Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1999-05-06
2002-12-03
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000
Reexamination Certificate
active
06489872
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to magnets for nuclear magnetic resonance applications, more specifically to magnets that generate at least one region with a relatively homogenous magnetic field at a position that is remotely located from the magnet assembly.
2. Background Art
A major use of magnets generating regions of uniform magnetic fields is in magnetic resonance. Nuclear magnetic resonance (“NMR”) is a well-established technique with applications fields such as physics, chemistry, mechanical engineering, civil engineering, nuclear engineering, petroleum engineering, food processing, pharmaceutical production, biology, and medicine. Most applications of NMR use a magnet to generate a uniform magnetic field. For superconducting magnets having a cylindrical bore, the uniform field region is within the bore whereas, for resistive electromagnets, the uniform field region is between the magnet's poles. In either configuration, a sample to be studied is confined to a uniform field region that is small compared to a characteristic dimension of the magnet.
Few NMR applications rely on “external” or “remote” uniform field regions in which the objects being examined are positioned to one side or outside of the NMR apparatus. U.S. Pat. No. 3,019,383 (“'383 Patent”) discloses an early remote apparatus for detecting subsurface liquids, such as water, from a position on the surface while using only the magnetic field of the Earth. A. G. Semenov, M. D. Schirov, A. V. Legchenko, A. I. Burshtein, and A. J. Pusep, further developed this approach in the 1980's (UK Patent Application GB2198540A, “Device for measuring parameters of underground mineral deposits,” filed May 29, 1986) with a more practical apparatus. Like the apparatus described in the '383 Patent, the apparatus of Semenov et al., does not use a man-made magnet; it relies solely on the NMR signal from protons in subsurface liquids processing in the weak magnetic field of the Earth.
NMR based solely on the Earth's magnetic field inherently yields a low signal-tonoise ratio (S/N), unless the sample size and corresponding number of nuclei are very large. S/N is an essential parameter in NMR; it is directly proportional to the number of atomic nuclei and to approximately the {fraction (3/2)} power of the magnetic field strength. Stronger magnetic fields align more spins and also make the spins precess faster, which leads to an increase in signal magnitude. NMR applications using the Earth's magnetic field overcome weak magnetic field limitations by examining a very large sample with a correspondingly large number of-atomic nuclei. A typical magnet used for NMR has a magnetic field strength that ranges from a fraction of Tesla to tens of Tesla. In contrast, the Earth's magnetic field strength is typically around ½ Gauss—approximately 100,000 times weaker than most commercially available NMR magnets. Applying the {fraction (3/2)} power criterion to the difference in magnitude between the Earth's field and commercial field magnet strengths equates to an Earth produced signal that is approximately 3×10
7
less than that of a commercial system for the same sample size. To compensate for the low signal strength, the apparatus of Semenov et al. uses a circular coil having a diameter of 100 meters that is placed on the ground to detect subsurface signals. The volume of the sample thus examined is approximately equal to a hemisphere having a diameter of 50 meters, equivalent to 3.3×10
10
cubic centimeters. When compared to an ordinary laboratory NMR sample of a few cubic centimeters, this represents an increase of 10
10
. Such an increase is sufficient to obtain useable signals despite the many adverse conditions of performing in situ environmental NMR.
The aforementioned example demonstrates the difficulty of performing subsurface or otherwise equivalent NMR experiments using Earth's field-based techniques for small sample volumes, such as those encountered when examining shallower depths (less than 50 m) that implies much smaller sample volumes. Therefore, a need exists for a NMR apparatus to perform experiments for remote regions having “mid-range” sample volumes between the “small range” (sub- to a few cubic centimeter dimensions of the laboratory apparatus) and the “large range” (millions of cubic centimeter dimensions of the Earth's field apparatus).
To meet the mid-range need, several classes of remote NMR have been developed that generate a magnetic field that is significantly stronger than the Earth's field. One subclass of such NMR apparatuses comprises magnets that are designed to fit inside holes to observe NMR signals from samples outside the hole. The most common application of this subclass is NMR downhole oilwell logging. Early versions of this subclass (as disclosed in U.S. Pat. No. 3,213,357, to Brown) used a NMR logging tool having a coil that was temporarily energized to create a magnetic field in the formation surrounding the bore hole. This field acted to orient, that is, to prepolarize, the spins belonging to the fluid of interest such as oil or water. The nuclear spins of the water around the bore hole were detected as they processed around the Earth's magnetic field immediately after the prepolarization field was turned off. Thus, this method used the applied magnetic field to prepolarize the spins and then used the weaker Earth's field to conduct the remainder of the experiment. The use of the prepolarizing field ameliorates the signal loss at the lower fields from the {fraction (3/2)} power dependence alluded to earlier; nevertheless, signal loss will still be incurred approximately at the ¾ power of the field strength.
Another scheme emerged in the 1980s, one that placed permanent magnets downhole to generate a magnetic field stronger than the Earth's field as disclosed in U.S. Pat. No. 4,350,955 to Jackson, et al. (“'955 Patent”). The '955 Patent describes a NMR apparatus with a magnet assembly having two cylindrical permanent magnets aligned along the bore hole axis. The apparatus of the '955 Patent projects a thin annular region of substantially uniform magnetic field outside the bore hole. Such schemes use both prepolarization and detection in the presence of magnetic fields stronger than Earth's field, about 0.5 Gauss, which gives rise to much better S/N. The {fraction (3/2)} power dependence of the signal on the field strength means that even a 5 Gauss field results in a 32-fold gain in S/N whereas a 50 Gauss field yields a factor of 1000 over S/N in the Earth's magnetic field and a 500 Gauss field leads to a gain of more than 30,000.
The '955 Patent states at col. 2, II. 64-68; col. 3, II. 1-11: “A well known requirement for generating an observable NMR signal is a relatively homogeneous magnetic field across a sample volume in order that the precessional frequencies of the nuclei within the sample will be relatively uniform. Previous attempts to “focus” a region of NMR sensitivity into the formation using a non-uniform field (e.g., a magnetic dipole) which decreases rapidly and monotonically with increasing distance from the axis suffer from the fact that the radial width of a sample volume within which the magnetic field homogeneity is good enough to support NMR is extremely small.”
This paragraph appears in the '955 Patent to Jackson, which the present application incorporates by reference.
U.S. Pat. No. 4,710,713 (“'713 Patent”), entitled “Nuclear Magnet Resonance Sensing Apparatus and Techniques”, to Strikman, issued Dec. 1, 1987, is an alternative scheme for bore hole NMR logging. This patent discloses a nuclear magnetic resonance sensing apparatus having one or more magnets to generate a magnetic field in a region remote therefrom. The one or more magnets define a longitudinal axis and the static field created by the magnets has a field direction substantially perpendicular to the longitudinal a
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Jackson Jasper A.
Baker Rod D.
Donovan Lincoln
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Pangrle Brian J.
Peacock Deborah A.
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