Al Frielink wrote:

> To All
> In making an S G Seismometer is there any reason why the coil/magnet
> combination couldn't be used as the sensor used in my Lehman?
>   I enjoy reading the e-mail from the PSN.
> Thanks.
> Al.

Al,

It would work for short period use, generally related to the
period of your S-G, but I suspect the longer period phases
would be greatly attenuated, due solely to the mechanical
nature of the S-G compared to the mechanical "amplification"
arrangement on the Lehman.

Most S-G's use sensors with position displacement sensing and
not coil and magnet induction; to sense the longer periods.

On the other hand, it would make it interesting to try, as I've
never heard of a coil and magnet induction S-G, as of this
writing.  I would think its "range", would be much greater than
a geophone due to its longer period....unless you get like a
1hz geophone....in comparison to the "normal" 8-10-12-20 hz
typical amateur geophone.

If you live in a seismic area, the coil and magnet might be
interesting to try; but if you don't, I would suggest trying other
methods of sensing.  If you live in a city/town with or near alot
of seismic noise sources, you will see alot of false triggers with
coil & magnet sensing.   With a displacement sensor, and
earthquakes "L" waves, real records will be much more obvious.

Personally, I think the S-G's are the easiest route for amateurs,
and with care in construction they can mimic the output of alot
of coil & magnet horizontal instruments.   The "zeroing" factor
is simple gravity for S-G's, and with its much smaller size, the
tilt factor and leveling adjustments is alot less frequent than a
"Lehman".  I use a linear (mostly) Hall sensor circuit myself,
created by Robert Lamb, in two S-G's.

Going further, one could even incorporate more than one type
of sensor in a single S-G, with additional lead in wires
compensation in construction.....you could try induction,
capacitive, Hall,  optical light sensors or any other possible
method combination.

Take care,

Meredith Lamb





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