PSN-L Email List Message

Subject: Re: High noise >10s
From: Geoff gmvoeth@.........
Date: Thu, 02 Aug 2012 12:46:30 +0000


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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