PSN-L Email List Message

Subject: Re: BBC news article about infrasound
From: John Hernlund hernlund@............
Date: Sun, 16 Feb 2003 12:23:07 -0800

This is quite interesting Arie. It reminds me of a talk I saw recently  
by a woman who uses the ionosphere as a seismometer of sorts. I figured  
some of you might be interested in this. The way it works is as  
follows: Small perturbations to the atmosphere just above the ground  
due to passing Rayleigh waves (love waves are pure shear, and can't  
propagate into a gas) propagate as low frequency sound waves upward  
into the atmosphere. Because the atmosphere becomes less dense  
approximately exponentially with height, the sound wave velocity  
decreases. Just like in the Earth, eg. slowing down of the waves due to  
sedimentary basins, etc., causes the waves to increase in amplitude in  
inverse proportion to their change in density. So these very slight  
sound waves at the surface due to Rayleigh seismic waves can obtain  
amplitudes of hundreds of meters in the upper atmosphere where the  
normal gas gives way to ions. This causes the ionosphere to oscillate  
up and down. This latter effect can be measured by either bouncing  
radar waves off the bottom of the ionosphere or using a satellite to  
measure the ion density from above. It is quite amazing how well the  
seismic records and ionospheric records correlate. She even had some  
records indicating that this effect can detect tsunamis as well. This  
atmospheric seismometer is by no means broadband, as there are  
complicated damping effects in the normal and ionized atmosphere. I was  
thinking this could give us a cheap way of measuring seismic surface  
waves on other planets, which is desperately needed to constrain the  
geophysics that we attempt to extrapolate to bodies with little or no  
data. But this is not an easy proposition, as the atmospheres and  
magnetic fields shaping the behavior of such things are profoundly  
different on each planet.

Regarding the propagation of infra-sound through the atmosphere I  
wonder if you have tried to model this as a wave guide? Certainly the  
amplitudes are there. I might expect that this type of thing would be  
very dispersive, as shorter wavelength energy begins cascading into  
longer wavelengths due to the geometric and damping effects in the  
upper atmosphere.


On Saturday, February 15, 2003, at 07:14 PM, Arie Verveer wrote:

> Regarding Infrasound;  over the last year I had been experimenting  
> with the
> detection of infrasound and found the following. The upper zonal winds  
> play
> an important roll in detecting infrasound from a distance source. The  
> sound
> is refracted as it goes through the atmosphere and the zonal winds can  
> act as
> a reflecting layer. (Typically around 50 km ). These zonal winds  
> change direction
> and velocity within the year and thus the detection of  infrasound  
> varies. Using the
> ocean as a infrasound source you can determine when Zonal winds change  
> direction
> ( On a season). Also their is Diurnal wind and local wind's. By the  
> way the earth is
> a
> reflecting layer, so infrasound can bounce between the 50 km region  
> and the earth and
> so propagates around the earth. The lower the frequency the further  
> the wave can
> propagate.
> I used a setup similar to " "  
> with a front end
> amplifier/filter
> and different sensor. You can monitor directly to the atmosphere but  
> wind will, and
> I mean "will" be a major problem. On "NO" wind days the results are  
> good, but let
> face it the
> wind blows more often then it doesn't, especially when the sensor is  
> so sensitive. I
> then
> tried using a acoustic pip array with some success. But my station is  
> located in a
> wooded area
> and the array size was limited. So I tended to record wind and its  
> turbulence. In the
> end I used
> 6 meters of very flexible thin walled silicon tube as the sensor head.  
> This was
> located in a
> building with many vents and dust filters. A pressure bleed was  
> located on the other
> side of
> the detectors diaphragm. Bingo, it worked well to winds unto 10 - 15  
> km per hour.
> So for most of the time I received a good signal. It detected mine  
> blasts some
> 160 km away. First you got the seismic signal the after some 7 to 10  
> minutes you got
> the
> infrasound.  Mainly around 9 minutes as the the signal reflected and  
> refracted in the
> atmosphere.
> I ran this configuration for about a 40 days before the seismic and  
> infrasound
> station was
> closed down.  I do believe you could record infrasound form a local  
> quake if the
> winds were
> right and the quake intensity was moderate. You do record local  
> infrasound from a
> quake
> as the seismic signature passes you. Though the signal to noise /  
> ratio is poor. As a
> thought,
> one could put a weight on the silicon tubing and maybe it could record  
> quakes. I
> wonder
> what the frequency response would be? Just an idea.
> Additionally I have recorded infrasound from meteors and local  
> explosions.
> So if anyone is thinking of setting up an infrasound station, let me  
> know and I'll
> pass on any info I have.
> Cheers
> Arie
> To stop spam my email is now broken in 2 bits. Join this "greensky" to
> "@............"
> without the "  ".
>> "Charles R. Patton" wrote:
>> FYI, some articles about infrasound.
>> main article on volcanoes and nuclear blast monitoring:
>> 2763657.stm
>> pictures of a monitoring site and the detector:
>> a miscellaneous link on infrasound:
>> All the BBC articles have additonal links to further reading.
>> Regards,
>> Charles R. Patton
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