- "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.
Eotvos
- 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.
Land ties
- 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.
Corrections
- The "uncorrected" gravity is corrected for meter drifting and for Eotvos effects to produce an "observed"
gravity measurement.
- 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
InfoBank
as:
|
scattered
|
< Activity-ID >.1##
files containing time sequential information along ship tracks.
|
|
analog
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physical materials, usually land tie information
|
|
metadata
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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:
- The
<Activity-ID>.1## files do not include navigation.
- Navigation must be obtained from
<Activity-ID>.0## or
<Activity-ID>.6## files.
- Navigation should therefore also be extracted when you retrieve these files.
- A "time"
key is associated with each record for correlation with other scattered information datasets.
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