donderdag 21 november 2013

Delfi-n3Xt baby sounds...

This morning I got up earlier today to attend the Delfi-n3Xt launch event in our faculty. Delfi-n3Xt is a nano-satellite (the size of a milk carton) which is built by students and staff of our faculty. Launch events are always exciting, because you don't know what will happen. But after two stressful hours we got the confirmation, "Nano-sat number 7 successfully deployed." This was followed by some cheerful cheers. However, was it turned on?

This blog started with some post about listening to satellites using the ground station on top of EWI (faculty of electronic engineering, mathematics and computer sciences). This ground station is mostly set up by a student which I assist in teaching him to track satellites. So this gives me the opportunity to get the telemetry data of the Delfi-n3Xt quite fast. It's good to be a teacher :). The next figure will show the satellite's first sounds in space:

Click on the figure to get a larger view. In the bottom left corner of the blue figure, an S-shaped curve can be noticed. This is the radio signal transmitted by the moving satellite in a frequency-time plot. The vertical axis is frequency and the horizontal axis is time. You clearly see the frequency of the satellite shift with time. This is of course the Doppler effect (please read my blog entries). 

These are the first baby sounds of the Delfi-n3Xt, the second Dutch university-constructed satellite in space. Fully operational! This clearly shows the capabilities of engineering students. Well done.

zondag 17 november 2013

The Earth rotates (except in Copenhagen)

Last week, I was visiting the beautiful capital of Denmark, Copenhagen. I was selected to participate in the International PhD Elite Course "Tectonics" at the University of Copenhagen (READY program), organized by Prof. Hans Thybo and Prof. Irina Artemieva. During this five day course, the participants (PhD students from all over the world and me :) ) were being taught by two experts in the field of tectonics, the wonders of geosciences. From Geology (I write Geology, because it was defined by the lecturer as one of the two only real sciences); Prof. Çelal Şengör from Turkey with a beautiful British-sounding way of lecturing. From Geophysics (capital G, because I disagree with Prof. Şengör ;)); Prof. Seth Stein from the USA, who also brought his wive, Carol Stein. She gave a very interesting view on the Mid-Continental Rift. These last five days were fantastic, because I learned a lot and met numerous interesting people. I recommend any PhD student participating in such a course. It will enlighten your brain. 

The course was given in the Geoscience building of the University of Copenhagen. While entering the main hall, I stumbled onto this view:

A large pendulum was swinging in the centre of the hall. The mass of the solid metal ball was 140 kg. This is the so called Pendulum of Foucault. It proves that the Earth is rotating. At first display in Paris 1851, the pendulum of Léon Focault was the first simple proof that the Earth was rotating in an easy-to-see experiment. The pendulum, swinging in inertial space, would rotate without putting any forces on it. This is because, not the pendulum is rotating, but the Earth, on which the observer is standing. Waiting long enough and you will see the pendulum plane of motion rotating. I recommend you to go and see one (see list), but because you are currently sitting behind your computer, I have a movie:

This video shows the slow swinging motion of the pendulum. I did not film long enough to show you the rotation. Also I think those large ventilation shafts in the floor disturbed the free-motion of the pendulum, because in the evening the plane of motion was in the same direction as in the morning. It was a little bit of a drawback, but I think it is still a cool way to show that the Earth is rotating, just turn off the air conditioning.

maandag 4 november 2013

Secrets of the deep ocean (a Vening Meinesz story)

No, this is not a blog about some random Sci-Fi movie, it really is a blog about the secrets of the deep ocean (sorry guys, no shiny sphere that gives you special powers when you touch it). Already as a kid I was fascinated about large structures in the deep depths of our oceans. I will show you some cool geological features and their origin in the deep depths of the South Atlantic Ocean. Fortunately, Vening Meinesz, the geophysicist I am talking about now for some time, sailed over this huge ocean onboard the K18 submarine (yes, yes, I know, again I am spamming you guys about this very interesting and cool scientist). 

After visited several ports along the eastern South American coastlines, the K18 and its occupants started from Mar del Plata the difficult crossing of the South Atlantic Ocean. Vening Meinesz wrote in Volume III of his "Gravity Expeditions at Sea", the following about this crossing:

The crossing of the Atlantic from Mar del Plata to Cape Town was a strenuous undertaking; it took us twenty-six days and during the first half part we three times ran into bad weather. Twice we struck a strong gale lasting a few days and the writer had to interrupt his observations because of the impossibility to dive in these rough seas. He nevertheless could make fifty-two gravity observations during this crossing. About halfway we touched for a few hours at the island of Tristan da Cunha but the time was too short and the landing difficulties too great for allowing Holweck-Lejay (another land-based gravity measurement device) observations on the island. He could, however, make a pendulum observation near the island over a depth of 1415 m and this result, combined with those obtained during the end of our preceding and the beginning of our further trip, gives valuable information about the isostatic equilibrium of the island. We likewise could make a sounding profile of the submarine slope; it will be given in the next volume of this publication.
     During the second half of the passage the route was chosen to cross the Walfish Ridge and a detailed gravity profile combined with soundings could be made. The ridge proved to be double here; the sounding profile will be represented in Volume IV. Near the African continent the route was directed towards a submarine promontory shown by the charts but we could not find it where it was plotted. The new edition of the Monaco Bathymetric Chart for the South Atlantic, which already contains our sounding profiles, has been rectified in this regard. 

So 52 valuable observations of the gravity field of Earth were made during this crossing, despite two strong gales. I made a figure of his and the sonar crew's measurements done during the crossing.

Click on the figure for a larger image. The black dots in the maps are the location of the measurements. You can clearly see the two stormy events, where it was impossible to dive (the lack of black dots). The top map shows the gravity anomalies modeled by current information and the bottom map shows the height and depth of the Earth. The two profiles underneath are the 2D representation of the gravity anomaly and depth, respectively, of the profile of the voyage. Again you can see the great accuracy of both Vening Meinesz's observations as well as the sonar depth soundings of the crew of the K18 submarine. 

Some cool features can be seen in these images. Despite the height of continents (orange to red in height maps), no large positive gravity anomalies are observed. This was, according to Vening Meinesz (and also according to me), proof that continents are in isostatic equilibrium. Furthermore, you can clearly see the spreading ridge in the middle of the ocean. This is one of the important observations that proves plate tectonics is a good representation of what is going on with the surface of the Earth. Vening Meinesz did not like this theory (in 1930 this theory was considerate to be fringe science), due to the second strong gale he failed to observe this crucial piece of evidence. Maybe, if the wind gods were more favorable, he had to change his mind about plate tectonics. However, if you look at the sonar depth measurements, it is noticeable that there is this symmetry in height (don't look at the short wavelength features). The deepest depths are observed close to the continental coasts and traveling towards the middle of the oceans, the height of the ocean floor goes up almost 1.5 km. This can be explained by plate tectonics very well (maybe some other time, otherwise google it!).

He writes about the second half of the crossing that they sail over the Walfish Ridge (Walvis Ridge mister Vening Meinesz, it is a South African name). I have already written a few words about this part of the voyage, its just one of my favorite deep sea structures on Earth. The Walvis Ridge is seen both in the bathymetry as the gravity anomaly maps, revealing a NorthWestern ridge towards the African continent. As discussed in one of my previous posts, these are remnants of the still active hot spot of Tristan da Cunha. Vening Meinesz sailed over a part of the Walvis Ridge that had a double ridge. This extra feature was created in the time (70 Ma) that the spreading ridge began to migrate westward. It is however (for me) unclear what happened 70 Ma ago. As we go back in time the continents Africa and South America move further towards each other, eventually connecting at the spreading ridge. It is concluded that the continents broke up astride, or in close proximity to the upwelling plume (which is now the hotspot) (O'Connor & Duncan, 1990). Large flood basalts are found both in Namibia (African) and Brazil (South America) that can be connected to the same upwelling plume. On the West part of the South Atlantic, another large deep ocean structure is visible, the Rio-Grande Rise. All these observations give more than enough evidence for the hot spot and the movement of oceanic plates over this upwelling plume. 

Vening Meinesz could not have deduced this from his measurements, but he did see the deep ocean structures with his device. This proved that global gravity measurements of Earth were very important and should be included in any geophysical research. Satellite missions like Champ, Grace and GOCE have since revealed more secrets of the deep depths of the oceans than any nerdy kid could dream off. 


J.M. O'Connor, R.A. Duncan, 1990, Journal of Geophysical Research, VOL. 95, NO. B11, PAGES 17,475-17,502