sketches or pictures would be very useful for the suggested magnet 
arrangements...

TIA

Ian

ChrisAtUpw@....... wrote:

> In a message dated 25/03/2006, gmvoeth@........... writes:
>
>     Can someone out there run down a list of best ways to increase the
>     signal to noise
>     ratio of a seismic sensor?
>
> Hi Geoff,
>  
>     Wow, a BIG subject! Difficult to cover everything.
>
>     Things like:
>     1. in a mass spring system use a larger mass
>
>     Above about 35 gm, the intrinsic kT noise of the mass is less than 
> natural noise. For 'safety', use maybe 5x to 10x this as minimum.
>     For a spring / mass system the ends of the spring need to be 
> straight wires effectively clamped. The same applies to suspension 
> systems. Good clamp design is important.
>     The mass should preferably be non magnetic.
>     No horse shoe or open pole magnets should be mounted on the 
> armature. This is just asking for trouble and you will get it! Mount 
> the sensor and damping magnets on the baseplate and put the coils and 
> damping plates on the arm. Note that carefully designed 'potcore' 
> magnets are used as the mass in some seismometers. These do not have a 
> high external magnetic field.
>
>     2. use a phased array
>
>     This would allow you to cancel much of the local environmental 
> noise if it is physically large, but it is a major undertaking. 
>
>     3. use low resistance's in the circuitry
>
>     Choose the amplifier noise impedance to roughly match the sensor 
> resistance.
>
>     4. use a narrow bandwidth
>
>     This can be fairly critical. However it varies with the local 
> noise and the range of the signal that you want to detect. For local 
> and near regional signals you need maybe 10 to 20 Hz. For teleseismic 
> quakes you may only need 2 to 3 Hz, or less.
>     In noisy situations you need properly matched fairly sharp cut-off 
> multipole filters.
>     You may also wish to provide a high pass filter to reduce VLF 1/f 
> noise.
>
>     5. use a high Q
>
>     Oscillators for driving sensor systems need to have very good 
> frequency and amplitude stability.
>
>     6. shield everything against anything not a seismic signal.
>
>     Agreed, but you do need a single effective earthing point, usually 
> linked to the input of the first opamp where you have the minimum signal. 
>
>     7. a high signal to noise ratio is not possible for the average
>     amateur due to cost
>        it might take a bunch of amateurs to pool their moneys and
>     talents to
>        create a decent seismic sensor, possibly such a thing is only
>     possible
>        at a university where both money and talent exists.
>
>     I disagree entirely.
>     Amateurs should be able to achieve instrument noise levels below 
> their local ambient seismic levels without great difficulty. 
>     The more likely problems are interference, RF, magnetic or static 
> electric, poor earthing, poor shielding, utility supply signals and 
> noise and local environmental noise commonly due to wind, lightning 
> and static electric discharge, human activity including ordinary 
> movement, cars, lorries and other road traffic, trains, road and 
> building work, quarrying, power plants, heavy machinery, mining.... 
> You need to try to identify the noise sources.
>     It may be that you need to treat your site as though it were a 
> heavy engineering factory and use isolating transformers and filters 
> for supplying your system and the computer, sealed metal cases, common 
> point earthing and braided screened connecting cables.
>     It is no accident that most seismic sites are well away from human 
> activity and fully encased borehole instruments are often used, buried 
> maybe 100 m below ground level. This eliminates a great deal of the 
> surface and weather related noise. 
>  
>     However, the design of sensor and damping systems is very 
> important. It is quite easy to increase the output of a system which 
> used a coil and a horse shoe magnet, by using a NdFeB magnet array and 
> a smaller coil. 
>     
>     For a Lehman, I use two 1/4" thick mild steel plates, 3.5" long by 
> 2" wide. The corners are drilled to take 1/4" mild steel set screws 
> 2.5" long with mild steel washers and nuts. The two plates are held 
> maybe 3/4" to 1" apart for a sensor, using three nuts on every bolt. 
> A bolt is firmly secured to one plate using a nut. The second plate is 
> mounted in between pairs of nuts on the free thread, to allow the 
> separation of the plates to be adjusted. Four NdFeB magnets 1/8" thick 
> by 1" square are mounted on the inside faces of the plates. A N+S 
> pair on one face is opposed by a S+N pair on the other. The sensor 
> coil is mounted in the high intensity magnetic field in the centre.  
>     This construction gives quite an effective magnetic and 
> electrostatic screen around the coil. The external stray field is low. 
> The sensitivity is high due to the high field and the response is 
> reasonably linear. If you use rectangular instead of round coils, it 
> can be made very highly linear over +/-1/2" movement.
>     For induced current damping, I use four NdFeB bar magnets 
> 1"x1/2"x1/4" thick in two opposing squares, with the same 1/4" steel 
> plate mounting. I use a copper damping plate, 1/16" to 1/8" thick as 
> appropriate, a bit over 2" wide and 2.5" free length. This allows the 
> arm to swing +/-1/2" without the edge of the copper plate overlapping 
> the edge of the 1" magnet square. The N+S join of the magnet pairs is 
> set perpendicular to the direction of motion. The damping is adjusted 
> by varying by the length of copper tongue overlapping the magnet 
> square and also by varying the separation of the 1/4" mild steel 
> plates and hence the magnet separations.
>
>     Does any know of a system out there with the
>     highest signal to noise ratio...if so can we see it ?
>
>     More then likely it is in military hands and is secret ?
>
>     More likely to be a commercial secret of the various seismometer 
> manufacturers! It takes quite a bit of effort to get the instrument 
> noise below the minimum earth noise levels. However, unless you are 
> very fortunate with your site, you are very unlikely to see seismic 
> noise levels even approaching that low.
>  
>     Try either wedging your sensor in a fixed position or substituting 
> a resistor for it and running the whole system for 24 hrs on a 
> weekday. This should demonstrate the amplitude and timing of many 
> interfering signals. But it won't pick up sensor movement due to 
> drafts, local magnetic field changes (including the earth, your 
> refrigerators, car, bicycle or mowing machine), ground movement or 
> tilt, insects or animals. Then you have the interesting task of 
> identifying these sources and eliminating the effects. Compare the 
> signals and timing with an active trace taken a seismically quiet day.
>  
>     Get onto the www and see how ham radio operators deal with 
> earthing in your area, also the utility company. There are large areas 
> of the US where the rocks and soils are dry and have very poor 
> electrical conductivity. Your house or mobile home wiring may be 
> sticking way up above the effective local 'earth' plane. 
>  
>     For noise coming in through the utility wiring, you can provide a 
> filter, but check that this has a good rejection from a few hundred Hz 
> up and is not simply an RF filter. The next stage is to use a 1:1 
> isolating transformer with a metal case and an electrostatic screen in 
> between the windings. Consider providing a separate Earth for 
> the screen, the case and the electronics. The more extreme alternative 
> is to use two batteries, one to drive the equipment and a laptop 
> computer while the other is charged on a separate power circuit. An 
> alternative is to use a large ferrite core transformer, similar to 
> that in a TV set, to provide a 15 kHz isolated charging circuit. You 
> can also use optical or radio links to transfer data to and from the 
> phone system.
>  
>     Hope that these comments / suggestions are of some help.
>  
>     Regards,
>  
>     Chris Chapman




  
  


sketches or pictures would be very useful for the suggested magnet
arrangements...



TIA



Ian



ChrisAtUpw@....... wrote:


  
  
  
  
In a message dated 25/03/2006, gmvoeth@........... writes:
  
Can someone out there run down a list of best ways to
increase the signal to noise

ratio of a seismic sensor?
  
Hi Geoff,
  
 
  
    Wow, a BIG subject! Difficult to cover everything.
  
Things like:

1. in a mass spring system use a larger mass
  
    Above about 35 gm, the intrinsic kT noise of the mass is
less than natural noise. For 'safety', use maybe 5x to 10x this as
minimum.
  
    For a spring / mass system the ends of the spring need to be
straight wires effectively clamped. The same applies to suspension
systems. Good clamp design is important. 
  
    The mass should preferably be non magnetic.
  
    No horse shoe or open pole magnets should be mounted on the
armature. This is just asking for trouble and you will get
it! Mount the sensor and damping magnets on the baseplate
and put the coils and damping plates on the arm. Note that carefully
designed 'potcore' magnets are used as the mass in some seismometers.
These do not have a high external magnetic field.
  
2. use a phased array
  
    This would allow you to cancel much of the
local environmental noise if it is physically large, but it is a major
undertaking. 
  
3. use low resistance's in the circuitry
  
    Choose the amplifier noise impedance to roughly match the
sensor resistance. 
  
4. use a narrow bandwidth
  
    This can be fairly critical. However it varies with the
local noise and the range of the signal that you want to detect. For
local and near regional signals you need maybe 10 to 20 Hz. For
teleseismic quakes you may only need 2 to 3 Hz, or less.
  
    In noisy situations you need properly matched fairly sharp
cut-off multipole filters. 
  
    You may also wish to provide a high pass filter to reduce
VLF 1/f noise.
  
5. use a high Q
  
    Oscillators for driving sensor systems need to have very
good frequency and amplitude stability.
  
6. shield everything against anything not a seismic signal.
  
    Agreed, but you do need a single effective earthing point,
usually linked to the input of the first opamp where you have the
minimum signal. 
  
7. a high signal to noise ratio is not possible for the
average amateur due to cost

   it might take a bunch of amateurs to pool their moneys and talents to

   create a decent seismic sensor, possibly such a thing is only
possible

   at a university where both money and talent exists.
  
    I disagree entirely. 
  
    Amateurs should be able to achieve instrument noise levels
below their local ambient seismic levels without great difficulty. 
  
    The more likely problems are interference, RF, magnetic or
static electric, poor earthing, poor shielding, utility supply signals
and noise and local environmental noise commonly due to wind, lightning
and static electric discharge, human activity including ordinary
movement, cars, lorries and other road traffic, trains, road and
building work, quarrying, power plants, heavy machinery, mining.... You
need to try to identify the noise sources.
  
    It may be that you need to treat your site as though it were
a heavy engineering factory and use isolating transformers and filters
for supplying your system and the computer, sealed metal cases, common
point earthing and braided screened connecting cables.
  
    It is no accident that most seismic sites are well away from
human activity and fully encased borehole instruments are often used,
buried maybe 100 m below ground level. This eliminates a great deal of
the surface and weather related noise. 
  
 
  
    However, the design of sensor and damping systems is very
important. It is quite easy to increase the output of a system which
used a coil and a horse shoe magnet, by using a NdFeB magnet array and
a smaller coil. 
  
    
  
    For a Lehman, I use two 1/4" thick mild steel plates, 3.5"
long by 2" wide. The corners are drilled to take 1/4" mild steel set
screws 2.5" long with mild steel washers and nuts. The two plates are
held maybe 3/4" to 1" apart for a sensor, using three nuts on every
bolt. A bolt is firmly secured to one plate using a nut. The second
plate is mounted in between pairs of nuts on the free thread, to allow
the separation of the plates to be adjusted. Four NdFeB magnets 1/8"
thick by 1" square are mounted on the inside faces of the plates. A N+S
pair on one face is opposed by a S+N pair on the other. The sensor coil
is mounted in the high intensity magnetic field in the centre.  
  
    This construction gives quite an effective magnetic and
electrostatic screen around the coil. The external stray field is low.
The sensitivity is high due to the high field and the response is
reasonably linear. If you use rectangular instead of round coils, it
can be made very highly linear over +/-1/2" movement.
  
    For induced current damping, I use four NdFeB bar magnets
1"x1/2"x1/4" thick in two opposing squares, with the same 1/4" steel
plate mounting. I use a copper damping plate, 1/16" to 1/8" thick as
appropriate, a bit over 2" wide and 2.5" free length. This allows the
arm to swing +/-1/2" without the edge of the copper plate overlapping
the edge of the 1" magnet square. The N+S join of the magnet pairs is
set perpendicular to the direction of motion. The damping is adjusted
by varying by the length of copper tongue overlapping the magnet square
and also by varying the separation of the 1/4" mild steel plates and
hence the magnet separations.
  
Does any know of a system out there with the

highest signal to noise ratio...if so can we see it ?

    

More then likely it is in military hands and is secret ?
  
    More likely to be a commercial secret of the various
seismometer manufacturers! It takes quite a bit of effort to get the
instrument noise below the minimum earth noise levels. However, unless
you are very fortunate with your site, you are very unlikely to see
seismic noise levels even approaching that low.
  
 
  
    Try either wedging your sensor in a fixed position or
substituting a resistor for it and running the whole system for 24 hrs
on a weekday. This should demonstrate the amplitude and timing of many
interfering signals. But it won't pick up sensor movement due to
drafts, local magnetic field changes (including the earth, your
refrigerators, car, bicycle or mowing machine), ground movement or
tilt, insects or animals. Then you have the interesting task of
identifying these sources and eliminating the effects. Compare the
signals and timing with an active trace taken a seismically quiet day.
  
 
  
    Get onto the www and see how ham radio operators deal with
earthing in your area, also the utility company. There are large areas
of the US where the rocks and soils are dry and have very poor
electrical conductivity. Your house or mobile home wiring may be
sticking way up above the effective local 'earth' plane. 
  
 
  
    For noise coming in through the utility wiring, you can
provide a filter, but check that this has a good rejection from a few
hundred Hz up and is not simply an RF filter. The next stage is to use
a 1:1 isolating transformer with a metal case and
an electrostatic screen in between the windings. Consider
providing a separate Earth for the screen, the case and the
electronics. The more extreme alternative is to use two batteries, one
to drive the equipment and a laptop computer while the other is charged
on a separate power circuit. An alternative is to use a large ferrite
core transformer, similar to that in a TV set, to provide a 15 kHz
isolated charging circuit. You can also use optical or radio links to
transfer data to and from the phone system.
  
 
  
    Hope that these comments / suggestions are of some help.
  
 
  
    Regards,
  
 
  
    Chris Chapman