PSN-L Email List Message

Subject: Re: Border Patrol Use Of Seismic Sensors
From: "Geoffrey" gmvoeth@...........
Date: Sun, 24 Sep 2006 13:34:34 -0700

>    Probably 8 to 10 Hz See _ 
> (  
>    HSJ, GS20 etc, are quoted for 'intruder detection',  but I have no direct 
> knowledge.

I have an HS 10 similar to the one shown
and like it best of any other I have tried.
You can get them down to 0.75Hz I believe
But they cost like $3000 USD new. You might get one used
and in questionable shape for 1/10 that cost.

> 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.

Im not sure this is true, You can create a vacuum
and eliminate all brownian motion I think
Like the old vacuum tubes you create the best
vacuum mechanically possible then use a "getter"
to eliminate as much as possible the remaining gasses.

then, Use some form of damping not involving brownian motions.

>    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.

All we are looking for here is an unquestionable mark
for first time of arrival and not the whole seismic signal.
That would be the Pn wave for regional/local I think.
P or Pp waves for teleseismic events.
One would have to coordinate/compare with others
to determine the validity of any received signals.

>    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.

Yes, I like this way too.

>    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. 

You are getting way too complex for what i call amateure here.
Amateures do not make money off their sport and may be only a layman
with an interest in whatever area.
What you talk about here is in the relm of the physicist
or his loyal engineers.

>    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 _ 
> (    We may need to provide a 'build it 
> yourself' general purpose design; maybe a  circuit board? 
>    Regards,
>    Chris Chapman

Whatever happened to that scientific idea that simpler is better and more reliable ?

Thanks for your response;

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