PSN-L Email List Message
Subject: Re: High noise >10s
From: "Tiago A." tiagoagre@.........
Date: Thu, 2 Aug 2012 16:19:11 +0000
Hi Geoff.
Increasing the sample rate to 125sps (-3dB at 36Hz), didint solve the issue
and increased the >10s noise..i will try to increase more the sample rate.
But increasing the sample rate reveals a very high count at 50Hz, i check
the earth connections it looks all ok..if i remove the geophone from the
ADC, i have +-2 counts of noise at 16bits and the 50Hz spike is gone.
Simply touching a wire from earth in any part of the geophone, will make
the geophone stiff, without responding.
SAC file with 15minutes at 125SPS
http://dl.dropbox.com/u/15802004/aa.sac
Netherless the 50Hz issue, getting only 3800ohm from a 5500ohm geophone is
a good indicator that something is wrong with the geophone itself. It still
responds ok to >0.1Hz, but <0.1Hz is just noise.
2012/8/2 Geoff
> if the geophone is OK then i understand
> the rdamp should be like 4x Rgeo.
> together the final R value is (Rd||Rg)
> the final Ge is like GRg/(Rd + Rg).
> If your geophone has a resistance
> more than like 1% off at room temperature
> you must consider it damaged somehow.
> Regards,
> geoff
>
>
> On 2012-07-30 14:34, Tiago A. wrote:
>
>> 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. geIm 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
>>
>>
>>
>
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Hi Geoff.
Increasing the sample rate to 125sps (-3dB at 36Hz), didin=
t solve the issue and increased the >10s noise..i will try to increase m=
ore the sample rate. But increasing the sample rate reveals a very high cou=
nt at 50Hz, i check the earth connections it looks all ok..if i remove the =
geophone from the ADC, i have +-2 counts of noise at 16bits and the 50Hz sp=
ike is gone. Simply touching a wire from earth in any part of the geophone,=
will make the geophone stiff, without responding.
SAC file with 15minutes at 125SPS
http://dl.dropbox.com/u/15802004/aa.sac
Nether=
less the 50Hz issue, getting only 3800ohm from a 5500ohm geophone is a good=
indicator that something is wrong with the geophone itself. It still respo=
nds ok to >0.1Hz, but <0.1Hz is just noise.
2012/8/2 Geoff
<=
;gmvoeth@.........>
if the geophone is OK then i understand
the rdamp should be like 4x Rgeo.
together the final R value is (Rd||Rg)
the final Ge is like GRg/(Rd + Rg).
If your geophone has a resistance
more than like 1% off at room temperature
you must consider it damaged somehow.
Regards,
geoff
On 2012-07-30 14:34, Tiago A. wrote:
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, are you using ?
The system (ADC) is a ADS1211P, the geophone is connected directly to
the ADC inputs with a 20cm wires. geIm 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.
=A0 =A0 From: Tiago A. tiagoagre@......... <mailto:tiagoagre@.........>
=A0 =A0 Sent: Mon, 30 Jul 2012 3:08
=A0 =A0 Subject: High noise >10s
=A0 =A0 Hi.
=A0 =A0 Im using a L4 1Hz geophone and for almost a month im struggling wit=
h
=A0 =A0 high noise from 10s up to ..., but from <10s it looks good. The<=
br>
=A0 =A0 noise comes and goes, this image is self explaining:
=A0 =A0 http://dl.dropbox.com/u/15802004/noise.png
=A0 =A0 Changed cables, re centered the geophone, but didint help..the
=A0 =A0 geophone is connected directly to a ADC without damping, external
=A0 =A0 amplification or filtering, i only use digital filtering from the
=A0 =A0 ADC. Before this issue start, i had a pretty clean spectrum >10s=
for
=A0 =A0 almost 6 months, which allowed me to received LQ and LR from some
=A0 =A0 strong events.
=A0 =A0 This sac contains 15minutes with high noise >10s, notice that at=
=A0 =A0 3:37 it recorded a Ml2.1 at 70km.
=A0 =A0 http://dl.dropbox.com/u/15802004/noise.sac
=A0 =A0 Hi Tiago,
=A0 =A0 =A0 =A0 =A0I am not sure that I understand what you have written - =
or what
=A0 =A0 system you are using.
=A0 =A0 You MUST damp a 1 Hz geophone correctly to get a flat output over 1=
Hz.
=A0 =A0 How are you getting your period expansion below 1 Hz down to 0.1 Hz=
?
=A0 =A0 Any period expansion DEPENDS on the geophone being correctly damped=
..
=A0 =A0 What is the noise on your ADC - when you connect it to a resistor
=A0 =A0 with the same value as the geophone ?
=A0 =A0 You do need a low noise metal film resistor.
=A0 =A0 What ADC / number of bits, are you using ?
=A0 =A0 Regards,
=A0 =A0 Chris Chapman
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