In a message dated 23/09/2006, gmvoeth@........... writes:

I  understand the US Border Patrol has used seismic sensors for a long time 
now  to watch certain areas of the border for intruders / interlopers and I was 
 wondering what is the sensors they are using (make and model or type or  
manufacturer)?
Hi Geoff,
 
    Probably 8 to 10 Hz See _http://www.geospacelp.com/industry2.shtml#geo_ 
(http://www.geospacelp.com/industry2.shtml#geo)  
    HSJ, GS20 etc, are quoted for 'intruder detection',  but I have no direct 
knowledge.

I can  imagine the military doing this too around nuclear sites and such
critical  areas where sensitive information is being collected / stored.

Why  can't a seismic sensor be built into a microchip somehow ?
    You can't get the low noise and the high  sensitivity, partly due to gas 
interactions in very small spaces and partly due  to intrinsic material noise. 
You also have Brownian noise / kT / frequency  considerations. This may limit 
you to weights of about an ounce.

Have the  experts in the field of science ever experimented here ?
    I would have expected them to have tried quite  hard. The hysteretic 
limitations of materials are fundamental and inherent,  so their success is likely 
to be limited.
 
    Horizontal seismometer noise may be limited by  the earth tides amongst 
other disturbances, which cyclically alter the slope of  the ground / angle of 
gravity slightly. This noise may be 20 dB above the  minimum vertical noise, 
but it is fairly broadband in it's effects. See the  annual noise plots from 
seismic stations.
 
    So we make a highly sensitive vertical sensor,  where the mass is 
balanced by a spring force. The dimensions of the  apparatus are temperature 
dependant and the spring constant is temperature  dependant, but neither are strictly 
linear, or of comparable magnitude. In  general, it may not be too difficult 
to reduce temperature effects by a factor  of 10, but any further improvement 
gets progressively much more difficult. Throw  in the fact that springs do not 
behave truly elastically and the whole problem  gets quite difficult. Springs 
with a very low temperature coefficient are  inherently magnetic, which can 
add other sources of noise. The STS1  probably represents about the best that 
can be done commercially.

Are  there any military secrets related to Geology ?
    Don't know of any, but I would doubt it, due to the  known fundamental 
limitations of the properties of materials.

Anyone  here know anything about the future of vibration sensors sensitive 
enough and  low noise enough down to or below the seismic noise level  ?


The only candidates that I know of are PZT  piezoelectric crystals and simple 
pendulum developments. PZT crystals have  quite a high temperature 
dependence. You need to hold the temperature very  constant.
 
    Since the output of coil / magnet induction sensors  goes to zero as the 
frequency goes to zero, the use of direct position sensors  has become 
commonplace. The use of very low drift / low noise / long  life semiconductor 
circuits is essential. Magnetic sensors (LVDT) may  be limited to about 10^-10 m due 
to their inherent Barkhausen (domain switching)  noise. Capacitative sensors 
may, with considerable effort, give another  couple of orders of magnitude 
sensitivity, but they still depend on the  expansion rate of materials for their 
stability. Fused quartz coated with  various metals and silver plated Invar are 
known plate materials.
 
    Using velocity damping derived by  differentiating the position signal is 
inherently noisy. Providing  electromagnetic damping directly is quieter and 
it is relatively easy to do with  NdFeB or Sm/Co magnet arrays.
 
    The move from coil springs of the LaCoste type to  leaf springs of the 
Streckheisen type enabled the 'parasitic vibration'  responses to be reduced. A 
bent sheet of copper plate close to a leaf spring may  be used to stabilise 
the temperature. Small NdFeB magnets may be stuck to the  leaf spring to provide 
inductive damping with the copper plate. 
 
    Some additional research may well be desirable on  suspension systems and 
on overall designs to minimise the effects of intrinsic  noise. The 
realisation that seismic sensors do not follow the 'standard' damping  curve has quite 
profound implications. Extending the period of 'simple' vertical  pendulums 
may offer significant advances, but high performance linear  capacitative 
sensors are needed. See _http://physics.mercer.edu/hpage/peters.html_ 
(http://physics.mercer.edu/hpage/peters.html)    We may need to provide a 'build it 
yourself' general purpose design; maybe a  circuit board? 
 
    Regards,
 
    Chris Chapman





In a message dated 23/09/2006, gmvoeth@........... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I=20
  understand the US Border Patrol has used seismic sensors for a long time n=
ow=20
  to watch certain areas of the border for intruders / interlopers and I was=
=20
  wondering what is the sensors they are using (make and model or type or=20
  manufacturer)?
Hi Geoff,
 
    Probably 8 to 10 Hz See http://www.geospacelp=
..com/industry2.shtml#geo 
    HSJ, GS20 etc, are quoted for 'intruder detecti=
on',=20
but I have no direct knowledge.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I can=20
  imagine the military doing this too around nuclear sites and such
criti=
cal=20
  areas where sensitive information is being collected / stored.

Why=20
  can't a seismic sensor be built into a microchip somehow ?
    You can't get the low noise and the high=20
sensitivity, partly due to gas interactions in very small spaces and partly=20=
due=20
to intrinsic material noise. You also have Brownian noise / kT / frequency=20
considerations. This may limit you to weights of about an ounce.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Have the=20
  experts in the field of science ever experimented here ?
    I would have expected them to have tried quite=20
hard. The hysteretic limitations of materials are fundamental and inher=
ent,=20
so their success is likely to be limited.
 
    Horizontal seismometer noise may be limite=
d by=20
the earth tides amongst other disturbances, which cyclically alter the slope=
 of=20
the ground / angle of gravity slightly. This noise may be 20 dB above the=20
minimum vertical noise, but it is fairly broadband in it's effects. See the=20
annual noise plots from seismic stations.
 
    So we make a highly sensitive vertical sensor,=20
where the mass is balanced by a spring force. The dimensions of the=20
apparatus are temperature dependant and the spring constant is temperature=20
dependant, but neither are strictly linear, or of comparable magnitude. In=20
general, it may not be too difficult to reduce temperature effects by a fact=
or=20
of 10, but any further improvement gets progressively much more difficult. T=
hrow=20
in the fact that springs do not behave truly elastically and the whole probl=
em=20
gets quite difficult. Springs with a very low temperature coefficient are=20
inherently magnetic, which can add other sources of noise. The STS1=20
probably represents about the best that can be done commercially.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Are=20
  there any military secrets related to Geology ?
    Don't know of any, but I would doubt it, due to=
 the=20
known fundamental limitations of the properties of materials.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Anyone=20
  here know anything about the future of vibration sensors sensitive enough=20=
and=20
  low noise enough down to or below the seismic noise level=20
?


    The only candidates that I know of are PZT=20
piezoelectric crystals and simple pendulum developments. PZT crystals h=
ave=20
quite a high temperature dependence. You need to hold the temperature v=
ery=20
constant.
 
    Since the output of coil / magnet induction sen=
sors=20
goes to zero as the frequency goes to zero, the use of direct position senso=
rs=20
has become commonplace. The use of very low drift / low noise / long=20
life semiconductor circuits is essential. Magnetic sensors (LVDT)=20=
may=20
be limited to about 10^-10 m due to their inherent Barkhausen (domain switch=
ing)=20
noise. Capacitative sensors may, with considerable effort, give another=
=20
couple of orders of magnitude sensitivity, but they still depend on the=20
expansion rate of materials for their stability. Fused quartz coated with=20
various metals and silver plated Invar are known plate materials.
 
    Using velocity damping derived by=20
differentiating the position signal is inherently noisy. Providing=20
electromagnetic damping directly is quieter and it is relatively easy to do=20=
with=20
NdFeB or Sm/Co magnet arrays.
 
    The move from coil springs of the LaCoste type=20=
to=20
leaf springs of the Streckheisen type enabled the 'parasitic vibration'=20
responses to be reduced. A bent sheet of copper plate close to a leaf spring=
 may=20
be used to stabilise the temperature. Small NdFeB magnets may be stuck to th=
e=20
leaf spring to provide inductive damping with the copper plate. 
 
    Some additional research may well be desirable=20=
on=20
suspension systems and on overall designs to minimise the effects of intrins=
ic=20
noise. The realisation that seismic sensors do not follow the 'standard' dam=
ping=20
curve has quite profound implications. Extending the period of 'simple' vert=
ical=20
pendulums may offer significant advances, but high performance linear=20
capacitative sensors are needed. See http://physics.mercer.e=
du/hpage/peters.html =20
We may need to provide a 'build it yourself' general purpose design; maybe a=
=20
circuit board? 
 
    Regards,
 
    Chris Chapman