Measuring and testing – Gravitational determination
Reexamination Certificate
2001-02-28
2002-12-17
Chapman, John E. (Department: 2856)
Measuring and testing
Gravitational determination
C505S843000, C505S846000
Reexamination Certificate
active
06494091
ABSTRACT:
The present invention relates to a measuring apparatus, and more precisely to a gravity meter. It uses a superconducting quantum interference device also known as SQUID.
DESCRIPTION OF RELEVANT PRIOR ART
There are three relevant types of prior art: gravimeters, SQUIDs and quantum gyroscopes.
1. Gravimeters
Gravimetry is an old art that has reached a level of precision and accuracy that few fields of science enjoy. There are different types of industries that are interested in gravimetry. A physicist might want to measure the variation of gravity with latitude, while a geophysicist will be interested in gravity in order to improve the current models of the interior of the Earth. The oil industry is also interested in gravity because a decrease in local gravity might indicate an oil field deep underground. This justifies the very large effort made into the development of equipment to obtain a more precise and accurate value of {right arrow over (g)} and {right arrow over (|g|)}.
There are basically three types of gravimeters: pendulum, spring and free-fall. The quest for ever more precise measurements has brought very many variations on the basic principle of an oscillating mass (pendulum), an elongated spring and free-falling objects. When measuring gravity, there are two types of measurements one is interested in: the absolute value of gravity at a given point and the variation of gravity with time at a given point. Different equipment will be used for these different measurements. Free-fall equipment clearly leads to an absolute value of gravity, while spring leads to a relative value of gravity. The-use of sprig equipment then requires the measurement of an absolute value of gravity at a given point that is used as a standard.
The modem free-fall equipment uses a laser beam directed on a retro-reflector that is in free-fall. The free-fall path is one arm of an interferometer, and one simply records the passage of interference fringes with time. From this information it is possible to extract the absolute value of g at the point where the free-fall occurred. Clearly, this value of g is the average value of g over the path of the free-fall Such devices have been designed for measurements on land (U.S. Pat. No. 3,727,462), in boreholes (U.S. Pat. No. 5,892,151) and in the water (U.S. Pat. No. 5,637,797).
The measurements of {right arrow over (f)} or {right arrow over (|g|)} have been so precise for the last 15 years, that time variation of {right arrow over (g)} have now been observed. The period of “oscillation” varies from seconds to hours, and this phenomenon is still not very well understood. One possible explanation is that the continents oscillate due to the atmospheric pressure.
Superconductivity has also been used to measure gravity. An early attempt at using superconductivity in gravimetry is shown in U.S. Pat. No. 3,424,006 where a superconducting floating element is magnetically suspended in a superconducting ring. The upper face of this element is used as a mirror and constitutes one arm of an interferometer. If g changes with time, the suspended element will rise or fall in the superconducting ring, and this will lead to a shift in the interference fringes. One simply has to record the position of the fringes with the time and then deduce the stability of {right arrow over (g)} with time.
A more recent attempt at the use superconductivity in gravimetry-is-shown in U.S. Pat. No. 5,962,781. In this patent, a superconducting string is used as an antenna connected to driving solenoids in resonance. If {right arrow over (g)} changes with time, the position of the string will slightly change and the resonance will be lost. Since the system is in resonance, it is very sensitive to any variation of position or variation of {right arrow over (g.)}
Another recent use of superconductivity in gravimetry is shown in the design of GWR Instruments Inc., San Diego, Calif., USA, where a spinning superconducting sphere is suspended in a magnetic field. This superconducting magnetic field is very stable and acts essentially as a spring to support the bulk of the sphere. A second magnetic field is provided by a coil and the position of the sphere is provided by an electronic circuit where one of the components is the sphere. If {right arrow over (g)} changes with time, the sphere will slightly move in the magnetic fields and feed-back circuit changes the current in the coil in order to bring back the sphere to its original position. The change in current is produced by a change in voltage, and the voltage is simply recorded every few seconds or minutes depending on the user. This is a very good system to measure the stability of {right arrow over (g)} with time, but is not capable of measuring the absolute value of {right arrow over (g)} since the bulk of the weight of the sphere is supported by the superconducting magnetic field. This equipment provides the most sensitive data of the to variations of {right arrow over (g)} with time.
Most of the known devices have limitations and deficiencies. For example, the device that uses a spring system will suffer from loss of stiffness of the spring over time and is very sensitive to temperature changes. Also, most, if not all of them measure either {right arrow over (|g|)} or {right arrow over (g)}(t), but not both. The free-fall apparatus using a laser beam reaches a high precision of {right arrow over (|g|)} at a given point only after several measurements are combined in order to reduce the statistical error. For example, the device known as JILA-2 manufactured by Micro-G Instruments, Boulder, Colo., USA, requires about 2000 falls which will take 2-3 hours to measure. This equipment is not designed to monitor {right arrow over (g)}(t), primarily due to the wear and tear of the equipment. The same comment applies to the spinning superconducting sphere, since it is designed to monitor {right arrow over (g)}(t), but cannot get {right arrow over (|g|)}.
Another problem with the previous equipment is that of vibration: the free-fall device is not particularly sensitive to vibrations since its reference beam is suspended by a spring to cancel the vibrations through a retro-action electronic system. The GWR gravimeter, however, is very sensitive to vibrations in view of a mass suspended in a magnetic field. A third problem is the weight and portability of the equipment. Clearly, it is an asset to have the equipment that is light and can be easily carried to any point on Earth. The free-fall device (such as the JILA-2) is light and portable, but GWR equipment is very heavy (about 1 ton) and cannot be carried easily.
The purpose of the present invention is to solve these problems and to provide a single apparatus capable of measuring both {right arrow over (g)}(t) and {right arrow over (|g|)} very quickly and precisely. The apparatus of the present invention will allow to measure {right arrow over (|g|)} at a given instant, then monitor it for a certain period of time by measuring g(t), and then measure {right arrow over (|g|)} again to check for consistency. Another advantage of the present invention is the fact that it is practically immune to vibrations, thus opening a new venue of studies in geophysics, variations of {right arrow over (|g|)} during an earthquake and many other possibilities. The present invention is also unaffected by temperature, and can be made relatively light through to the use of a superconductors at high critical temperatures, thus greatly reducing the costs and bulkiness of the refrigerating equipment.
2. SQUIDS
Superconducting quantum interference devices (SQUIDs) are based on the quantization of the magnetic flux through a superconducting loop and on the Josephson effect; they are used mostly as very sensitive magnetic field detectors. In general, a SQUID is a loop of superconducting wire where one has built one or more Josephson junctions. When used as a magnetic field sensor, they are most o
Chapman John E.
Shvartsman Mila
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