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 ParameterGX: 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

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