PSN-L Message - Subject: Re: My first Sensor

In a message dated 21/09/2006, tchannel@.............. writes: Hi, I am building a horizontal pendulum sensor, my first attempt. I built my own amp and filter. I configured the Low pass filter to Low pass 2.5Hz. Before I installed the filter, the sensor would pick up footsteps or stomps on the concrete floor, now nothing creates a trace, except If I blow on the beam it develops a nice trace. No earthquakes since I installed the filter so I don't know if its going to work. Hi Ted, I always pays to ask! What sort of period have you selected for the pendulum? Have you set up the damping to near 0.7 critical? Are you using oil or magnetic damping? 1 I could use some advise, if the amp and filter seem to be okay or if I have over done something. 2.5 Hz lowpass is on the low side. It is more common to use 10 Hz for quiet locations and maybe 5 Hz for urban sites. The filter actually delays the signal passing through it and you want to keep this to less than 0.2 sec. What type of filter are you using and how many poles? Teleseismic quakes usually have P waves of about 1 Hz and S waves fo about 0.5 Hz. Regional quakes usually have P waves less than 3 Hz, but local quakes may have components over 20 Hz. The earth absorbs the higher frequencies preferentially. 2 Another issue is, I am overwhelmed with confusion about "Seconds vs Hz" I understand how to work with them and what they mean. But the more I read the more confused I get. The events come in low frequencies, 10 to 20seconds is that 0.1Hz. and .05Hz? Does that make everything above those Hz, or should I say, below those periods unwanted? You can see I need help. Correct. It is just easier to use 10 sec as opposed to 0.1 Hz. 3 Using AmaSies, what values do I set the Low pass filter, and what values do I set the high pass filter, for enhancing an enlarged detail trace? Do I use both a low and high together.........Some examples would be helpful. The little I have use them has just been trial and error, to get the best picture. Since you have a real filter at 2.5 Hz, there isn't much point setting the lowpass filter above this.You may wish to leave the high pass filter unselected. The P and S waves may have quite low amplitudes. It is common to pick up the large amplitude Love waves over 10 sec and then search back for the P and S waves. You get a microseism background from the ocean between 4 and 8 sec, so you may want to filter above and below this. Can you take an unfiltered trace of background and call up an FFT plot of amplitude versus frequency? The ocean background peak should show up clearly. The period is a bit different for the west and east coasts. In the middle of the USA, you may see two peaks. Regards, Chris Chapman

In a message dated 21/09/2006, tchannel@.............. writes:

<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>

Hi, I am building a horizontal pendulum s= ensor,=20 my first attempt. I built my own amp and filter. I configured=20= the=20 Low pass filter to Low pass 2.5Hz. Before I installed the filter, th= e=20 sensor would pick up footsteps or stomps on the concrete floor, now nothin= g=20 creates a trace, except If I blow on th= e beam=20 it develops a nice trace. No earthquakes since I installed the= =20 filter so I don't know if its going to work.

Hi Ted,

I always pays to ask!

What sort of period have you selected for the=20 pendulum? Have you set up the damping to near 0.7 critical? Are you using oi= l or=20 magnetic damping?

1 I could use som= e advise, if=20 the amp and filter seem to be okay or if I have over done=20 something.

2.5 Hz lowpass is on the low side. I= t is=20 more common to use 10 Hz for quiet locations and maybe 5 Hz for urban=20 sites. The filter actually delays the signal passing through it and you= =20 want to keep this to less than 0.2 sec.=20

What type of filter are you using and how many=20 poles?

Teleseismic quakes usually have P waves of abou= t 1=20 Hz and S waves fo about 0.5 Hz. Regional quakes usually have P waves less th= an 3=20 Hz, but local quakes may have components over 20 Hz. The earth absorbs = the=20 higher frequencies preferentially.

<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>

2 Another issue is, I am overwhelme= d with=20 confusion about "Seconds vs Hz" I understand how to work with them a= nd=20 what they mean. But the more I re= ad the=20 more confused I get. The events come in low frequencies, 10 to=20 20seconds is that 0.1Hz. and .05Hz? Does that make= =20 everything above those Hz, or should I say, below those periods=20 unwanted? You can see I need help.

Correct. It is just easier to use 10 sec as opp= osed=20 to 0.1 Hz.

<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>

3 Using AmaSies, what values do I s= et the=20 Low pass filter, and what values do I set the high pass filter, for enhanc= ing=20 an enlarged detail trace? Do I use both a low and high=20 together.........Some examples would be helpful. The little I=20= have=20 use them has just been trial and error, to get the best=20 picture.

Since you have a real filter at 2.5 Hz, there i= sn't=20 much point setting the lowpass filter above this.You may wish to leave the h= igh=20 pass filter unselected. The P and S waves may have quite low amplitudes.&nbs= p;It=20 is common to pick up the large amplitude Love waves over 10 sec and then sea= rch=20 back for the P and S waves.

You get a microseism background from the ocean=20 between 4 and 8 sec, so you may want to filter above and below this. Can you= =20 take an unfiltered trace of background and call up an FFT plot of amplitude=20 versus frequency? The ocean background peak should show up clearly. The peri= od=20 is a bit different for the west and east coasts. In the middle of the USA, y= ou=20 may see two peaks.

Regards,

Chris Chapman

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