Coastal & Marine Geology InfoBank
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This documents InfoBank's usage of "gravity".
- "Gravity" (as used within the CMG InfoBank ) is a measure of the attraction exerted by the earth on a mass.
- Local (land or platform based) gravimetric measurements are made using a portable instrument.
- Worldwide measurements are made using satellites.
- The acceleration of an object toward the earth is measured.
- The amount of this acceleration can be affected by movements of the device (e.g., the platform).
- To minimize movements due to roll, pitch, yawl of the platform, the gravimeter is positioned at the center of the platform.
- To remove accelerations caused by other platform movement a correction known as an Eotvos value is computed.
- The Eotvos value is not at all based on the gravity measured, it is based on the velocity, heading, and latitude
of the platform.
- Gravity is a very weak force and necessitates highly sensitive equipment for collection.
- These gravimeters usually "drift" over time (i.e.: returning to the exact same place a few weeks later
might give a slightly different reading).
- "Land ties" are made whenever possible at the start and end of a cruise.
- If these measurements are at the same location, then any difference between them is the meter drift.
- A linear adjustment to all measurements throughout the time between measurements is then made.
- If the land ties are at different locations they need to be correlated with a worldwide gravity datum.
- Then meter drift can be computed.
- The "uncorrected" gravity is corrected for meter drifting and for Eotvos effects to produce an "observed"
- This is a relative, not absolute number.
- Adjusting for the difference between the observed values at land ties and the worldwide datum values there,
converts the observed values to absolute numbers.
- Correcting for the elevation of the gravimeter relative to sea level creates a"free air" value.
- Correcting for additional attraction of land mass above sea level creates a "Bouguer" value.
Gravity information occurs in the
< Activity-ID >.1##
files containing time sequential information along ship tracks.
physical materials, usually land tie information
information about gravimetric analog paper rolls, inventory of gear, operation of gear, magnetic tapes,
land ties, and paper printer listings
InfoBank gravimetric data file names:
InfoBank gravimetric metadata file names:
"We can lick gravity, but sometimes the paperwork is overwhelming." --
Wernher Von Braun
"A little less levity here please." --