Re: [Raspberry Shake Community Forum] Re: geophone in borehole? Electronic Stability

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chris...@aol.com

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Dec 25, 2016, 7:26:52 AM12/25/16
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Hi Angel, 

    An extension cable adds capacity across the input terminals. HOW MUCH added capacity can the feed back RS input accept before it oscillates, please ? This is a serious consideration - the geophone has quite a large inductance, which could give an LC resonant circuit !

    Regards, 

    Chris
From: Angel Rodriguez <an...@osop.com.pa>
To: raspberryshake <raspber...@googlegroups.com>
Sent: Sat, Dec 24, 2016 11:50 pm
Subject: Re: [Raspberry Shake Community Forum] Re: geophone in borehole?

Hi John,

Merry Christmas!

There are lots of youtube video on how to drill a hole by hand.  Try something like "drilling for water by hand"  I have a small borehole that goes down about 10 meters.  I have been giving this some thought.  I think that I am gong to gimble mine.  Who know when I will have time.

Keep us informed

Regards,

Angel


On Sat, Dec 24, 2016 at 6:32 PM, John Stuart <qxst...@gmail.com> wrote:
I achieved a significant reduction in noise level when I moved my RShake to a small 2'x2' concrete pad not in contact with my house, driveway, or street foundations & curbs (see "building a Seismic Vault" thread.)

I would still like to reduce the street traffic vibrations, so a borehole might work.  I do have an old soil bore analysis, so I expect the 'bedrock' sandstone is about 10 to 12 ft deep.  But first, I'd like to experiment with going down just a few feet.  Here's an idea I had for something simple.

The RShake's geophone can slip right down inside a 1" Sch.40 PVC pipe.  A 3/4" PVC slip plug is a press-fit inside the 1" pipe, but if you heat the pipe in boiling water, it will slip in and make a dandy end cap with flush sides.  

      

So by moving the geophone a few feet underground, I would hope to eliminate the thermal expansion popping I am recording from the plastic case exposed to the ambient temperature swings, and maybe some of the vehicle traffic noise.

But how to drill a hole?
If I can water-jet and pound a hole deep enough with a round wooded tree stake (and then pull it out), the above capped 1" PVC would fit down the hole.  Or I could drill a deeper and larger hole, to fit a 1-1/4" PVC pipe with end-cap, then cement it in place like a well casing for the 1"PVC geophone pipe. There's also YouTube's numerous water well drilling videos. But,,, will the seismic coupling between the soil and a small 1" pipe be as good as between the soil and a 2' x 2' concrete slab??  If I get down to the sandstone rock, will the street traffic become stronger?  

John Stuart (RFD2F)
Lafayette, CA

  .










On Monday, December 12, 2016 at 4:56:09 PM UTC-8, John Beale wrote:
Just thinking, as that's much easier than doing... Right now with the sensor on the concrete slab floor of my garage, I get a good bit of noise from the street out front (60 feet away) and the next street over which is 200 feet away.  I don't imagine it would be easy to do, but let's just say I was willing to consider drilling a geophone-diameter hole in the backyard to locate the sensor underground.  I believe the water table in my location is 40 feet down so I wouldn't go that far. Let's say I could somehow drill a 20 foot deep hole.  Would I see significantly less street noise at that depth? If the noise level is strictly proportional to distance, I would guess not, as the distance down is much less than the horizontal distance to the source.  I believe the soil in this area is just sandy clay (USGS map says "Alluvium"), no bedrock to be found.
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Angel Rodriguez

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Dec 25, 2016, 7:54:49 AM12/25/16
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Chris,

I will have to ask Dave or Brett.  I think that we could check with Brett's model.

Angel


On Sun, Dec 25, 2016 at 7:26 AM, chrisatupw via RaspberryShake <raspber...@googlegroups.com> wrote:
Hi Angel, 

    An extension cable adds capacity across the input terminals. HOW MUCH added capacity can the feed back RS input accept before it oscillates, please ? This is a serious consideration - the geophone has quite a large inductance, which could give an LC resonant circuit !

    Regards, 
    Chris
From: Angel Rodriguez <an...@osop.com.pa>
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Brett N

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Dec 25, 2016, 8:41:58 AM12/25/16
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On Sunday, December 25, 2016 at 7:54:49 AM UTC-5, Angel Rodriguez wrote:
Chris,

I will have to ask Dave or Brett.  I think that we could check with Brett's model.

Angel
 
Angel,

I ran a Spice gain plot for the Shake, using increasing capacitance values in parallel with the geophone, up to the point that I could see an effect.  It took ~10nF (huge) before I could see much of anything.  Even that large amount of capacitance had no visible effect up to 10kHz and only a little up to 1MHz.  http://bnordgren.org/seismo/RSSparCap.pdf  
Cable capacitance is not going to be a problem.

What it shows is not surprising, since the geophone impedance is rather low and the period-extension feedback is only acting at low frequencies.

Also, a few Ohms of cable resistance shouldn't hurt much, considering that the resistance variability of the geophones is likely to be greater than what any reasonable length of wire might add.  I would be careful, though, to try to avoid external noise pickup by using (shielded?) twisted-pair wire for the down-hole connection.   

Brett

chris...@aol.com

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Dec 25, 2016, 9:29:33 AM12/25/16
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Hi Angel, 

    You can only calculate so much ! I would have put my switched capacitor box across the input to my RP - BUT - we have NOT YET received our RPs here in the UK ! There will be an LC resonance due to the capacity between the wire windings themselves and the frame. This is a serious consideration for anyone who wants to locate the geophone remotely. With a bit of luck, all the problems will have been sorted out by the time we get ours ! It is probably our Customs and Excise Service... (!) Do you have the Courier name and any tracking numbers, please ? 

    Regards, 

    Chris
From: Angel Rodriguez <an...@osop.com.pa>
To: raspberryshake <raspber...@googlegroups.com>; Brett Nordgren <bret...@bnordgren.org>
Sent: Sun, Dec 25, 2016 12:54 pm
Subject: Re: [Raspberry Shake Community Forum] Re: geophone in borehole? Electronic Stability

Chris,

I will have to ask Dave or Brett.  I think that we could check with Brett's model.

Angel
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Branden Christensen

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Dec 25, 2016, 9:40:30 AM12/25/16
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Sorry Chris. No tracking numbers. Please hang in there.

On Dec 25, 2016 09:29, "chrisatupw via RaspberryShake" <raspber...@googlegroups.com> wrote:
Hi Angel, 

    You can only calculate so much ! I would have put my switched capacitor box across the input to my RP - BUT - we have NOT YET received our RPs here in the UK ! There will be an LC resonance due to the capacity between the wire windings themselves and the frame. This is a serious consideration for anyone who wants to locate the geophone remotely. With a bit of luck, all the problems will have been sorted out by the time we get ours ! It is probably our Customs and Excise Service... (!) Do you have the Courier name and any tracking numbers, please ? 

    Regards, 

    Chris
From: Angel Rodriguez <an...@osop.com.pa>
To: raspberryshake <raspberryshake@googlegroups.com>; Brett Nordgren <bret...@bnordgren.org>

Sent: Sun, Dec 25, 2016 12:54 pm
Subject: Re: [Raspberry Shake Community Forum] Re: geophone in borehole? Electronic Stability

Chris,

I will have to ask Dave or Brett.  I think that we could check with Brett's model.

Angel
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Angel Rodriguez

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Dec 25, 2016, 9:46:45 AM12/25/16
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Chris,

We have tracking numbers but they are useless since we can not see them on the web page.  They might work for the Postal System but then that does not help us.  The are basically Postal EMS Service.

Some must have arrived since we see triangles on the map that are new to us.

Angel


On Sun, Dec 25, 2016 at 9:29 AM, chrisatupw via RaspberryShake <raspber...@googlegroups.com> wrote:
Hi Angel, 

    You can only calculate so much ! I would have put my switched capacitor box across the input to my RP - BUT - we have NOT YET received our RPs here in the UK ! There will be an LC resonance due to the capacity between the wire windings themselves and the frame. This is a serious consideration for anyone who wants to locate the geophone remotely. With a bit of luck, all the problems will have been sorted out by the time we get ours ! It is probably our Customs and Excise Service... (!) Do you have the Courier name and any tracking numbers, please ? 

    Regards, 

    Chris
From: Angel Rodriguez <an...@osop.com.pa>
To: raspberryshake <raspberryshake@googlegroups.com>; Brett Nordgren <bret...@bnordgren.org>

Sent: Sun, Dec 25, 2016 12:54 pm
Subject: Re: [Raspberry Shake Community Forum] Re: geophone in borehole? Electronic Stability

Chris,

I will have to ask Dave or Brett.  I think that we could check with Brett's model.

Angel
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chris...@aol.com

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Dec 25, 2016, 12:13:21 PM12/25/16
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Hi Brett, 

    It is NOT any GAIN change, but the PHASE change which is CRITICAL, when adding capacity to a geophone in a force feedback circuit. You need to prevent ANY oscillation. What Inductance / Capacitance values have you assumed for the geophone, please ? I have not yet received my RP, so I can't measure them. 
From memory, the inductance of my old type 4.5 Hz geophones was well over 1 Henry (2.3 H?). However, they are currently in a sealed case with damping resistors soldered on and I am not going to disturb them ! 
     http://bnordgren.org/seismo/RSSparCap.pdf gives me a "Not Found" error - it doesn't seem to be on line at this time.

    Regards, 

    Chris

From: Brett N <marc...@verizon.net>
To: RaspberryShake <raspber...@googlegroups.com>
Cc: brett3nt <bret...@bnordgren.org>; angel <an...@osop.com.pa>
Sent: Sun, Dec 25, 2016 1:41 pm
Subject: Re: [Raspberry Shake Community Forum] Re: geophone in borehole? Electronic Stability

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John Stuart

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Dec 25, 2016, 9:16:21 PM12/25/16
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I just made some measurements of two geophones that are supplied with the RShake,
I used an Agilent U1733C LCR Meter set at 100 Hz (lowest) test frequency, and measured just the geophone (not connected to the Shake board)..

L=   55uH     62uH
C=   46uF     41uF
R=   387ohm 390ohm
Z=   388ohm 392ohm

John Stuart
Lafayette, CA

John Stuart

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Dec 26, 2016, 10:10:31 PM12/26/16
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Earlier today I tested a 15 ft. length of Belden 9841 #24 shielded pair cable.between the geophone and Shake board.

Then I shook the shake on my doctor's weighing scale / shake table,with  5um displacement, at 0.5, 1, 2, 3, 4, and 5 Hz, with and without the shield connected at the RPi end (only).
I saw no additional noise or oscillations on Swarm's wave, or spectrum views. Sensitivity appeared to be the same as with the original short geophone leads. I also tried turning on several room light dimmers at 50%, and saw no evidence of 60 / 120 Hz buzz pickup.

Test data visible on RShake RFE6E , 2016/12/26, between 23:10 and 23:16

John Stuart
Lafayette, CA

Branden Christensen

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Dec 26, 2016, 10:13:31 PM12/26/16
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Very cool news! Thanks for sharing John! I wonder how long the cable could be before the Sensitivity fell off. 


Branden Christensen
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chris...@aol.com

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Dec 27, 2016, 8:27:02 AM12/27/16
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    Good work !

    Chris

From: John Stuart <qxst...@gmail.com>
To: RaspberryShake <raspber...@googlegroups.com>
Sent: Tue, Dec 27, 2016 3:10 am
Subject: Re: [Raspberry Shake Community Forum] Re: geophone in borehole? Electronic Stability

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Brett N

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Dec 27, 2016, 10:32:08 AM12/27/16
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Chris,

Yes, you can only calculate so much, but if you know what you are doing and have a good model, simulation can be extremely helpful.  So far, the Spice model has successfully predicted a number of subtle effects that have previously been dealt with in the design.  The Spice model I used for the geophone is at    bnordgren.org/seismo/RSSGeo.pdf     Its parameters are shown there.  The coil inductance was as measured with the mass locked.  The other values came from measurements and the manufacturer's specifications.

Regarding the significance of my capacitance-added gain plot,   http://bnordgren.org/seismo/RSSparCap.pdf   it is well known that the closed-loop transfer function (gain) of a circuit that has inadequate feedback margins will show a peak at frequencies of potential oscillation.  As the margins are decreased, the peak increases in height, until, when they reach zero, it goes to infinity and the loop oscillates.  Since the gain plot showed no such peaks I inferred that the margins were excellent, even with the added capacitance.

However, just to satisfy myself, I also decided to examine the feedback loops in the Shake.  Such simulations are not so simple, so, given the lack of gain peaking, I hadn't bothered previously.  See: R.D. Middlebrook "Measurement of Loop Gain in Feedback Systems" Int. J. Elect. 1975, v38, n4.  Unlike force-balance instruments, there is no overall feedback loop, only the feedback around individual op-amp stages.  The geophone impedance, in parallel with any wiring capacitance does affect the loop gain of the circuit that does the period extension.  With no added capacitance, its gain crossover is at 35KHz with phase margin of 88 deg. while with 10nF of wiring capacitance added, gain crossover goes to 38KHz with phase margin of 56 deg.  IThat means that in both cases, the loop will be extremely stable.

.Brett

Brett N

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Dec 28, 2016, 6:24:40 AM12/28/16
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Chris,

I played around some more with the model and find that with something around 24nF in parallel with the geophone, it will oscillate at ~68kHz.  If you can stay well below 25nF, there should be no problem.  That shouldn't be difficult to maintain with cable lengths one is likely to see.

Unusual case, in that it's the gain margin that gets you.  With 10nF the phase margin was comfortable at 56 deg, but the gain margin was only about 4dB. 

Brett


Brett N

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Dec 28, 2016, 6:52:56 AM12/28/16
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Branden,

With John's Belden 9841 cable, with the shield grounded, you reach 10nF capacitance at 132 m.  That comes with a resistance of 10.4 Ohms.  Plugging those into the model, you get an analog gain @5Hz of 82.5, vs 82.8 with no cable, an insignificant change.  However, 10nF might be a bit closer to oscillation than I'd like to see.  Maybe limit to 100m max, with that cable, which has 7.5nF. 

Brett


On Monday, December 26, 2016 at 10:13:31 PM UTC-5, Branden Christensen wrote:
Very cool news! Thanks for sharing John! I wonder how long the cable could be before the Sensitivity fell off. 


Branden Christensen
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On Mon, Dec 26, 2016 at 10:10 PM, John Stuart <qxst...@gmail.com> wrote:
Earlier today I tested a 15 ft. length of Belden 9841 #24 shielded pair cable.between the geophone and Shake board.

Then I shook the shake on my doctor's weighing scale / shake table,with  5um displacement, at 0.5, 1, 2, 3, 4, and 5 Hz, with and without the shield connected at the RPi end (only).
I saw no additional noise or oscillations on Swarm's wave, or spectrum views. Sensitivity appeared to be the same as with the original short geophone leads. I also tried turning on several room light dimmers at 50%, and saw no evidence of 60 / 120 Hz buzz pickup.

Test data visible on RShake RFE6E , 2016/12/26, between 23:10 and 23:16

John Stuart
Lafayette, CA

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chris...@aol.com

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Dec 28, 2016, 1:33:26 PM12/28/16
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Hi Brett, 

    Thanks ! 
    The specification for the geophone is 395 Ohm +/- 5 Ohm. The Tc of Resistance for Copper wire is 0.0038 / C Deg, so a 20 C Deg change gives a resistance change of ~30 Ohms. Over the +20 C to -40 C range, the resistance change could be ~91 Ohms. Could this be a limitation ?  
    If the 10 Hz geophone resistance was 360 Ohms, or less, the circuit might well oscillate. 
    Roughly what -ve R value was designed into the RaspberryShake, please ?  
    I have used -ve resistance inputs for geophones to give a current output, but I have always allowed quite   substantial error margins on -R.

    I can't seem to contact you at marc...@verizon.net - mail gets returned.

    Regards, 

    Chris

From: Brett N <marc...@verizon.net>
To: RaspberryShake <raspber...@googlegroups.com>
Sent: Wed, Dec 28, 2016 11:24 am
Subject: Re: [Raspberry Shake Community Forum] Re: geophone in borehole? Electronic Stability

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