Thanks for the answer Chris.

   I am not sure that I understand what you have written - or what system
you are using. What ADC / number of bits, are you using ?

The system (ADC) is a ADS1211P, the geophone is connected directly to the
ADC inputs with a 20cm wires. Im using the A/D set to 22bits with the PGA
set to 2x. With a Decimate Factor of 5600 with Turbo Mode 8, gives a sample
rate of 27.86, which sets -3dB at 8Hz (3rd order digital filter) .

You MUST damp a 1 Hz geophone correctly to get a flat output over 1 Hz.
What is the value of the damping resistor should i use? Its a 5500ohm
geophone. But the strangest thing is i can only measure 3800ohm (tested
today), i think when i got the geophone it had a higher resistance. This
geophone has be altered, the previous owner changed the original Bendix 6/8
pin connector to a 4 pin Bendix connector.

How are you getting your period expansion below 1 Hz down to 0.1 Hz ?
Only using AmaSeis. One of the few times i could get down to 0.03Hz was in
the Mexico M 7.4 earthquake (~7500km). It was offline in the Sumatra
earthquakes. I live in a small island in the middle of the ocean, so i have
always a lot of noise in the 2~5s period.

What is the noise on your ADC - when you connect it to a resistor with the
same value as the geophone ?
I can check that in the next days.


Thanks.

Regards,
Tiago.



2012/7/30 

> 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
> noise from 10s up to ..., but from <10s it looks good. The noise comes and
> goes, 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
> filtering, 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.
> http://dl.dropbox.com/u/15802004/noise.sac
>
> Hi Tiago,
>
>     I am not sure that I understand what 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 correctly damped.
> What is the noise on your ADC - when you connect it to 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
>
Thanks for the answer Chris.

=A0=A0 I am not sure that I understand what you have written=
 - or what system you are using. What ADC / number of bits,=A0are you using ? 


The system (ADC) is a ADS1211P, the geophone is connected directly to t=
he ADC inputs with a 20cm wires. Im using the A/D set to 22bits with the PG=
A set to 2x. With a Decimate Factor of 5600 with Turbo Mode 8, gives a samp=
le rate of 27.86, which sets -3dB at 8Hz (3rd order digital filter) .


You MUST damp a 1=
 Hz geophone correctly to get a flat output over 1 Hz. 
Wh=
at is the value of the damping resistor should i use? Its a 5500ohm geophon=
e. But the strangest thing is i can only measure 3800ohm (tested today), i =
think when i got the geophone it had a higher resistance. This geophone has=
 be altered, the previous owner changed the original Bendix 6/8 pin connect=
or to a 4 pin Bendix connector. 


How are you getti=
ng your period expansion below 1 Hz down to 0.1 Hz ?
Only =
using AmaSeis. One of the few times i could get down to 0.03Hz was in the M=
exico M 7.4 earthquake (~7500km). It was offline in the Sumatra earthquakes=
.. I live in a small island in the middle of the ocean, so i have always a l=
ot of noise in the 2~5s period.


What is the noise=
 on your ADC - when you connect it to a resistor with the same value as the=
 geophone ? 
I can check that in the next days.

Thanks.


Regards,
Tiago.



2012/7/30 =
 <chrisatupw@.......>


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 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=A0



Hi Tiago, 

=A0

=A0=A0=A0 I am not sure that I understand what you ha=
ve 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.=A0

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,=A0are you using ? <=
/div>


=A0

=
Regards,

=
=A0

=
Chris Chapman



=20