vrijdag 16 mei 2014

Gravity Art: "The Face of the Earth viewed by a Gravity Scientist"

Last week I visited a conference which had the theme: "The Face of the Earth". At the conference were very inspiring talks and interesting scientists. It gave me new inspiration for my blog and my PhD research. It also gave me artistic creativity. So enjoy my art work titled: "The Face of the Earth viewed by a gravity scientist!" (click on figures to enlarge)

The figure represents the free air anomaly of the Earth. This anomaly is the deviation of the gravity signal from the main ellipsoidal signal (the 9.81 m/s^2 you learned in high school). The colors represent the magnitude of the deviation. So more mass is red and blue means less mass then the main signal (green is zero). But you don't want to talk numbers, you want to enjoy the colors, because that is art all about. The above picture is of course the old continent Africa. You can clearly see some nice features in the gravity field. In the middle of Africa (Congo) is a large blue area, showing the location of the Congo Basin and the old craton beneath it. Also you see the characteristic gravity signal of mid oceanic ridges, where the dynamic mantle pushes material up, creating a gravity high (reddish/orange). Furthermore, the large collision zone between Africa and Eurasia is viewed as large red band of mountainous land. Finally, the subduction zone near Indonesia is seen as a very thin ribbon of blue and red. What else can you see?

Another one I made for my Auntie Down Under:

She wanted to see more colors so I played a little with the colormap. Can you see cool details? For Example, I noticed small red/white blobs just offshore the west coast of Australia. Those are marine volcanos, which I did not know they existed (but now I found them on Google maps). Furthermore, I filtered the effect of mantle convection and deep density anomalies to get a better view on the crustal structures.

Can you see the differences (colormap is the same)?

Let me know what area of Earth you would like to see Gravity Artified, and I will see what I can do!

zaterdag 10 mei 2014

Gravity shows the shape of Earth (I need your help!)

I need your help! A few posts ago, I tried to measure the shape of the Earth using my acceleration chips in my macbook. After a quick sensitivity analyses I found out that the precision of the chips was not sufficient to do this exercise. However, I really want to see if I can measure the curvature of the Earth by looking at its gravity field. So instead of high-tech measurements with my macbook, I want to see if a low-tech solution (a string pendulum) is more accurate.

The basic physics behind a pendulum measurement. The relationship between the period (T) of the pendulum and its length (l) and the local gravity (g) is (yes, high school again):

Well, this is not entirely correct, but for this experiment (keep the amplitude low!!!) it is good enough (take a look here to get a better relationship). So, to measure the local gravity (g) we need to know the length of the pendulum (half-measure!!!) and the period of the pendulum.

So, what is my set-up? A piece of string with a weight of some sort attached to the end, such that it can move freely. Also I have a half-measure to measure the length of the string. For the experts out there, a temperature and air pressure measure instrument, which could be handy to determine the drag of the pendulum.

Schematic of the pendulum experiment obtained from Wikipedia
As we all learned in high school, the amount of mass does not influence the period of a pendulum. However, it does influence the amount of loss due to drag (ballistic coefficient, see my blogpost), so use a weight with substantial mass. I use a mass around 5 kg. Finally, some sort of stopwatch or clock!

Oh, and know at which latitude you do the experiment (as accurate as possible). Find the latitude on a map, GPS device or use Google Earth. We want to see the relation between gravity and latitude!

Lets start the experiment!

  1. Measure the length of the pendulum (rotation point to center of mass of the weight). Do this every measurement, because this could change due to temperature differences. This measurement must be done with a precision of 4 digits. So for a string of 1 meter, the absolute length must be know within a mm.
  2. (This one is for the experts!) Look at the temperature and pressure instruments. Write them down.
  3. Let your pendulum swing! Don't start with a too large angle (losses are strongest), but also not to small (difficult to measure period accurately). Lets say, 5-10 degrees.
  4. Start the stopwatch when the pendulum has no velocity, so at one of its highest points. This is much easier to time exactly, then when the pendulum is moving very fast (in the middle of the swing). Let the pendulum swing 50 times (or 10, however this will not give enough precision in your time measurement) and divide the measure period by 50 (or 10). This reduces the timing error and makes the measurement more precise.
  5. Calculate the measured gravity and repeat this experiment several times (calculate the mean and standard deviation).

My results are listed below:

Latitude: N 51 deg 59' 04.2'' ±0.2''
Height: 7 ±5m

Measurement 1: 9.8155 m/s^2
Measurement 2: 9.8012 m/s^2
Measurement 3: 9.8212 m/s^2
Measurement 4: 9.8126 m/s^2
Measurement 5: 9.8254 m/s^2
Measurement 6: 9.8083 m/s^2

Final gravity measurement: 9.814 ± 0.0087 m/s^2 (one standard deviation)

Now It is up to you! Can you do the same and send me your measurements? Lets see if we can get a global coverage (at all latitude) and try to measure the shape of the Earth!

Good luck!