Information About ™Gravimetry |
| CATEGORIES ABOUT GRAVIMETRY | |
| gravimetry | |
| geodesygravimetry | |
| geodesy | |
| geophysics | |
| geodesy | |
| gravity | |
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UNITS OF MEASUREMENT Gravity is usually measured in units of Acceleration . In the SI system of units, the standard unit of acceleration is 1 Metre Per Second Squared (abbreviated as m/s2). Other units include the Gal (sometimes known as a ''galileo'', in either case with symbol Gal), which equals 1 Centimetre per second squared, and the Gee (''g''n), equal to 9.80665 m/s2. The value of the ''g''n approximately equals the acceleration due to gravity at the Earth's surface (although the actual acceleration ''g'' varies fractionally from place to place). HOW GRAVITY IS MEASURED An instrument used to measure gravity is known as a gravimeter, or Gravitometer . Since General Relativity regards the effects of gravity as indistinguishable from the effects of Acceleration , gravimeters may be regarded as special purpose Accelerometer s. Many Weighing Scale s may be regarded as simple gravimeters. In one common form, a Spring is used to counteract the force of gravity pulling on an object. The change in length of the spring may be calibrated to the force required to balance the gravitational pull. The resulting measurement may be made in units of force (such as the Newton ), but is more commonly made in units of Gal s. More sophisticated gravimeters are used when precise measurements are needed. When measuring the Earth's gravitational field, measurements are made to the precision of microGals to find density variations in the rocks making up the Earth. Several types of gravimeters exist for making these measurements, including some that are essentially refined versions of the spring scale described above. These measurements are used to define Gravity Anomalies . Besides . The study of gravity changes belongs to Geodynamics . All modern gravimeters use specially-designed Quartz Zero-length Spring s to support the test mass. Zero length springs do not follow Hooke's Law , instead they have a force proportional to their length. The special property of these springs is that a vertical pendulum can be designed with a period approaching a thousand seconds. This detunes the test mass from most local vibration and mechanical noise, increasing the sensitivity and utility of the gravimeter. The springs are quartz so that magnetic and electric fields do not affect measurements. The test mass is sealed in an air-tight container so that tiny changes of barometric pressure from blowing wind and other weather do not change the buoyancy of the test mass in air. Gravimeters have been designed to mount in vehicles, including aircraft and ships. These special gravimeters isolate acceleration from the movement of the vehicle, and subtract it from measurements. The acceleration of the vehicles is often hundreds or thousands of times stronger than the changes being measured. HISTORY The modern gravimeter was developed by Lucien LaCoste and Arnold Romberg in 1936. They also invented most subsequent refinements, including the ship-mounted gravimeter, in 1965, temperature-resistant instruments for deep boreholes, and lightweight hand-carried instruments. Most of their designs remain in use (2005) with refinements in data collection and data processing. SEE ALSO |
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