PSN-L Email List Message

Subject: Re: Designing a new vertical sensor
From: "Geoffrey" gmvoeth@...........
Date: Fri, 3 Apr 2009 17:49:37 -0700

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.


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