PSN-L Email List Message

Subject: Re: Homade Geophone
From: "Geoff" gmvoeth@...........
Date: Tue, 9 Aug 2005 19:03:49 -0700

Chris Chapman

>    The strain gauge a) will not be anything like  sensitive enough b) it is 
> quite fragile and won't take overloads c) as primarily  a resistive device, it 
> has a fairly high power dissipation d) they are not  exactly cheap.

I have seen delicate scales before where the mass
under test hoovers and often I thought that would
make a wonderful EQ sensor like the globe which is
suspended in thin air by magnetic/optic feedback loop
the mass is held stationary and you would measure the
electrical signal used to keep the mass hoovering.
You might reduce the power reqired by such a thing
through the use of springs and levers using the magnetic
forces only for the final gram or two of weight.
I have never seen such an eq detector before.

> Do I  need a special preamp for the piezo element ?
>    Does your amplifier have a low noise CMOS input  opamp with a controlled 
> DC impedance of 100 M Ohms? 
>    Are you just starting seismology and electronics? 

I am using op177G opamps and like them very much
but they are not CMOS.
I like to use the preamp and postamp as
a differential amplifier using the smallest
resistors possible to do the job.
The filter is between the two so no amps will
be jammable before the desired signals reach their destination.

I think if you used two of your weighted crystals
centered about ground and properly phased
the diff amp might work just fine.

I have no formal training in op amps
or advanced electronics I have picked up
most of what I know through unguided research.

Remember those old car radios that used
iron slugs to tune the radio I would think such
a scheme might also make for a great FM  type
motion detector but because of the possibility of
RFI being generated by the RF circuits I will not
try such a thing.

I have been playing around with seismic stuff
for about 10 years now experimenting from
time to time with a breadboard but this has
been intermittent at best because other things
always seem to interfere with my efforts.
It is a way I like to spend my time and money
so that is why I mess around with it.

My current geophone sits on the same slab
as the mobile home I live in with awnings coupling
wind noise to the slab also as well as footsteps
from anyone in the area.
Vehicular noise is common as well as heavy construction
and the bedrock maybe 800 feet deep covered by
valley fill. It is not a good location for a receiver
but still I am able to receive teleseismic signals
when things are quiet.
I would put the geophone underground but I must
contend with a septic tank and high voltage
power lines about 60 feet away so the underground
option is not possible so it seems.

I had a special slab put in just for a seismometer
but my mobile home was changed so the heat pump now
sets within 10 feet of the slab making unacceptable noise that will
swamp/jam the preamp when the machine turns on not to mention
the power lines for the heat pump run parallel to the geophone cable.

My problems are great and my resources are meager
but I sure have one heck of a time trying for the best
system to receive those EQs.


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