Infrasonic sounds in the earth
The rumbling of Mount Etna on Sicily can be measured as far afield as De Bilt. Infrasound from the volcano is very low frequency sound travelling through the atmosphere. It is inaudible to the human ear yet still measurable. So says seismologist Läslo Evers of the Royal Netherlands Meteorological Institute (KNMI), who is researching infrasound and conducts measurements using special equipment. On Wednesday 8 March, he will deliver his inaugural lecture as part-time Professor of Seismo-Acoustics at TU Delft.
Prof. Evers envisions countless new applications for seismo-acoustic data. 'We should not be deaf to what we can all hear'. Sub-oceanic microphones can provide a wealth of information that is difficult to obtain in any other way, such as the temperature of deeper waters.
Prof. Evers is the Netherlands' leading specialist in the field of infrasound. It is something we have all experienced at some time: some distance from a disco or loud party, the high pitches are inaudible and all we can hear are the chest-pounding bass tones. The same applies to infrasound. It can be measured using micobarometers, which register very slight fluctuations in air pressure, and low-frequency microphones, which register sounds lower than 20 Hz. It can also be measured with a strongly amplified loudspeaker in a well-sealed room. That would enable you to register all the signals of supersonic flights above the North Sea, for example.'
The sound-monitoring sub-oceanic microphones and sensors are part of the International Monitoring System (IMS) that was implemented for the verification of the Comprehensive Nuclear Test-Ban Treaty (CTBT). While the primary purpose of this unique monitoring network is to detect any nuclear testing, it can also be used to gather long series of climatological data through deeper ocean monitoring. Together with TU Delft, the KNMI has developed a method of measuring the temperature on the basis of sound waves.
'We can examine whether certain properties of the medium the sound waves travel through change at all over time: the temperature of the deep ocean, for example. Climate researchers are under the impression that the temperature of the atmosphere has not risen in recent years but that the oceans are indeed warming. If that is the case, where is that heat actually going? We know that the speed of sound in the ocean varies significantly depending on the temperature of the water. Thus a sound wave travelling faster indicates a higher temperature in that particular area. Studies of this are currently in progress.’
'Among other things, I am also investigating the loss of ice from icebergs on Greenland. We would like to establish whether the rate of this loss is increasing or decreasing. And there are still more possible applications. We have discovered, for instance, that icebergs are easier to monitor using underwater signals. The position of floating icebergs from satellite observations can easily be lost in Antarctica, for example, where it is often cloudy. And yet icebergs, especially the big ones, pose a real risk to shipping in that area.'
Prof. Evers believes that the TU Delft chair in Seismo-Accoustics offers a multitude of new opportunities for education and research in Applied Geophysics. 'There is a wealth of IMS data that could also be used for research purposes.'
‘In the TU Delft department of Geoscience & Engineering, our research field verges on that of Geoscience & Remote Sensing. We hope to create a bridge between the two departments to enable us to cooperate and/or collectively study sound sources. This is what makes TU Delft such an ideal place to further develop knowledge in this field.’
'To me, the most attractive element of all these techniques is that we merely listen. We do not transmit any signals into the world which subsequently have to be received. It is a passive process: you set up a microphone somewhere on Earth and all you have to do is listen.´
8 March 2017, 15:00, TU Delft Aula building
Inaugural address by Prof. L.G. Evers (Faculty of CEG) Seismo-Acoustics
Contact Prof. Läslo Evers (030) 2206 335, firstname.lastname@example.org, L.G.Evers@tudelft.nl
Harry Geurts, Media Relations Officer, KNMI , (030) 22 06 317, Harry.Geurts@knmi.nl
Roy Meijer, Science Communications Adviser, TU Delft, +31 (0)15 278 1751, R.E.T.Meijer@tudelft.nl