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USGS CMG "Gravity" Definition

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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.

Gravity

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 physical materials, usually land tie information
    metadata 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:
    <Activity-ID>.10# Raw uncorrected readings from digital gravimeters
    <Activity-ID>.11# Raw uncorrected readings from analog gravimeters
    <Activity-ID>.12# Edited gravity information
    <Activity-ID>.13# Control file (usually for removing turns)
    <Activity-ID>.14# Land ties
    <Activity-ID>.15# Free Air Anomaly information with turns removed
    <Activity-ID>.16# Free Air Anomaly information
    <Activity-ID>.17# Non-surface free air anomaly information (includes the meter depth)
    <Activity-ID>.18# Free Air Anomaly and Bouguer Anomaly information

  • InfoBank gravimetric metadata file names:
    <Activity-ID>.bathgrav bathymetry + gravity systems metadata
    <Activity-ID>.bathgravmag bathymetry + gravity + magnetics systems metadata
    <Activity-ID>.bathgravmagnav bathymetry + gravity + magnetics + navigation systems metadata
    <Activity-ID>.bathgravnav bathymetry + gravity + navigation systems metadata
    <Activity-ID>.gradiometer gradiometer systems metadata
    <Activity-ID>.gravity gravity (only) systems metadata
    <Activity-ID>.gravmag gravity + magnetics systems metadata
    <Activity-ID>.landtie gravity land tie metadata
    <Activity-ID>.meter1 gravimeter #1 metadata
    <Activity-ID>.meter2 gravimeter #2 metadata
    <Activity-ID>.meter32 gravimeter #S32 metadata
    <Activity-ID>.meter41 gravimeter #S41 metadata
    <Activity-ID>.straitline straitline systems metadata


    "We can lick gravity, but sometimes the paperwork is overwhelming." -- Wernher Von Braun
    "A little less levity here please." -- Bruce Sherwood


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    Page Last Modified: Mon Nov 4 03:37:29 PST 2013  (chd)