A submarine voyage into the gravity field of Earth

Plate tectonics is a theory that is only 60 years old. Before this time, people were seeing Earth as a solid un-deformable piece of rock. Imaging you being a geophysicist in those days (ok, if you are currently not a geophysicist this is even more difficult, but in comparison try to explain the wonders of the internet to your grandparents.), having trouble to explain most of the things that where observed.

In those times a civil engineer, educated in Delft (my university :) ), went on a submarine voyage, measuring the gravity field of Earth. Wow, they should make a movie about this (I think they even did)! With his own designed pendulum equipment, the Dutch engineer could measure the gravity field with an accuracy up to a few mGal (which is very impressive, just trust me). Who is this brave, 2 meter tall (which I like because I am the same height), submarine-sailing hero? Born on 30 July 1887 in The Hague, he was named Felix Andries Vening Meinesz.

I am currently working in a project to describe and explain the measurements of Vening Meinesz during one of his famous voyages. The project is making me feel like the Indiana Jones of the Geophysicists, going through old notebooks, carefully reading all the details and trying to get a sense of what he did and when he did it (ok without the whip and the cowboy hat, but still). 

Today, I give you a small sneak preview of my work. It is not finished, but still I think it is cool. The part of his voyage we look at is when he sailed over the Walvis Ridge (yes, it is not Whale Ridge, but the Dutch or South African Walvis), where he did several measurements.

The Walvis Ridge is a large sea mount chain, west of South Africa. It has a great effect on escaped eddies from the Indian Ocean (maybe I will explain this in later posts). The Ridge is formed by hotspot vulcanism (see my post about Hawaii), still today forming the island Tristan da Cunha. After visiting South America, Vening Meinesz, onboard the submarine Hr. Ms. K18, sailed to Cape Town, visiting Tristan da Cunha and crossing the Walvis Ridge. On his way he did 2-4 measurements a day, observing the variations in the gravity field due to masses like Walvis Ridge. This is what he saw:

I made a plot of his measurements and subtracted the ellipsoidal shape of the Earth, this results in the free-air anomaly (deviations from the main signal, you can see so much more!!). In the top figure are the measurements of Vening Meinesz (interpolated), with the black error bars. In red is a state-of-the-art satellite gravity field, having a much smaller resolution and thus dampening the signal, removing high-wavelength features. Features that Vening Meinesz could observe in the submarine, because he was much closer to the source. The bottom plot shows the bathymetry of that part of the Atlantic ocean, clearly showing the elevation difference of the Walvis Ridge. The red diamonds are the locations of the submarine during the measurements. The captain really tried to sail close to the ocean floor. I say,  "Kuddos for him and his sonar-crew, not hitting anything!" (ok, at some points they are below the ocean floor, but blame this on the uncertainty of the bathymetry model, not on the skills of the crew of Hr. Ms. K 18). 

Still today, Vening Meinesz's measurements are very impressive, obtained only by watching the swinging of three pendulums.