PSN-L Email List Message

Subject: Re: High noise >10s
From: chrisatupw@.......
Date: Mon, 30 Jul 2012 07:42:55 -0400 (EDT)



From: Tiago A. tiagoagre@.........
Sent: Mon, 30 Jul 2012 3:08
Subject: High noise >10s


Hi.

Im using a L4 1Hz geophone and for almost a month im struggling with high n=
oise from 10s up to ..., but from <10s it looks good. The noise comes and g=
oes, this image is self explaining:

http://dl.dropbox.com/u/15802004/noise.png

Changed cables, re centered the geophone, but didint help..the geophone is =
connected directly to a ADC without damping, external amplification or filt=
ering, i only use digital filtering from the ADC. Before this issue start, =
i had a pretty clean spectrum >10s for almost 6 months, which allowed me to=
 received LQ and LR from some strong events.

This sac contains 15minutes with high noise >10s, notice that at 3:37 it re=
corded a Ml2.1 at 70km.
http://dl.dropbox.com/u/15802004/noise.sac=20

Hi Tiago,=20

    I am not sure that I understand what you have written - or what system =
you are using.=20
You MUST damp a 1 Hz geophone correctly to get a flat output over 1 Hz.=20
How are you getting your period expansion below 1 Hz down to 0.1 Hz ?
Any period expansion DEPENDS on the geophone being correctly damped.=20
What is the noise on your ADC - when you connect it to a resistor with the =
same value as the geophone ?=20
You do need a low noise metal film resistor.
What ADC / number of bits, are you using ?=20


Regards,

Chris Chapman
=20

From: Tiago A. tiagoagre@....... com
Sent: Mon, 30 Jul 2012 3:08
Subject: High noise >10s
Hi.

Im using a L4 1Hz geophone and for almost a month im struggling with high n= oise from 10s up to ..., but from <10s it looks good. The noise comes an= d goes, this image is self explaining:

ht= tp://dl.dropbox.com/u/15802004/noise.png

Changed cables, re centered the geophone, but didint help..the geophone is = connected directly to a ADC without damping, external amplification or filt= ering, i only use digital filtering from the ADC. Before this issue start, = i had a pretty clean spectrum >10s for almost 6 months, which allowed me= to received LQ and LR from some strong events.

This sac contains 15minutes with high noise >10s, notice that at 3:37 it= recorded a Ml2.1 at 70km.
ht= tp://dl.dropbox.com/u/15802004/noise.sac 

Hi Tiago,
 
    I am not sure that I understand wh= at you have written - or what system you are using.
You MUST damp a 1 Hz geophone correctly to get a flat= output over 1 Hz.
How are you getting your period expansion below 1 Hz = down to 0.1 Hz ?
Any period expansion DEPENDS on the geophone being co= rrectly damped. 
What is the noise on your ADC - when you connect it t= o a resistor with the same value as the geophone ?
You do need a low noise metal film resistor.
What ADC / number of bits, are you using ?
 
Regards,
 
Chris Chapman

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