"MGD77" (Marine Geophysical Data Exchange Format) is an exchange format for marine geophysical data relating to bathymetry, gravimetry and magnetometry.
- Reference Gravity Stations
The measurements are tied to the values of the Reference Gravity Stations of the International Gravity Standardization Net 1971 (IGSN 71).
Ties are also made to stations which are not included in the IGSN 71. The gravity values expressed in the POTSDAM system are corrected to be included in the IGSN 71 reference system.
For more information see the base ties and the website of the International Gravimetric Bureau
- Drift correction
i.e. Gv = measurement by the gravimeter at time T.
Prior to the cruise, a tie is made in port n°1 where we measure:
- Gv1, the gravimetric measurement value in port
- Grefnav1, the measurement of the IGSN 71 point of reference in relation to the gravimeter onboard the vessel
- T1, the time (or date) of the measurement
A new tie is made in port n°2 after the cruise, where we measure:
- Gv2, the gravimetric measurement value in port
- Grefnav2, the measurement of the IGSN 71 point of reference in relation to the gravimeter onboard the vessel
- T2, the time (or date) of the measurement
therefore Gzero1 = Grefnav1-Gv1 and Gzero2 = Grefnav2-Gv2 the zero values of the marine gravimeter in ports 1 and 2 respectively [not to be confused with G0, G1 and G2 of terrestrial gravimeters which appear in the base ties).
The time drift, which is assumed to be constant throughout the cruise, is:
(1) Der = (Gzero2 - Gzero1) / (T2 - T1)
To calculate the absolute gravity Gabs from the measured gravity Gv at time T, the following calculation is made:
(2) Gabs = Gzero1 + Gv x Der x (T - T1)
Note to users of CARAIBES software: The correspondence with the data import settings is:
- "Gravity" = Grefnav = Gnav in base ties
- "Observed value"= Gv
- Variation in the spring stiffness coefficient
The spring stiffness coefficient evolves. This creates errors: the greater the latitude of the trajectory, the greater the error. This evolution must therefore be taken into account in the absolute gravity calculation from gravimeter measurements. A stiffness coefficient Kstiffness is therefore introduced:
Formulas (1) and (2) above become:
(3) Der = ((Grefnav2 - Grefnav1) + Kstiffness x (Gv2 - Gv1)) / (T2 - T1)
(4) Gabs = Gzero1 + Kstiffness x Gv + Der x (T - T1)
- Sea state and gravimeter recording times
- KSS 30
Real-time acquisition is performed with the sea state parameter of the KSS 30 "Sea State" = 2, which corresponds to a "smooth" sea, whatever the actual sea state. The corresponding real-time anti-aliasing Bessel filters and the time constant of the gravimeter delay the recording of the measurement. Based on GENAVIR analyses, a 120-second delay should be deducted from the recording time of real-time measurements. This is the value that is currently applied by SISMER to KSS 30 data. The filtering times obtained more recently by SHOM are:
- Sea State 2: 109.3 s
- Sea State 3: 132.8 s
- Sea State 4: 181.8 s
- KSS 31
From 2000 to July 2007, the KSS 31 was run at "Sea State" = 3 (except during the BLASON 2 cruise from 9/8/2002 to 9/9/2002 when SS = 2). The Bessel filters and the time constant of the gravimeter introduce a recording delay. The time delay applied by SISMER is 130 seconds.
From July 2007, the KSS 31 was run at "Sea State" = 2. The time delay applied by SISMER is 120 seconds. The filtering times obtained by SHOM are:
- Sea State 2: 101.0 s
- Sea State 3: 124.5 s
- Sea State 4: 173.5 s
Henceforth, KSS31 is run at "Sea State" = 2.
The BGM5 filters differ from those of the KSS and do not introduce a recording delay. On the other hand, the time attributed to the filtered measurement is deferred with a 304.5 second delay.
- Diurnal variation correction
The data are provided without correction of diurnal variations whether in the form of raw data or data converted to the international Marine Geophysical Data Exchange Format (MGD).
- Calculation of magnetic anomalies
The measurements in Marine Geophysical Data Exchange Format (MGD) are provided with magnetic anomaly values calculated from the most recently published International Geomagnetic Reference Fields (IGRF) at the date of file creation.
For more information see the websites of International Union of Geodesy and Geophysics (IUGG) and NOAA.
- Difference between the ship's position and the magnetometer's position
The raw data are not georeferenced. To do so we must:
- interpolate the positions of measurements over time from the navigation file validated by Ifremer/Genavir
- correct the position according to the length of magnetometer cable paid out. This length varies according to ships and the equipment used.
For all further enquiries, please contact the SISMER service (firstname.lastname@example.org).