Thursday, April 5, 2012

Miss Gravity

As my colleagues of the team awesome call me Miss Gravity, I feel obligated to write something about this topic. Actually, on the ship, we do not only realize seismic measurements. We also measure the value of the gravity field, the magnetic field, and the bathymetry along the trajectory of the ship. The whole data, collected from the different cruises allows us to obtain maps of these different quantities with a better resolution. The gravity value that we measure contains a lot of information… that only a specialist can decipher. As shown on the attached figure (sorry it’s in French but I am going to help you. This figure has been realized by Olivier de Viron, a brilliant researcher in gravimetry at the Institut de Physique du Globe de Paris), the value of the gravity is a series of numbers, and each of them is associated with a particular process:

9.8 represent the acceleration applied on each mass at the Earth’s surface. It results from the gravitational acceleration created by the newtonian attraction of the whole mass of the Earth and the one from the solar system (mainly the Moon and the Sun), and the centrifugal acceleration due to the Earth’s rotation. If the Earth were spherical, immobile and homogenous, this value would be constant at its surface. Because of the rotation, of the topography and the presence of density heterogeneities, this value varies.

9.81 Aplatissement = flattening. The rotation deforms the Earth: it is flattened at poles. The second decimal contains this deformation.

9.814 Montagnes & fosses océaniques = Mountains and oceanic trench. The third decimal is necessary to have access to these contributions in the gravity field. From now on, we can talk about gravity anomalies. Indeed, if we consider the gravity field associated to the spheroid (i.e. the Earth after deformation) as a reference field, the local variations of the gravity produce by the Earth’s topography and density heterogeneities constitute gravity anomalies compared with this model.

9.8142 Anomalies de densité internes = internal density anomalies. A fourth decimal must be added if we want to better know the large density anomalies responsible for the geodynamical processes occurring at the Earth’s surface (Are they negative or positive?, What is their size?, What is their nature: thermal or chemical origin?). This is my job.

9.81426 Grands reservoirs = large reservoir of water. We reach the fifth decimal. Now we can talk in mGals, unit used in gravimetry in general.

9.814262 variations temporelles des marées = temporal variations associated with tides. The action of the Moon on the Earth is measurable from the sixth decimal.

9.8142627 grandes constructions = buildings. The presence of human beings is, indeed, visible in the gravity field!

Thus the gravity data keep a lot of secrets that they would really like to be discovered! 

1 comment:

  1. An excellent way of explaining gravity anomalies! I'll use the figure in my introductory geophysics class from now on.