PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Infrasound research activities in Kochi University of Technology– Infrasound observation network by using multiple comprehensivesensors and its application for disaster mitigation as well as safe
human life Masa-yuki YAMAMOTO1
1Kochi University of Technology, Japan

ABSTRACTInfrasound research activities in Kochi University of Technology will be presented. Infrasound observation
network by using multiple comprehensive-type infrasound sensors was established recently in Kochi, Japan.
Main purpose of the sensing network is for disaster mitigation/prevention mainly for tsunami, indicating agood application of infrasound sensing. If we can detect infrasonic waves before coming real tsunami waves
to sea shore, it can effectively be used for saving human lives from the flood of sea water. However, thesensors can also detect many types of natural and artificial low-frequency audible sound and infrasound. Itmeans the network sensing system can be applied to safe human life by means of emergency level alert tolocal citizens like weather forecasts. Moreover, we recently tried to evaluate wind turbine low-frequency
audible/infrasonic noise affection for human health. In a vicinity of two wind turbines we measured soundfield environment by using audible/infrasonic sensors together with biological information sensing systemwith multiple methods with respect to more than 30 human subjects/testers. With recent progress of
biological information sensor technology, we can use them freely outside but many kinds of artifacts can also
affect real signals of heart beat and brain wave measurements like our noise hearings.
Keywords: Infrasound, Sensor network, Tsunami, Disaster mitigation, Wind turbines

1. INTRODUCTION
Infrasound is known as large wavelength sonic wave that frequency is below the human audible
lower limit of 20 Hz. The infrasound can be used for possible remote-sensing in atmosphere especiallyfor the huge scale explosive events. In the infrasound laboratory in Kochi University of Technology, it
has been used for detection of Hayabusa reentry in 2010 (1) as well as some icy environmental proxymonitoring in Antarctic (2), for example.
If we imagine the infrasound of 1 Hz with assuming the sound speed of 340 m/s, the wavelength
becomes also 340 m hence the half-wavelength becomes 170 m. If we imagine again that a massiveiron plate with 170 m square is suddenly uplifted, the plate can push the air on the plane to upward thenit makes infrasound of 1 Hz because of the resonance between the scale size of uplifted plate and thehalf-wavelength of the sound. It is like a huge drum or a huge speaker on the surface of the plate.
Therefore, in an analogy to the larger scale size, if we have any geophysical events like landslide,avalanche, thunder, volcanic eruption, earthquake, and tsunami, resonance infrasound can effectivelygenerated on their surface and propagate in the atmosphere. As the infrasound can propagate with thespeed of sound as same as the audible sound, it can be used as the remote sensing in the Earth’satmosphere. Moreover, infrasound or low frequency sound itself hascharacteristics of long distantpropagation because damping/attenuating by molecule viscosity is inverse proportional to a square offrequency. In nature, it was observed that the infrasound induced by tsunami just after the main shockof the 3.11 Tohoku earthquake (M=9.0) in 2011 propagated more than half globe of the Earth. Here weshow a possibility of using infrasound for disaster mitigation for estimating source region and energyof some geophysical events. We currently tried to construct a prefectural level dense infrasound sensor
network in Kochi, Japan for testing such activities.
Another topic is for investigating the wind turbine noise in infrasound and low-frequency audible
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Yamamoto.masa-yuki@kochi-tech.ac.jp

range. The infrasound sensor can be used with some biological information sensing devices that caneasily in use outdoors. We investigated the correlation between the degrees of stress/relaxation at theboth sites of experimental places with/without wind turbines for 29 subject persons for the experiment.

2. DENSE INFRASOUND SENSOR NETWORK FOR DISASTER MITIGTION
2.1 Silent Sound Generated by a 100 km Scale Loud Speaker on Ocean Surface
When the huge earthquake happens at the deep location in the crust of the Earth beneath the seabed,
uplift or subsidence is occurred, then it makes also uplift or subsidence on ocean surface, triggering thetsunami. As the speed of tsunami wave is proportional to the square root of the ocean depth, it canbecame effectively slow when it reaches continental shelf to sea shore region and sea water coming
afterward can catch up the former water then makes the wave amplitude higher and attack the sea coast
as a severe disaster. At the same time, induced infrasound can propagate in the atmosphere to the sea
coast with a speed of sound without any slowdown (Figure 1). Thus, if we can observe the variation of
the infrasound signal at along the coast lines, some information of the tsunami source area might beobtained before the tsunami attacks. Since the region of uplift/subsidence is about 100 km scale for thehuge scale tsunami, 1 wavelength of resonant infrasound should be about 200 km, namely 1 period of
600 s. The frequency of this infrasound is 1/600 Hz, about 1.6 mHz.
In order to detect the infrasound induced by the tsunami source area, we developed an infrasoundsensor with SAYA Co. in 2015. The specification of the SAYA ADXII-INF01C sensor is as shown in
Figure 1. Important thing is to measure 1 mHz order pressure change for tsunami infrasound, whereas
if the cases of volcanic eruptions, for example, frequency range is about 2-order higher than that oftsunami because of the scale size of a volcanic crater. It is like an Earth scale pipe organ.

Figure 1 – Infrasound sensor and possible mechanism of infrasound generation by tsunami
2.2 Sensor Network Distribution in Japan
When we imagine the detection of the tsunami induced infrasound signal for disaster mitigation,
dense sensor network should be constructed depends on the scale of corresponding geophysical events.
As for the tsunami disaster mitigation we need to expand our infrasound sensors as in a scale of 25 km,
therefore we designed 3-scale arrayed distribution for dense sensor network in our region. Here, weconstructed 2 sets of 2 km scale arrays, some sets of 8 km scale ones, and 2 sets of 25 km scale ones.
Inside Kochi prefecture shown in an orange rectangle in Figure 2, we distributed 15 infrasoundsensors in 2017. The sensor network was spread to the other area in Japan as is also shown in the figure(yellow filled circles). With combining multiple-site infrasound observations, it can be expected thatan appropriate scale and shape of the tsunami uplift region will be able to be measured with using theremote infrasonic sounding. Currently we are trying to know the infrasound signal pattern to bemeasured at any of the expecting observatories in Japan area by using the precise seabed topographic
map information with having the largest assumption of the Nankai Trough earthquake.

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The sensor itself has a measurement function not only of the infrasound sensor with 5 Hz sampling
but also of MEMS 3-axis accelerometer, thermometer, barometer, and GPS time, thus we call it acomprehensive infrasound sensor. The data handling by using a network rooter with a SIM card isapplied for usual operation, which can be confirmed on web pages in quasi real time (3).

Figure 2 – Infrasound sensor network established in Japan as of May 2019

Figure 3 – Infrasound signal of Mt. Aso volcanic eruption by arrayed sensor network in Kochi
2.3 Infrasound sensing and simulation in upper atmosphere
In 2017, we developed a small microphone type infrasound sensor SAYA INF03 in order to try tomeasure the infrasound aboard a large balloon of ESA BEXUS-24 launched from ESRANGE, Sweden
and successfully measured low-frequency audible sound and infrasound up to 11 km. Then, theinfrasound microphone type infrasound sensor was extended to the rocket-borne type and successfully

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launched on May 4, 2019 to the thermosphere at about 113 km by using the first venture company
sounding rocket (Interstellar Technologies, Momo3) reached to the space (above 100 km) in Japan.
The infrasound sensor detected fluctuated sound pressure change induced by fireworks that wereexploded at about 100 m from the ground prior to the rocket launch.
As for the sound propagation in the Earth’s atmosphere, it has been investigated since the space era
from 1950’s. Currently, we have already studied on atmospheric parameter profiles in basics and some
atmospheric model can also be used for scientific researches. However, if we try to investigate theprecise propagation process of the sound/infrasound in the middle and upper atmosphere. It is clearlyaffected by the vertical profile of the local wind velocity and direction as well as atmospherictemperature.
We investigated some of the dataset of infrasound signal observed at Kochi area during
2016-2018, with searching any volcanic eruption signals detected on the multiple-site infrasoundrecords. Moreover, we have developed a sound propagation model in 3D space with using someavailable model atmosphere with changing local wind profiles. The comparison between thesimulated infrasound propagation and the real observation datasets provides us some fruitful
information in sound propagation mechanism in rafefied upper atmosphere.
Grid search algorithm was also applied to the volcanic eruption datasets previously observed in
Kyushu Island by using 3 infrasound sensors, and successfully found the location of Mt. Sakurajima
with the developed algorithm (4).
2.4 The other application of infrasound sensing
As for the other application of the infrasound sensing, we recently tried to obtain environmental
information about the infrasound generated at the wind power plant and propagated in the atmospherein the vicinity of the wind turbines. The wind turbine is known as one of the existing artificial
sound/infrasound source, however, due to the limitation of available infrasound sensors, theinformation about the wind turbine infrasound is limited. In the previous studies, even in the situationof lower amplitude of the low-frequency sound range rather than the ISO sensitivity curve of thehuman audible sound, some reports described possible effect of the low-frequency sound or infrasoundon the human health, nevertheless the sound cannot be listened to.
We developed a recorder system with using an electroencephalography (EEG) sensor for brain
waves as well as the pulse wave sensor (PWS). Since the both of EEG and PWS are low-cost andportable sensors, we can use the recorder system with the both sensors outdoor, thus we can conduct
biological information sensing at the appropriate places with/without wind turbines (5). The result israther variated than that we expected before and we have to summarize it with having medium values
for 29 subjects, however, the slight tendency of the infrasound influence was found although it might
be still argued.

3. CONCLUSIONSIn this paper we introduced the recent research activities with infrasound sensing in Kochi
University of Technology. We developed some types of infrasound sensors with SAYA Co. since 2015
and distributed 30 comprehensive infrasound sensors of ADXII-INF01C, etc. with making arrayedsensor network in 2 km, 8 km, and 25 km scale in Kochi in 2017. Many geophysical events as volcanic
eruptions, thunder storms, typhoon passages, meteorites, etc. have been recorded in multiple-sitearrayed observation and successful analyses of source direction/position finding by using grid searchalgorithm as well as comparison to 3D simulation of infrasound propagation in middle and upperatmosphere.

ACKNOWLEDGEMENTS
The author is grateful to Mr. A. Yokota for developing several types of infrasound sensors as well as
Drs. H. Saito, Y. Kai and K. Nakajima for providing their precise simulation results in infrasoundpropagation in upper atmosphere and tsunami infrasound generation and its propagation. The authorand his colleagues in infrasound laboratory, Kochi University of Technology express their sincere
gratitude to all the collaborators in infrasound observation network in Japan. The author thanks Dr. Y.
Kakinami for infrasound sensor operation in Hokkaido. This research is partly supported by funding
agencies/programs of SECOM Science and Technology Foundation, MEXT/JSPS KAKENHI
Grant-in-Aid for Scientific Research (B) No. 17H02062, MIC/SCOPE (175009003)

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REFERENCES
1. Yamamoto M.-Y, Ishihara Y, Hiramatsu Y, Kitamura K, Ueda M, Shiba Y, Furumoto M, Fujita K.
Detection of Acoustic/Infrasonic/Seismic Waves Generated by Hypersonic Re-Entry of theHAYABUSA Capsule and Fragmented Parts of the Spacecraft. Publications of the Astronomical Societyof Japan. 2011;63(5):971-8.
2. Yamamoto M.-Y, Ishihara Y, Kanao M. Infrasonic waves in Antarctica: A new proxy for Monitoring
Polar Environment. International Journal of Geosciences. 4:797-802.
3. Infrasound Laboratory, Kochi University of Technology. Kochi University of Technology InfrasoundObservation Network System. 2018; http://infrasound.kochi-tech.ac.jp/infrasound/graph.php.
4. Saito H, Sorimachi R, Yamamoto M.-Y. Grid search method estimating wave source positions. JpGU
2018; May 2018; Chiba, Japan.
5. Nagamatsu M, Yamamoto M.-Y. Influence evaluation of infrasound by using both of biologicalinformation and infrasound sensors in the vicinity of wind turbine facilities. Proc ICA 19; 9-13
September 2019; Aachen, Germany.

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