PSN-L Email List Message

Subject: Re: Detecting micro-earthquakes
From: ChrisAtUpw@.......
Date: Fri, 10 Feb 2006 13:28:54 EST


In a message dated 10/02/2006, rsparks@.......... writes:

While I  have become familiar with my own seismometer and capabilities, I am 
looking at  a different world when looking at your files. First is the 200 
samples  per second which provides resolution of events as high as 100 Hz. That 
is  very good.
Hi Jon,
 
    Your amplifiers have a cut-off at ~20 Hz. The  minimum sample rate is 40 
sps. 200 sps is not necessary and could use a lot of  memory. Can you use 60 
to 100 sps?

Next is  the 4.5 Hz geophone which has a peak response at about 4.5 Hz. I 
expect  it to detect local quakes very well. The expectations for large distant 
quakes  are less.
    It will not have a peaked response if it is  correctly loaded. There 
should be a ~2.7 K Ohm resistor soldered across the  output terminals of each 
geophone.

What  surprised me was the large amount of energy found at 10 Hz, declining 
up  to about 20 Hz. I did not expect to see that when I applied the FFT 
algorithm  from Winquake to each file, and for the most part, to each subdivided time 
 period within each file. What I expected to see was the ocean wave noise  at 
0.2 Hz but at a much reduced peak compared to my own seismometer which has  a 
natural period of about 3 1/5 to 4 seconds.   
    You don't see microseisms at about 6 sec with a 4.5  Hz geophone. The 
response will be down by a factor of over 700. If you are  on the coast you will 
likely see higher frequency noise associated with breaking  waves. 

I am  curious to know if you can explain the 10 Hz energy source? Also ocean 
waves?  Maybe a nearby flag pole and flag waving in the wind?
    The Strouhal number is between 0.2 and 0.4,  depending on the 'bluntness' 
of the object; S=fxD/v, where f is the vortex  shedding frequency, D is the 
width of the object and v is the velocity. Vortices  are shed alternately from 
either side of the object. The force on an object is  at f/2 Hz, so a strong 
20 m/sec, 76 kph wind will give D = 20 / 100, D  ~ 0.2 m. 
    I suspect that the 10 Hz could be resonances  in air ducts in a central 
heating system? Maybe a fan? The velocity of  sound is about 330 m/s, so 10 Hz 
has a wavelength of about 33 m. Do you have any  ducts or corridors about 33, 
16.5 or 8.25 m long?
 
    To see the response, choose a section of  recording with no quake and 
click on the FFT tab in the top tool  bar. You will see a plot of the amplitude 
against the period. 

The  second surprise was the large noise count, up in the +/- 300 unit range. 
 It is nothing to be alarmed about, only different from my system. I wonder  
what the noise floor is when the seismometer is disconnected from the  
electronics?


You need to substitute 390 Ohm metal film resistors  for the geophones to do 
this test. Can you buy electronic components  easily?
 
    Regards,
 
    Chris Chapman





In a message dated 10/02/2006, rsparks@.......... writes:
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>While I=20 have become familiar with my own seismometer and capabilities, I am lookin= g at=20 a different world when looking at your files. First is the 200 sample= s=20 per second which provides resolution of events as high as 100 Hz. Tha= t is=20 very good.
Hi Jon,
 
    Your amplifiers have a cut-off at ~20 Hz. The=20 minimum sample rate is 40 sps. 200 sps is not necessary and could use a lot=20= of=20 memory. Can you use 60 to 100 sps?
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>Next is=20 the 4.5 Hz geophone which has a peak response at about 4.5 Hz. I expe= ct=20 it to detect local quakes very well. The expectations for large distant qu= akes=20 are less.
    It will not have a peaked response if it is=20 correctly loaded. There should be a ~2.7 K Ohm resistor soldered across the=20 output terminals of each geophone.
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>What=20 surprised me was the large amount of energy found at 10 Hz, declining= up=20 to about 20 Hz. I did not expect to see that when I applied the FFT algori= thm=20 from Winquake to each file, and for the most part, to each subdivided time= =20 period within each file. What I expected to see was the ocean wave no= ise=20 at 0.2 Hz but at a much reduced peak compared to my own seismometer which=20= has=20 a natural period of about 3 1/5 to 4 seconds.  
    You don't see microseisms at about 6 sec with a= 4.5=20 Hz geophone. The response will be down by a factor of over 700. If you=20= are=20 on the coast you will likely see higher frequency noise associated with brea= king=20 waves. 
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>I am=20 curious to know if you can explain the 10 Hz energy source? Also ocean wav= es?=20 Maybe a nearby flag pole and flag waving in the wind?
    The Strouhal number is between 0.2 and 0.4= ,=20 depending on the 'bluntness' of the object; S=3DfxD/v, where f is the vortex= =20 shedding frequency, D is the width of the object and v is the velocity. Vort= ices=20 are shed alternately from either side of the object. The force on an object=20= is=20 at f/2 Hz, so a strong 20 m/sec, 76 kph wind will give D =3D 20 /=20= 100, D=20 ~ 0.2 m.
    I suspect that the 10 Hz could be resonanc= es=20 in air ducts in a central heating system? Maybe a fan? The velocity of=20 sound is about 330 m/s, so 10 Hz has a wavelength of about 33 m. Do you have= any=20 ducts or corridors about 33, 16.5 or 8.25 m long?
 
    To see the response, choose a section of=20 recording with no quake and click on the FFT tab in the top t= ool=20 bar. You will see a plot of the amplitude against the period. 
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>The=20 second surprise was the large noise count, up in the +/- 300 unit ran= ge.=20 It is nothing to be alarmed about, only different from my system. I wonder= =20 what the noise floor is when the seismometer is disconnected from the=20 electronics?
    You need to substitute 390 Ohm metal film resis= tors=20 for the geophones to do this test. Can you buy electronic components=20 easily?
 
    Regards,
 
    Chris Chapman

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