Help! A huge oil slick is approaching the coast!
Oil spill monitoring by airborne surveillance
Aircraft from some countries that border the Mediterranean Sea regularly patrol their coast, monitoring ship movements and marine pollution. In open sea, however, surveillance is either very scarce or completely absent.
Where the marine environment is particularly delicate, like in the enclosed Mediterranean, monitoring from space is vital.
The image displayed was taken by the ERS satellite, built and launched by ESA. There is a radar sensor onboard, able to watch the Earth's surface even through clouds and in the darkness. Radar images look similar to photographs, but their interpretation is different (see more information on radar technology).
In a radar image, the ocean is not blue but black and white, and may also appear to have a lot of strange features. As a general rule, the brighter the sea surface, the rougher the sea (because wind speed is higher). But oil on the water dampens the rough surface and it becomes smooth again (and dark, as is shown in the image).
Black and white radar image of the ocean
But our image is coloured! This is because it is composed of 3 images taken on 3 different days. In order to distinguish the information coming from each of these days, each image was given a different colour:
Coloured radar image of the ocean
All images were acquired at the same time of the day - 10:20 GMT. If all 3 images were identical, we would expect a black and white image, because in each point of the image we would find an equal amount of blue, green, and red. And every change would be seen as colour: e.g. a little less blue with constant green and red would turn the colour from grey to yellow, etc.
- 13 Sept 1991 is shown in red
- 19 Sept 1991 is shown in green
- 25 Sept 1991 is shown in blue
The colours in the image are therefore linked to changes in radar response or backscatter within the period of time between the first and last image to be acquired. Although it is a very short period, backscatter from the sea may change rapidly because of wind conditions. The magenta over the sea indicates stronger winds on 13 and 25 September. The horizontal blue/magenta line is a wind-front, bringing a change in weather on 25 September.
Click here to open the lenses exercise.
You can analyse the single black and white images and the map more closely by clicking on the map thumbnail to the left. Be patient, it needs some time to download! When you have finally got it, look out for the four squares in the top left corner. The one with the white corner is the map. Click on the upper left small white corner square and drag the 'lens' near the coast. By dragging the lower right corner you can open the lens more and read where you are: it is a map of the area. Do the same with the red, green, and blue lens. One by one, pick them at the top left corner and drag them slowly over the sea (and over the oil spill). Open them more by dragging the lower right corner.
On which date do you think the oil was spilled? On the bar below the image, you can see the geographical location of the cursor position. Place the cursor in the middle of the oil spill and read the location. By clicking and holding the left button of your mouse and then moving your mouse, you can measure the distance - it is displayed on the bar next to the position. Cool, isn't it? Now you have all the tools necessary to make your observations and measurements and to fill in the form below...
In case you do not see the indications of the cursor on the lower left window bar, you can download the image by clicking here to download the image, and use the LEOWorks software for doing the exercise. There you can split the image into the three originally black and white images (red is of 13 September, green is of 19 September, and blue is of 25 September) and you can look up the position of the cursor (View - Cursor Position/Value) and also measure the distances (Image - Measure tool).
Oil slick approaching the coast
In the lower right corner, you can see another oil slick! It is magenta in colour. On the black and white image of the day the individual satellite image was taken, this slick appears in black! Why? Because the oil dampens the waves and the surface is smooth here. A smooth surface appears dark or black in a radar image.
But oil slicks drift with the current and also with the winds. In our case, the drifting is about 320 metres per hour. The current is drifting west. When will the oil reach the coast? We have to do something!
Complete the exercise (click here for a printable version of the exercise)