PSN-L Email List Message
Subject: Re: Instrument quality - microseisms
From: ChrisAtUpw@.......
Date: Thu, 22 Oct 2009 19:32:59 EDT
In a message dated 22/10/2009 15:12:02 GMT Daylight Time,
lconklin@............ writes:
The other thing I would love to hear an explanation for is why the
microseism noise is so narrow band. When it is strong, it looks virtually
sinusoidal on the monitor, and an FFT of the data shows a very narrow peak. Does
anyone know why that is?
Hi Larry,
Check
_http://oceanworld.tamu.edu/resources/ocng_textbook/chapter16/chapter16_01.htm_
(http://oceanworld.tamu.edu/resources/ocng_textbook/chapter16/chapter16_01.htm)
and
_http://oceanworld.tamu.edu/resources/ocng_textbook/chapter16/chapter16_04.htm_
(http://oceanworld.tamu.edu/resources/ocng_textbook/chapter16/chapter16_04.htm)
Microseisms are generated by ocean waves, wave action on coasts, cold
fronts, storms, hurricanes.... there are several possible causes, some more
local than others. At lower wind velocities, you get a much wider range of
ocean wave periods and low amplitudes - nearer 'noise'. As the wind
velocity and the wave fetch increase, the period increases and the distribution
becomes very much more peaked. It may take 1000 wavelengths to fully develop
an ocean wave. The 'losenges' that you occasionally see are interference
effects from two or more sources. The low amplitude microseisms may have
periods of 5 to 7 seconds. At high amplitudes you may see waves of 10 to 12
seconds with well defined beats. One source on the east coast is likely to be
in the Atlantic south of Greenland. There is a rough patch of sea with a
long fetch about 1,500 km east of Labrador at the moment, with ~20 ft waves.
_http://www.oceanweather.com/data/_ (http://www.oceanweather.com/data/)
Regards,
Chris Chapman
In a message dated 22/10/2009 15:12:02 GMT Daylight Time,=20
lconklin@............ writes:
The=20
other thing I would love to hear an explanation for is why the microseis=
m=20
noise is so narrow band. When it is strong, it looks virtually=20
sinusoidal on the monitor, and an FFT of the data shows a very narrow=20
peak. Does anyone know why that is?
Hi Larry,
Microseisms are generated by ocean waves, wav=
e=20
action on coasts, cold fronts, storms, hurricanes.... there are several po=
ssible=20
causes, some more local than others. At lower wind velocities, you get=20
a much wider range of ocean wave periods and low amplitudes - nearer=
=20
'noise'. As the wind velocity and the wave fetch increase, the period incr=
eases=20
and the distribution becomes very much more peaked. It may take 1000 wavel=
engths=20
to fully develop an ocean wave. The 'losenges' that you occasionally see=
are=20
interference effects from two or more sources. The low amplitude microseis=
ms may=20
have periods of 5 to 7 seconds. At high amplitudes you may see waves of 10=
to 12=20
seconds with well defined beats. One source on the east coast is like=
ly to=20
be in the Atlantic south of Greenland. There is a rough patch of sea with=
a long=20
fetch about 1,500 km east of Labrador at the moment, with ~20 ft=
=20
waves.
http://www.oceanweather.com/dat=
a/
Regards,
Chris Chapman
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