PSN-L Email List Message

Subject: Re: Designing a new vertical sensor
From: "tchannel" tchannel@............
Date: Fri, 3 Apr 2009 19:42:04 -0600

Thanks, Geoffrey,   I will read your suggestions and notes carefully.


----- Original Message ----- 
From: "Geoffrey" 
Sent: Friday, April 03, 2009 6:49 PM
Subject: Re: Designing a new vertical sensor

> NOTES From A layman (Vs Expert):
> I believe that increasing the mass
> will increase the signal to noise ratio but not sure.
> I used an old 500Lb. seismometer once that
> had a 100+ lb AVADUPOISE. ( I think ) mass and it would receive
> even the surface waves well yet it was a 1 sec device.
> Best vertical I ever tried.
> USE Pb ( PlumBum ) as the mass.
> A typical soda can of Lead.
> Possibly 8 + LBS avd. or around 1 gallon of water.
> I believe that using a balanced sensor will
> make the electronics easier to build by
> providing a proper input for a op amp diff amp.
> As well as doubling the signal level
> and somewhat lowering the noise level.
> Noise will cancel 50% of the time but also
> increase 50% time so possibly there is no
> noise advantage to this.
> A balanced sensor is two coils/magnets wired in
> additive fashion with the center point grounded
> making a single three wire sensor.
> (SORT OF CLASS "B" push pull kind of thing but not exactly that)
> But both must be matched as close as possible to being
> identical. Both sensors can use the same mass
> or be two separate single devices on the same
> foundation (platform). Cable should be 100%
> shielded and possibly the preamp located at the sensor
> itself with maybe 50Ft of cable at most.
> The shield should probably be cut just before reaching the
> sensor but not sure. If everything is insulated from EARTH you
> might surround even the sensor in the electrical ground.
> Grounding is important to eliminate static and RFI
> noise as well as the 60hz power hum that might originate
> from power lines.
> Use magnetic damping separate from the sensor
> and you will get better output from the sensor.
> IF you could place the mass in a vacuum
> there would be no buoyancy effects for
> changes in air density on the mass.
> The greater the A/d Resolution the lower the overall
> gain needs to be in the electronics.
> Filter Low Pass the output so that the Aliasing
> Freq occurs at 0Db or less of electrical gain if possible.
> Match all your components as closely as possible
> in the preamp to avoid baseline drifting and common
> mode problems ( same signal on the two main signal leads ) .
> Double regulate the power supply with at least
> three+ volts between the output and the source.
> So if you got 5 volts out you need at very least 8
> volts in.
> Anything at all that varies could affect the input
> into your A/D converter. And all you want is pure signal.
> The best of all would possibly be a three axis signal that is
> mathematically combined to create a single magnitude.
> But then you are not concerning yourself with the
> three dimensions but only phases and first time
> of arrival which is what interests myself the most.
> Some people believe that the sensor magnet is fixed
> to the physical ground
> is the best way to go but not sure here since I have
> had excellent results with short period devices
> the other way around.
> If you could string 100 devices together
> 50 on each side of Signal ground
> And locate them on the same platform
> just think of the sensitivity
> that might produce. But who has that
> kind of monies
> It seems to me that all kinds of waves
> may be realized in the vertical since
> when you squeeze or relax forces in the earth
> bulges will be realized at the surface
> as vertical motions.
> ( Pardon my terms I am only a layman :-)  )
> I think you are not interested in the electronics
> but not sure about that. Need the tools.Expensive.
> (designing/Making Your Own or building premade kits)
> But you can do exactly what you want this way.
> I do not recommend this unless you have great amount time
> on your hands.
> geoff
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