Hi Tiago,

Take the geophone and lie it on its side and see what things look like.

Angel




On 08/02/2012 04:19 PM, Tiago A. wrote:
> 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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