In a message dated 02/12/05, gmvoeth@........... writes:

> Maybe one of you seismic gurus can invent a tornado detector
> using a seismometer and a fft program somehow.

Hi Geoff,

       Two US scientists were publicised as trying to do just that shortly 
after this severe tornado. You ought to be able to pick up severe turbulence and 
local thunder, but maybe the 10 Hz cutoff of most seismic systems will limit 
you. The seismic background noise certainly increases as a cold front or a 
storm system passes by. Deciding what is an incipient tornado and what are 
'normal' storm system noise signals may be more difficult.

       You will also see a lot of wind related noise. Any structure 
protruding into the airflow generates a string of von Karman votrices downwind, to a 
distance over 100 x the height of the object. The frequency is proportional to 
the windspeed and inversely proportional to the width of the object. This 
applies from 'singing' phone wires to whole mountains. Trees and tall buildings 
couple their oscillations into ground motion at seismic frequencies.

       You can also couple up an amplifier + audio recorder to a geophone and 
actually listen to the environmental and other signals in the ground. When I 
get an unexpected / unexplained background signal, I 'listen in'. I can hear 
the tyre noise of cars on the main road from over 1,000 yards away. An 
apparently random, but fairly large local signal turned out to be when heavy lorry 
tyres ran over a depressed water drain in the road. Wind noise from trees are all 
too frequent events.

       You might have better luck for tornado warning using a large area 
Infrasound microphone, similar to those used for CTBT detectors. Tornadoes 
generate a lot of LF / VLF noise. If you buy one of the 60 to 80 nF capacity Piezo 
Sounder disks and use a FET input opamp with a 100 M Ohm input, you should get 
down to below 0.03 Hz, no problem. Piezo Sounder disks are made to convert 
electrical signals into sound waves, but they work very nicely converting LF sound 
waves into electrical signals!  

       A two Litre thermos filled with low density polyurethane foam makes 
quite a good backing volume. You need to provide a capillary leak into the 
flask, so that very slow atmospheric pressure changes do not build up over time. 
The response of the disk will be from several hundred Hz down to the low 
frequency cutoff due to the opamp input RC time constant. 

       You can fit a low pass filter to the input. This may consist of a 
rigid container, like a couple of large glass drinks bottles with a length of tube 
on the input and the output to set the roll off frequency. The direct analogy 
is a resistor connected to a capacitor followed by a resistor to the backing 
volume capacitor. Ideally, the filter capacity would be several times the size 
of the backing volume. These sort of systems are used in gliders for 
variometer lift / sink air pressure change indication, filtering out much of the 
higher frequency air turbulence signals.

       Incidentally, you can make a quite useful seismic sensor by fitting a 
weight to the centre of one of these disks. With a FET input opamp, they can 
out perform a geophone.

       Regards,

       Chris Chapman
       
        
In a message=20=
dated 02/12/05, gmvoeth@........... writes:



Maybe one of you seismic gu=
rus can invent a tornado detector

using a seismometer and a fft program somehow.




Hi Geoff,



       Two US scientists were publicised a=
s trying to do just that shortly after this severe tornado. You ought to be=20=
able to pick up severe turbulence and local thunder, but maybe the 10 Hz cut=
off of most seismic systems will limit you. The seismic background noise cer=
tainly increases as a cold front or a storm system passes by. Deciding what=20=
is an incipient tornado and what are 'normal' storm system noise signals may=
 be more difficult.



       You will also see a lot of wind rel=
ated noise. Any structure protruding into the airflow generates a string of=20=
von Karman votrices downwind, to a distance over 100 x the height of the obj=
ect. The frequency is proportional to the windspeed and inversely proportion=
al to the width of the object. This applies from 'singing' phone wires to wh=
ole mountains. Trees and tall buildings couple their oscillations into groun=
d motion at seismic frequencies.



       You can also couple up an amplifier=
 + audio recorder to a geophone and actually listen to the environmental and=
 other signals in the ground. When I get an unexpected / unexplained backgro=
und signal, I 'listen in'. I can hear the tyre noise of cars on the main roa=
d from over 1,000 yards away. An apparently random, but fairly large local s=
ignal turned out to be when heavy lorry tyres ran over a depressed water dra=
in in the road. Wind noise from trees are all too frequent events.



       You might have better luck for torn=
ado warning using a large area Infrasound microphone, similar to those used=20=
for CTBT detectors. Tornadoes generate a lot of LF / VLF noise. If you buy o=
ne of the 60 to 80 nF capacity Piezo Sounder disks and use a FET input opamp=
 with a 100 M Ohm input, you should get down to below 0.03 Hz, no problem. P=
iezo Sounder disks are made to convert electrical signals into sound waves,=20=
but they work very nicely converting LF sound waves into electrical signals!=
  



       A two Litre thermos filled with low=
 density polyurethane foam makes quite a good backing volume. You need to pr=
ovide a capillary leak into the flask, so that very slow atmospheric pressur=
e changes do not build up over time. The response of the disk will be from s=
everal hundred Hz down to the low frequency cutoff due to the opamp input RC=
 time constant.=20



       You can fit a low pass filter to th=
e input. This may consist of a rigid container, like a couple of large glass=
 drinks bottles with a length of tube on the input and the output to set the=
 roll off frequency. The direct analogy is a resistor connected to a capacit=
or followed by a resistor to the backing volume capacitor. Ideally, the filt=
er capacity would be several times the size of the backing volume. These sor=
t of systems are used in gliders for variometer lift / sink air pressure cha=
nge indication, filtering out much of the higher frequency air turbulence si=
gnals.



       Incidentally, you can make a quite=20=
useful seismic sensor by fitting a weight to the centre of one of these disk=
s. With a FET input opamp, they can out perform a geophone.



       Regards,



       Chris Chapman