Brett Nordgren brett3nt@............. Sent: Tue, 16 Aug 2011 18:14 Subject: Re: tin cries - dithering Hi Chris, ****Thanks for your suggestion. I understand your=20 approach, though implementing it will be a bit difficult. Hi Brett, That rather depends on how you go about it !!=20 An alternative might be to fit two coils close to=20 the centre of the spring and just excite them ?=20 . ***Normally, to get a decent idea of long-period noise we=20 have to record for at least a day, and preferably several.=20 And to separate instrument noise unambiguously from true=20 ground noise we would need to record three instruments and=20 use correlation techniques to distinguish their individual=20 noise contributions. See: leeman, Wettum and Trampert Three-Channel Correlation Analysis: A New Technique to=20 Measure Instrumental Noise of Digitizers and Seismic Sensors" SSA v96 n1 p258 . ***As it stands, at the lowest frequencies, the noise spectrum=20 we normally see rises smoothly as 1/f and looks almost exactly=20 like the best instruments except, of course, it's not as low. =20 It is entirely possible that some of the noise we see may be=20 true ground noise. We just don't have a quiet enough location=20 to tell without playing the correlation game, which could be=20 a pain. =20 . You don't happen to have a well locally in which you could float the se= ismometer? =20 That is if you want absolute readings, but they will still=20 be variable. If you just want to check if the noise is lower,=20 you might have a reference seismometer and an experimental=20 seismometer and look at the long period noise of both ? =20 ****If anyone could suggest what is the magnitude of the=20 effect on the spring characteristics, I could at least model=20 the instrument response to such stepwise force changes.=20 Someone must have studied this and come up with some real=20 numbers. . You have traces of a sensor with an Annealed spring=20 and an As Supplied spring. ****Wouldn't the six-second microseisms be an effective=20 dither mechanism? They're usually large compared with=20 everything else. I don't think so. You are using a FORCE FEEDBACK=20 SEISMOMETER ! The increase in the strain of the spring=20 may be << 1% of the indicated signal. You are trying=20 to keep the position of the mass constant with respect=20 to the frame. How much feedback are you using?=20 Regards, Chris= Brett Nordgren brett3nt@bnordgren= ..org
Sent: Tue, 16 Aug 2011 18:14
Subject: Re: tin cries - dithering
Hi Chris, ****Thanks for y= our suggestion. I understand yourapproach, though implementing it wil= l be a bit difficult. . Hi Brett,That rather depends on how you go about it !!An alternative might= be to fit two coils close tothe centre of the spri= ng and just excite them ?<= TT>. ****Normally, to get a decent idea of long-period= noise wehave = to record for at least a day,and preferably seve= ral. And= to separateinstrument noise u= nambiguously from true <= /TT> ground = TT>noise we = would need to record three instruments= FONT> anduse correlation techniques to distin= guish their individual <= /TT> noisecontributions. See: Sleeman, Wettum = and Trampert "Three-Channel C= orrelation Analysis: A New Technique to Measure = Instrumental Noise ofDigitizers and Sei= smic Sensors" BSSA v96 n1 p258 . .= ****As it stands= , at the = lowest frequencies, the = noise spectrumwe normal= ly see rises smoothly as 1/f and= looks almost ex= actlyli= ke the best instruments except, of course, it's not as low.It = is entirely possiblethat some of= the noise we see may be<= /FONT> true ground noise. We just don't have a quiet enough location= to tell with= out playing the correlation game, which<= /FONT> could bea pain..You don't happen to have a well loca= lly in which you could float the seismometer?That is if you want absolute readings, but they will st= ill be v= ariable. If you just want to check if the noise is lower,you might have a referenc= e seismometer and an experimentalseismometer and look at the long period noise of = both ?<= /FONT> ****If anyone co= uld suggest what is the magnitude= of theeffect on the spring characteristics, I could at least modeltheinstrument response to such stepwise force changes. Someone must have studied <= FONT size=3D2>this and come up with some= realnumbers..You have traces of a sensor with an Annealed spring and an As Supp= lied spring.****Wouldn't the six-second microseisms = be an effective<= /FONT>dither= FONT> mechanism? They're usually large compare= d witheverything else. .I don't think so. You are using a FORCE FEEDBACK <= /FONT>SEISMOMET= ER ! The increase in the strain of the sprin= gmay be= << 1% of the indicated signal. You are trying = FONT>to keep th= e position of the mass constant with respectto the frame.How much feedback are you using? = TT>Regards,= Chris