GB2376751A - Gravity meter - Google Patents

Abstract

A gravity meter (10) comprises a casing (12), a vacuum tube (14) mounted in the casing in a vibration-free manner, a sensor mechanism within the vacuum tube, the sensor mechanism comprising two masses (M1, M2) of different size positioned at different heights and acting on the respective arms of a beam balance (24), and means (34, 40) to null tilting of the beam balance by controlled movement of the lower mass (M2). Movement of the beam balance (24) is detected optically.

Description

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GRAVITY METER This invention relates to gravity meters, and is particularly concerned with continuously recording gravity meters.

The concept of gravity meters has been known for some considerable time. Their object is to measure smal changes in the acceleration of a mass due to gravity, known as"g".

However most such meters have been expensive to manufacture and are unsuitable for long-term installation in the field.

Gravity meters are used inter alia by geophysicists, especially for monitoring volcanic activity and for oil exploration. They do not normally measure the absolute value of g but changes in its value. They can be installed at a particular site for long-term measurements, or can be taken from site to site, taking measurements at each.

It is an object of the present invention to provide a gravity meter which can be made at lower cost, which is reliable and accurate in operation, and which is portable.

The concept underlying the gravity meter is that, from the definition of gravity, a mass of any given size will weigh more closer to the centre of the earth than it does at a greater distance from the centre. Therefore, if one has a mass at 0 meters elevation (sea level) and if one raises the mass by 1 meter to +1 meter above sea level, then the mass should have decreased in weight.

The vertical gravity meter of the present invention operates by measuring the difference in weight (therefore gravity) between two masses which are suspended one above the other in a vertical plane. The instrument measures the differential of gravity acceleration exerted on the two masses. The vertical gravity meter of the present invention finds particular use in the detection and measurement of gravity anomalies.

In accordance with the invention there is provided a

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vertical gravity meter comprising a casing, a vacuum tube mounted in the casing in a vibration-free manner, a sensor mechanism mounted within the vacuum tube, the sensor mechanism

C 0 rL-Lp r-'S 4 comprising two masses of different size positioned at different vertical heights and acting on the respective arms of a beam balance, and means to null tilting of the beam balance by controlled movement of the lower mass.

Preferably, the means to null tilting comprises spring means which exerts torque on a pivotable support for the lower mass, thereby to raise or lower the support to restore the position of the lower mass.

The spring means preferably extends between the pivotable support and a carriage which is displaceable along a micrometer screw in a controlled manner.

The beam balance can be a mechanical beam whose movement is measured optically.

In order that the invention may be more fully understood, one presently preferred embodiment of vertical gravity meter will now be described by way of example and with reference to the accompanying drawings. In the drawings: Fig. 1 is a schematic vertical section through a vertical gravity meter in accordance with the present invention; and, Fig. 2 is an illustration, on an enlarged scale, of the lower end of the meter of Fig. 1, to show more details of the adjusting mechanism.

Referring first to Fig. 1, there is shown a vertical gravity meter indicated generally at 10. The meter comprises an outer casing 12 within which is suspended a vacuum tube 14. The outer casing 12 and vacuum tube 14 are both generally cylindrical in shape. The vacuum tube 14 is suspended within the outer casing by upper and lower seismic dampers 16 which extend between the upper and lower ends respectively of the vacuum tube and the adjacent internal wall of the casing. The seismic dampers 16 are vibration damping springs whose purpose is to absorb vibration so that if micro-seismicity occurs, the

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outer casing 12 will move up and down but the inner vacuum tube will remain stationary.

The outer casing 12 provides thermal insulation and, in use, is positioned vertically, surrounded by sand 18 in a purpose-built concrete plinth 20. The purpose of the sand 18 is to absorb vibration.

Within the vacuum tube 14 is mounted the sensor arrangement, as will now be described. Adjacent to the upper end of the vacuum tube 14 there is provided a horizontal platform 22. Within the chamber above this platform 22 is positioned a beam balance 24, the position of which is arranged to be detected optically. Adjacent to one end of the beam is positioned a fixed mass Ml. Adjacent to the other end of the beam is secured the upper end of a wire filament 26 whose lower end carries a second mass M2. The two masses are of different size, with mass Ml being much smaller than mass M2. The smaller mass Ml is attached directly to the end of the beam, while the larger mass M2 is suspended by the wire filament 26 at a distance d below the other arm of the beam.

Because mass M2 is greater than mass Ml, and because mass M2 is closer to the centre of the earth than mass Ml, the beam 24 will tilt down at the side of mass M2. The degree by which the beam tilts is registered by an optical sensor (not shown) and can be continuously recorded.

On installation of the meter, the gravity differential between mass Ml and mass M2 is"nulled"or zeroed in the following way. As shown in Fig. 2, the larger mass M2 which is suspended from the wire filament 26 is attached to one end of a beam 28 which is supported at the other end by a frictionless pivot hinge 30 which is located just above a horizontal strut 32 of a sensitivity carriage which is indicated generally at 34. The sensitivity carriage 34 also comprises an upper horizontal strut 36. The two horizontal struts 32 and 36 are joined at their ends, as indicated by the rectangle of broken lines in Fig. 2, and the whole of the

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sensitivity carriage 34 is moveable up and down a pair of vertical support rods 38.

Adjacent to the mass M2 the beam 28 is connected to one end of a spring 40 or elastic polymer, whose other end is connected to a spring carriage 42 which is positioned above the beam pivot hinge 30. The spring carriage 42 is moveable back and forth along a horizontal micrometer screw 44. The position of the spring carriage 42 on the micrometer screw 44 is controlled through a gearbox indicated schematically at 46 and a turning dial 48. By rotating the turning dial 48, the position of the spring carriage 42 is moved. By moving the spring carriage 42 back and forth along the micrometer screw 44, the torque exerted on the beam 28 by the spring 40 can be increased or decreased. The beam 28 can thus be raised or lowered. Consequently, the weight differential which is registered via the beam balance 24 between mass Ml and mass M2 can be nulled by increasing or decreasing the spring torque. In other words, the initial tilting of the beam balance 24 can be zeroed by the use of the zeroing spring.

The residual weight difference between mass Ml and mass M2 is balanced by the torque from the spring 40 so that the beam of the beam balance 24 becomes horizontal and in effect "believes"that the mass (and therefore weight) of Ml is equal to that of M2.

It is important to be able to increase or decrease the sensitivity of the instrument. The instrument may need to be made more or less sensitive depending upon the gravity anomalies which are being targeted. To achieve this, the whole of the sensitivity carriage 34 can be moved in the vertical plane so that the distance d between the two masses Ml and M2 can be increased or decreased, thereby causing the weight differential between Ml and M2 to increase or decrease respectively.

After the instrument has been installed and has been nulled or zeroed in the manner described above, a measure is

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first taken of the vertical gravity gradient, using a conventional gravity meter. The instrument is then calibrated against that measurement by raising and lowering the vertical gravity meter through one meter. As the instrument is raised from its zeroed or nulled position, the differential is in favour of mass Ml (as the spring 40 contracts), whereas as the instrument is lowered from its zeroed or nulled position, and the spring 40 stretches, the differential is in favour of mass M2. By taking measurements at the aforesaid raised and lowered positions one has a measure of the gravity differential per metre, which can then be used to calculate the gravity differential when operational measurements are taken.

Claims (10)

CLAIMS:

1. A gravity meter comprising a casing, a vacuum tube mounted in the casing in a vibration-free manner, a sensor

mechanism mounted, within the vacuum tube, the seiiHor mechanism comprising two masses of different size positioned at different vertical heights and acting on the respective arms of a beam balance, and means to null tilting of the beam balance by controlled movement of the lower mass.

2. A gravity meter according to claim 1, in which the means to null tilting comprises spring means.

3. A gravity meter according to claim 2, in which the spring means exerts torque on a pivotable support for the lower mass, thereby to raise or lower the support to restore the position of the lower mass.

4. A gravity meter according to claim 3, in which the spring means extends between the pivotable support and a carriage which is displaceable along a micrometer screw in a controlled manner.

S. A gravity meter according to any preceding claim, in which the beam balance is a mechanical beam whose movement is measured optically.

6. A gravity meter according to any preceding claim, in which the lower mass is substantially greater than the upper mass.

7. A gravity meter according to claim 6, in which the upper mass is mounted on the beam and the lower mass is suspended below the beam.

8. A gravity meter according to claim 4, in which the carriage is displaceable vertically to change the distance between the two masses.

9. A gravity meter according to any preceding claim, in which the vacuum tube is suspended by upper and lower seismic dampers.

10. A gravity meter substantially as hereinbefore described with reference to the accompanying drawings.