Eötvös Correction

Use the Moving Platform Gravity > Corrections by Steps > Eötvös Correction menu option, (geogxnet.dll(Geosoft.GX.Gravity.EotvosCorrection;Run)*), to calculate the Eötvös correction.

Eötvös Correction dialog options

Eötvös equation

Select one of the three variations of the Eötvös correction. The difference between the three is in the order of 0.2 mgals. Select one of:

  • Exact

  • Glicken

  • Harlan

The correction equations are found in the Application Notes.

Script Parameters: EOTVOS_CORRECTION.EQUATION [0- Exact(default, 1- Glicken, 2 – Harlan]

Time channel

Select the time channel from the drop-down list. The default name is "Time".

Script Parameters: EOTVOS_CORRECTION.TIME

Units

Define if the above time channel is in units of hours or seconds.

  • The Eötvös equation requires the time in hours. If your time is in seconds it will be converted on-the-fly for the calculations.

    • Script Parameter: EOTVOS_CORRECTION.UNITS[0- Hours, 1- Seconds]

    Latitude channel

    Select the latitude channel from the drop-down list.

    By default, if there is a channel named "Latitude" in the database, it will be selected.

    Script Parameter: EOTVOS_CORRECTION.LATITUDE

    Heading channel

    Provide the heading channel name. If the heading channel already exists, it will be used. If it does not exist, it will be generated and used. It will also be saved to the database.

    Script Parameter: EOTVOS_CORRECTION.HEADING

    Velocity channel

    Provide the velocity channel name. If the velocity channel already exists, it will be used. If it does not exist, it will be generated and used. It will also be saved to the database in units of knots.

    Script Parameter: EOTVOS_CORRECTION.VELOCITY

    Altitude channel

    For airborne surveys, select the channel containing the altitude above sea level.

    Script Parameter: EOTVOS_CORRECTION.ALTITUDE

    Output Eötvös channel

    Specify the output Eötvös correction channel name.

    Script Parameter: EOTVOS_CORRECTION.OUTPUT

    Application Notes

    *The GX tool will search in the "...\Geosoft\Desktop Applications \gx" folder. The GX.Net tools, however, are embedded in the geogxnet.dll located in the "...\Geosoft\Desktop Applications \bin" folder. If running this GX interactively, bypassing the menu, first change the folder to point to the "bin" folder, then supply the GX.Net tool in the specified format.

    Earth’s rotation produces an outward centrifugal acceleration. Objects moving across the rotating Earth experience an additional acceleration related to the east-west component of the velocity, with the effect being greatest at the equator and decreasing with latitude. A body moving east (in the direction that Earth spins) experiences an increase in centrifugal acceleration, while the opposite is true for a body moving west. The vertical component of this acceleration and a small acceleration related to the motion of the curved earth is known as the Eötvös effect. This effect can be as large as 30 mGals and should be accounted for in moving platform surveys.

    The Eötvös effect is defined as:

    The first term corresponds to the Eötvös effect; the second term is a refinement, which is much smaller in amplitude. To take into account Earth’s ellipsoidal shape, R can be expanded into Earth’s major axis, Earth’s flattening, the latitude, and the observation height to yield the Exact equation below. Two other published variations for Eötvös effect are also offered.

    [Exact]:

    [Harlan, 1968]:

    [Glicken, 1962]:

    Where:

    E: Eötvös correction

    Ws: Angular velocity of Earth’s rotation = 7.2921158533 E-5 rad/sec

    φ: Latitude

    νe & νn: Velocities in easting & northing directions calculated from the heading and velocity channels

    h: Observation height

    e: Correction for Earth’s flattening towards the poles = 0.0818191908426

    R: Earth’ radius

    ra: Earth’s major axis = 6378137.0 m

    rb: Earth’s minor axis = 6356752.3141 m

    ε: Earth’s eccentricity = (ra-rb)/(ra)

    V: Velocity in knots

    References

    • R. B. Harlan, "Eötvös corrections for airborne gravimetry", Journal of Geophysics Research, vol 73, no 14 (July 15, 1968).
    • M. Glicken, "Eötvös corrections for a moving gravity meter", Geophysics, vol 27, no 4 (1962), pp. 531-533.