PSN-L Email List Message

Subject: RE: seismograph item?
From: "Doug Crice" dcrice@............
Date: Fri, 6 Dec 2002 06:19:58 -0800

The SMA-1 looks complete to me.  The sensors are those odd devices on
the left in the photograph.  The accelerometer in an SMA-1 is just a
damped mass with a small mirror attached. In the presence of
acceleration, the mirror rotates and moves the spot of light on the
film. The film is moving, and thus a seismograph (or more properly, an
accelerograph) is drawn on the film. An event marker put time ticks on
the film at 100 ms intervals.

It is a 3-component system recording on 100 mm film in a canister. The
canister can be removed and opened in a dark space for developing. The
unit is triggered into activity by a separate sensor (also built into
the unit) and keeps recording for the duration of the event (or until it
runs out of film).

The sensitivity is on the order of +/- 1 g. And of course its purpose is
to record vibrations from strong earthquakes. I remember that quite a
stir was created when an SMA-1 recorded an acceleration of 1.01 g during
the Paicoma Dam earthquake in Southern California. Up until then, it was
believed that the maximum possible acceleration in any earthquake was
about 1/4 g.

The SMA-2 was the next generation and replaced the film with magnetic
tape recording. Kinemetrics figured out how to get an electrical signal
from the damping current in those optical sensors and record it on tape
(analog FM).

Thousands of these units were installed around the world to study ground
spectral amplification and structural response in earthquakes. Every
high rise in the LA area got three units: one in the basement, one in
the center, and one in the roof to look at how the building reacted to
the earthquake. They were wired together for synchronized timing so you
see how the roof zigged when the basement zagged.

The cases are rigid cast aluminum, air and water tight, and are worth
the price for the case alone, but it would be a shame to dismantle these
classic instruments.

Doug Crice
Geostuff/GeoRadar Inc.
12996 Somerset Drive                phone 1-530-274-4445
Grass Valley,  CA  95945  USA    fax 1-530-274-4446

-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@...............
On Behalf Of meredithlamb
Sent: Thursday, December 05, 2002 10:35 PM
To: psn-l@..............
Subject: Re: seismograph item?

Hi Bob/s, and all,

Not familar with it, but viewed the ebay pic's.  It looks like
only the electronics/film medium there with the case opened.
The larger round oblong object looks to be the film container.
Presume the actual sensors are (not there) normally separate
and connected via a cable plug-in on the case.  So, its without
the sensors I presume.

Note the same seller has two SMA-2's up for bid also, but they
appear to be without any sensors either.  Ebay #1923227423

Take care, Meredith

hammond wrote:

> Bob, it's a strong motion accelerograph which uses film as the
> medium.
> The unit gets triggered if the ground acceleration exceeds a set value
> then records
> the ground motion on a wide film strip.  These are still widely used
> superceded by
> digital strong motion recorders.
> Bob Hammond
> Public Seismic Network - Alaska
> Fairbanks
> At 05:09 PM 12/5/2002, you wrote:
> >Hi gang,
> >   Ebay offers:
> >Kinemetrics SMA-1 Seismograph Event Indicator
> >Item # 1923224052 ends Dec 10
> >   I have no idea what this is but it looks complicated.
> >Bob Barns


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