Marine geophysics
Mis à jour le 19/07/2024Presentation
The work carried out by the Shom's marine geophysics department involves studying and characterising the Earth's gravity field (gravimetry) and the earth's magnetic field (geomagnetism).
This work is part of production and research and development (R&D) activities for civil and military purposes.
The Shom develops its activities in marine geophysics at all levels: from the acquisition, processing and archiving of data to their use for modeling and map production.
The R&D activities aim to improve acquisition and processing methods by equipping themselves with innovative instruments and appropriate software tools. The work carried out is promoted in the form of publications and during scientific conferences.
The Shom contributes to training and maintaining a high level of skills in marine geophysics, both internally (Shom school and staff) and externally (ENSTA Bretagne, UBO).
Applications
Knowledge of variations in magnetic and gravimetric fields has applications in the ocean environment, ranging from the detection of anthropogenic objects (wrecks, obstructions, etc.) to the mapping of gravity fields, and including the study of the tide terrestrial and the effect of oceanic overload.
This list of applications is of course not exhaustive.
Object detection
Objects are detected using a magnetometer towed behind a boat. This magnetometer continuously records the intensity of the Earth's total magnetic field, which is the sum of various internal (geodynamic), external (solar activity) and lithospheric (geology and metallic anthropogenic objects) contributions. Measuring the total field makes it possible to detect the presence of objects lying (or buried) on the seabed (wrecks, munitions, etc.).
Once an object has been detected by the magnetometer, a more specific search is carried out using other instruments (lateral sonar, multibeam echosounder) to determine its nature and geographical position on the nautical chart.
This object detection is therefore important to ensure safe navigation.
Modelling the gravity field
Gravimetry is the measurement of variations in the Earth's gravity field. Gravimetric measurements do not incorporate major temporal effects, but regional variations are superimposed on a local signature of the topography.
These variations can be measured at low resolution by satellite or at high resolution by plane, boat or on land.
The Shom's geophysics department studies these variations in gravity in the marine environment, in order to calculate models and produce maps.
These models contribute to our knowledge of the seabed and, in particular, enable us to quantify gravimetric anomalies linked to relief (e.g. seamounts, ridges, etc.).
The study of ocean overload
The attraction of the stars (mainly the moon and the sun) moves the masses of water (these are the oceanic tides) and deforms the earth which rises or sinks (these are the terrestrial tides).
An additional action is the deformation induced by the effect of tides on the earth's crust. In the Armorican massif, during high equinox tides, the ground can rise and sink by around 20 cm, under the effect of what is called oceanic overload.
These ground movements are recorded using very precise gravimeters for at least several days and thus make it possible to specify the ocean overload predicted by theoretical models. The figure below shows that theoretical models underestimate the effect of ocean overload in Brest by around 15%.
Partners
Shom is active within the national and international scientific community in geophysics and geodesy. He is a member of the CNFGG (French National Committee for Geodesy and Geophysics), the GRGS (Spatial Geodesy Research Group) and the BGI (International Gravimetric Bureau). It also works in collaboration with academic and institutional partners (IUEM, ENSTA, ONERA, etc.)