In a message dated 02/01/2010, brett3nt@............. writes:

I agree  that the low frequency peak might very well be the atmospheric 
stuff  proposed by Chris, though verticals are mostly immune to the 
wind-caused  ground tilting that is so annoying with horizontals. 
Hi  Brett,
 
    Sure, but a vertical is sensitive to air density  changes and wind 
generates these only too easily from many Hz downwards.
    
His question about  the pressure cover is a good one.  Normally the 
atmospheric 
noise we  see on the vertical is in the frequency range of minutes and is 
reduced by a  factor of 10 or 20 by adding a sealed cover.  If your 
vertical 
is small  enough to be covered by a cooking pot, that would work nicely.  A 
 
typical setup would be a very solid base slab, like 2-4" thick  
granite  with an inverted pot or similar container sealed down over  
it.  The granite base is because pressure changes will cause anything  much 
less solid to flex under the seismo and generate noise.  Our  experience 
was 
that 3/4" aluminum flexes way too much.  For sealing  things, including 
cable openings, Dave Nelson has found that the putty-like  windshield 
sealant seems to work nicely.
_http://www.detailandstripes.com/3mwirorise08.html_ 
(http://www.detailandstripes.com/3mwirorise08.html)    It sticks 
things together very well, remains workable and is designed to not  squeeze 
out from between the windshield and car frame or in our case the  base slab 
and pressure container.
 
    I can buy a similar product called BluTack or  WhiteTack in flat sheet 
form about 1/8" thick. See 
_http://www.artech-electronics.com/us/products/accesso/blutack.html_ 
(http://www.artech-electronics.com/us/products/accesso/blutack.html)  

If a vertical has any long period sensitivity  at all, it should be 
pressure sealed.



Or alternatively compensated? See 
_http://eost.u-strasbg.fr/alessia/papers/SundaySeismometer.pdf_ 
(http://eost.u-strasbg.fr/alessia/papers/SundaySeismometer.pdf)  for  the Press-Ewing glass float compensated vertical 
seismometer. How about using an  Aluminum Drinks Can?
 
    Regards,
 
    Chris Chapman





In a message dated 02/01/2010, brett3nt@............. writes:
I agree=20
  that the low frequency peak might very well be the atmospheric 
stuff=
=20
  proposed by Chris, though verticals are mostly immune to the 
wind-ca=
used=20
  ground tilting that is so annoying with horizontals. 
Hi  Brett,
 
    Sure, but a vertical is sensitive to air dens=
ity=20
changes and wind generates these only too easily from many Hz downwards.
    
    His ques=
tion about=20
the pressure cover is a good one.  Normally the atmospheric 
noise=
 we=20
see on the vertical is in the frequency range of minutes and is 
reduce=
d by a=20
factor of 10 or 20 by adding a sealed cover.  If your vertical 
is=
 small=20
enough to be covered by a cooking pot, that would work nicely.  A=20

typical setup would be a very solid base slab, like 2-4" thick=20

granite  with an inverted pot or similar container sealed down ov=
er=20

it.  The granite base is because pressure changes will cause anyt=
hing=20
much 
less solid to flex under the seismo and generate noise.  Our=
=20
experience was 
that 3/4" aluminum flexes way too much.  For seali=
ng=20
things, including 
cable openings, Dave Nelson has found that the putty=
-like=20
windshield 
sealant seems to work nicely.
    http://www.deta=
ilandstripes.com/3mwirorise08.html =20
It sticks 
things together very well, remains workable and is designed=
 to not=20
squeeze 
out from between the windshield and car frame or in our case=
 the=20
base slab 
and pressure container.
 
    I can buy a similar product called Bl=
uTack or=20
WhiteTack in flat sheet form about 1/8" thick. See http://www.artech-electronics.com/us/products/accesso/blutack.html&n=
bsp;


    If a vertical has any long period sensiti=
vity=20
at all, it should be pressure sealed.


 
    Or alternatively compensated? See htt=
p://eost.u-strasbg.fr/alessia/papers/SundaySeismometer.pdf for=20
the Press-Ewing glass float compensated vertical seismometer. How about us=
ing an=20
Aluminum Drinks Can?
 
    Regards,
 
    Chris Chapman