Principles of Gravimeters
A gravimeter is simply a very precise weighing machine used to find the weight of a certain lump of metal or other material at a series of stations distributed over the area being surveyed. Since the weight of an object is its response to the Earth's gravitational attraction, this weight will be slightly affected by the nature of subsurface materials at the place of measurement. It will be slightly larger, for example, at stations where the subsurface material is of higher density or where dense material comes closer to the surface. The changes in weight are so small that the weighing machine must be capable of detecting changes of the order of one part in ten million.
Gravimeters must not only resolve small variations in the gravity field, but must also remain stable over a large range of values and environmental conditions. It is therefore required that practical instruments must be highly precise, portable, robust, simple to use and relatively inexpensive. All gravity meters have inherent limitations, however, and the particular equipment deployed must be consistent with survey objectives. There are clear distinctions between making absolute measurements of gravity and measuring relative gravity, or variation from place to place.
Likewise, modern land gravimeters are of two basic types – absolute and relative. Absolute gravimeters time the free fall of a body in a vacuum, using lasers and optical interferometry, to obtain accuracies of better than 0.01 mGals under favourable conditions. Whereas, absolute gravimeters are transportable, they are still quite bulky and time consuming to set up and read. Typically, only one absolute reading can be made in a single working day. Relative gravimeters are devices which can only measure differences in gravity from station to station. Those in use today, all rely on the elongation of a spring which supports a proof mass. When gravity changes, the force on the proof mass will likewise change, and this will be reflected in a change in the length of the supporting spring. The position of the proof mass is sensed by one means or another, and the amount of external force required to bring it back to a standard position provides a measure of the gravity value at the station, relative to other stations.
Absolute gravimeters are being routinely employed in establishing the absolute values of gravity at selected stations, which then may serve as base stations for relative gravity surveys, so that the results of the latter may then be expressed in absolute gravity terms, with almost the same accuracy as the values for the absolute stations themselves. Precise measurements with absolute gravimeters is quite an art. In any significant field survey, absolute gravimeter bases are established and are made accessible to the relative gravimeter surveys.
There are two basic types of field portable relative gravimeters with different spring balance configurations. These are known as astatic or unstable, and stable types. The astatic gravimeters operate in a state close to unstable equilibrium, which gives them great mechanical sensitivity. The stable gravimeters are simpler in mechanical principles, but require much higher precision of sensing of the position of the proof mass.
Gravimetric surveys for different applications may have different requirements in respect of precision of measurement. This implies that there may be scope for gravimeters with different performance specifications. We may designate two levels of performance by the terms standard and microgravity. Manufacturers of gravimeters offer portable relative gravimeters suited to each of these levels.
Instruments used for standard surveys are essentially mechanical devices, using optical or electronic means for determination of the proof mass position and manual or electrostatic restoration of the mass to its null position. Portable land gravimeters manufactured and marketed between 1950 to 1989 incorporated this mode of operation. Instruments used for microgravity surveys do not use springs because of the inherent limitations of creep of the material (steel or quartz) used for fabricating the springs. The most accurate relative gravimeters are superconducting gravimeters, which operate by suspending a liquid helium cooled diamagnetic superconducting niobium sphere in an extremely stable magnetic field. The current required to generate the magnetic field that suspends the niobium sphere is proportional to the strength of the Earth's gravitational field. The superconducting gravimeter achieves sensitivities of one nanogal, one thousandth of one billionth (10-12) of the Earth surface gravity.
Microgravity instruments incorporate extensive electronics in their design for sensing of the proof mass position and its restoration to the null position with great precision. The gravity signal is recorded and processed in the solid state memory by embedded software. In these instruments, corrections for tilt errors, for long term drift, for the temperature of the sensor and for earth tides are applied in real time. These gravimeters communicate through an RS-232 port with computers, printers and modems, for data dumping, processing and presentation. All modern gravimeters employ an extremely stable inertial platform to compensate for the masking effects of motion and vibration, a difficult engineering feat.
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