficient homogeneity to perform a NMR process in a working region.
2. Description of the Related Art
Magnet assemblies for generating homogeneous fields for the purposes of NMR are well known in a variety of fields but the present invention is particularly concerned, although not exclusively, with well logging applications. In these applications, the working region must be positioned outside the magnet assembly and in the surrounding rock in order to obtain NMR information from the rock. An early system of this type is described in U.S. Pat. No. 4,350,955.
In this arrangement, a pair of axially opposed magnets produce a radial field which has a maximum value and a region of uniformity over an annular volume whose radius is determined by the separation of the magnets. A variation on this scheme is described in WO-A-92/07279 whereby the volume of the homogeneous region is increased to improve the sensitivity of the NMR measurement.
These and similar systems are cylindrically symmetric. In some circumstances it may be desirable to deploy a well-logging instrument which takes the form of a "pad tool". These circumstances are, for example, when the size of the hole is much greater than the diameter of the tool, or when the hole is grossly irregular in section, and it is desired to exclude the response of the drilling fluid. Such a pad tool has been described in U.S. Pat. No. 4,933,638. The type of magnet system described in this has the drawback that the "throw" (i.e. the distance from the apparatus to the sensitive region) and the dimensions of the sensitive region have to be small compared to the dimensions of the magnet system in the transverse plane. This restricts the usefulness of this system.
SUMMARY OF THE INVENTION
In accordance with the present invention, a magnet assembly comprises a pair of substantially aligned, spaced apart first magnets having like poles facing each other; and a second magnet positioned adjacent the space between the first magnets with its magnetic axis transverse to the alignment axis of the first magnets whereby a magnetic field of sufficient homogeneity to perform a nuclear magnetic resonance (NMR) process is generated in a working region laterally offset from the first and second magnets.
We have devised a new magnet assembly which can generate a working region having a homogeneous magnetic field which is asymmetric relative to the magnet assembly and offset from the magnet assembly, the degree of offset being greater than has previously been possible while the lateral dimension of the working region is also greater than has previously been achieved.
Preferably, the second magnet is laterally offset from the alignment axis of the first magnets.
The second magnet could comprise a single magnet or a number of suitably positioned subsidiary magnets.
Typically, particularly for well-logging applications, the magnets comprise permanent magnets although electrical coils operating in a resistive or superconducting mode could also be used.
In one example, the boundary of the working region nearest to the magnet assembly is spaced by more than substantially 10 cm from the alignment axis of the first magnets. Preferably, also, the working region has a lateral dimension of at least 10 cm.
The invention also provides a NMR probe comprising a magnet assembly according to the invention; and a RF magnetic field generator for generating a substantially homogeneous RF magnetic field within the working region.
The RF magnetic field generator may be based on a conventional RF coil of the type described in U.S. Pat. No. 4,933,638 or as described in more detail below.
As has already been mentioned, the invention has particular application in the field of well logging. However, there are many other applications, including food monitoring in which the assembly is inserted into a storage silo or the like, or where, for example, the assembly is positioned to projec
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Barrera Raymond M.
Oxford Instruments Limited
Phillips Michael W.
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