A blue helicopter.

Using earthquake detectors, scientists were able to count the blades on this helicopter!

Image: Alain Rioux

Iceland is a remote and beautiful island, brimming with volcanoes. Volcanic eruptions give the Earth an almighty shake, so it’s no surprise that Iceland has lots of earthquake measuring seismometers. But you might be surprised to find out what these instruments are picking up.

Recently, a team of scientists placed several seismometers around an Icelandic volcano. As the data came in, they found some strange vibrations. They were weak and fast, and they seemed to come from very close to the surface of the Earth.

To get mote data, the scientists found recordings from another, permanent earthquake detector about 15 kilometres away. This detector also measured the same vibrations, only it got them four minutes earlier. After some head scratching, the scientists realised they were actually detecting helicopters flying overhead!

The team wondered how much information they could get from these seismic readings of helicopters. Using advanced signal processing techniques, they realised they could work out the speed, direction, altitude and location of nearby helicopters. They could even use their measurements to count the number of blades on each helicopter’s main rotor!

Seismometers are designed to measure earthquakes, but they measure all kinds of vibrations in the Earth. They can detect anything from nearby kangaroos to distant nuclear bomb tests. Once scientists know what each vibration looks like on their instruments, they can filter them out. That means fewer confused scientists, and a much better understanding of how Earth works!

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