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,
 
    Check http://oceanworld.tamu.edu/resources/ocng_textbook/chapter16/= chapter16_01.htm
    and http://oceanworld.tamu.edu/resources/ocng_textbook/chapter16/= chapter16_04.htm
 
    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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