Electricity: measuring and testing – Particle precession resonance – Using well logging device
Patent
1995-06-30
1997-10-21
Arana, Louis M.
Electricity: measuring and testing
Particle precession resonance
Using well logging device
324318, G01V 300
Patent
active
056800444
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an oscillating magnetic field generating assembly for use with a magnet assembly which generates a magnetic field in a working region externally of the magnet assembly having a unformity suitable for performing an NMR experiment.
2. Related Art
Magnet assemblies have been developed in recent years for inspecting bore holes for oil bearing rock. A typical example of such an assembly is described in U.S. Pat. No. 4,350,955 in which a pair of permanent magnets are provided spaced apart with their north-south axes aligned and with like poles facing one another. This arrangement has the advantage that the oscillating magnetic field generating assembly which has to be provided can be in the form of a RF coil positioned in the space between the magnets. More recently, a modified form of this arrangement has been described in WO92/07279.
One of the problems with these arrangements which has been discovered in use that there is a significant variation in the magnetic field generated by the oscillating magnetic field generating assembly (conventionally known as the B.sub.1 -field) over the working region. This becomes particularly significant when the radial extent of the working region is increased. This has two effects on the NMR measurement.
The first effect is the non-uniform excitation of the spins. This means that in a typical spin-echo measurement 90.degree., 180.degree. etc pulses are not defined, and so optimum excitation cannot be obtained over the whole volume. This both reduces the signal strength and confuses the interpretation.
The second effect is that the signal which is received by the RF coil is weighted in favour of the nearer parts of the sensitive volume. This again confuses the interpretation of the data.
Both of these untoward effects can in principle be overcome by complex pulse sequences, but these are difficult to design and implement and may restrict the range of measurements which can be made.
In accordance with one aspect of the present invention, an oscillating magnetic field generating assembly for use with a magnet assembly which generates a magnetic field in a working region externally of the magnet assembly having a uniformity suitable for performing an NMR experiment, comprises a set of electrical coils connected to an oscillating electrical source, the coils being arranged such that the magnetic field generated by the oscillating magnetic field assembly is also sufficiently uniform within the working region to perform an NMR experiment.
The inventors have for the first time considered the oscillating magnetic field generating assembly in terms of the magnetic field which it generates and have realised that the problems set out above can be overcome by designing the assembly such that the magnetic field which it generates within the working region is sufficiently uniform to perform an NMR experiment.
The degree of uniformity required of the field generated by the oscillating magnetic field generating assembly is much less than that normally required of the magnet assembly. Typically, an oscillating magnetic field having uniformity of the order of 25% across the working region is acceptable whereas the uniformity required of the main magnetic field generated by the magnet assembly is usually parts per million, for example 100 parts per million or better.
Conveniently, the set of electrical coils is arranged such that at least the first order variation of magnetic field with distance is substantially zero. This is most easily arranged if the coils are substantially coaxial and, where a set of four electrical coils are provided, where the axially inner coils are each positioned about a mid-point between the axially outer coils at a distance from the mid-point substantially equal to the radius of the axially outer coils.
In some circumstances, a problem can arise with this geometry, particularly using spaced apart electrical coils, namely that the coils can detect noise signals. Preferably, therefore, the e
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Hanley Peter
McDougall Ian Leitch
Arana Louis M.
Oxford Instruments PLC
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