| Gravity Meter, often called a "Gravimeter" | - Gravimeter |
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Above: InView on another mission. Measurements of minute ( 1:200,000,000 ) changes in the gravitational field over a region can indicate the location of:
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The measurement relies on precisely timed zero-crossings.
The uncertainty in the wavelength of a standards grade HeNe laser stabilised on the i (a 13 ) component of the 11-5 R(127) hyperfine transition of the 127 I 2 Iodine molecule is 2.1E-11. The error in the precision Cesium atomic clock is around 1E-12. http://cires.colorado.edu/~bilham/FG5operation.html
Note that the gravitational field strength decreases by about 3uGal with a 10 mm increase in height. - from www.lacosteromberg.com/images/fg5schem.jpg
The Scintrex CG-5 from www.scintrexltd.com has a resolution of 1 uGal, and a standard deviation of less than 5 uGal. The “Gal” unit is named after “Galileo”, and equals 1 cm/s 2 . The gravitational constant = g = 980.665 Gal.
The accuracy of the FG-5 shown on the topof the opposite page is 2.1 uGal, equivalent to 6 mm change in altitude. Sander Geophysics Limited report a resolution for their airborne AIRGrav system from 150 … 300 uGal. To put these figures in context, the earth’s gravitational field strength is 980,600,000 uGal. The major challenge is to have an extremely low vibration level in the aircraft to enable accurate microgravity measurements. An alternative approach is to measure the gradient in the Earth’s gravitational field using a completely different and expensive instrument, called a “gradiometer”.
Microgravity data (A) and model (B) showing anomalous area. Areas of low gravity anomalies (100 to 175 µGals) were identified in five of the survey lines. The anomalous areas are centred within the collapse area and are aligned with the strike of the mine. An example of the data along one of the profile lines, is shown above. - from www.fhwa.dot.gov/mine/ky0316.htm (link no longer exists)
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