Installing the AS-1 Seimometer:
1.
Assemble the AS-1 seismometer as described in the instructions provided with
the
seismometer. The seismometer will work best in a quiet environment.
The most
important factor is placing it on a solid floor: a concrete slab basement, or
first
level is best. However, the seismometer will work nearly anywhere at
lower gain
setting, including on an upper level floor of the building and on a table top
(for
testing and demonstration). Because the instrument is not very sensitive
to high
frequency signals, vibrations from walking near the seismometer are small
unless
one is walking within a couple of meters of the seismometer. For this
reason, it is
desirable to place the connected computer and display at least 2 meters away
from
the seismometer.
2.
Be sure to level the base with the leveling screws and the boom by moving the
level bubble and a washer along the top surface of the boom until the boom is
approximately level. This process, and the addition of masses (washers
on the
bolt at the end of the boom), must be repeated when oil (use 10W40 motor oil)
is
added to the container and the washer extending downwards from the boom is
positioned in the oil for damping. Place a drop of the motor oil on the
hinge of
the boom (be very careful of the sharp edge and do not damage the edge) to
help
prevent friction on the hinge. Be sure that the magnet and coil
assemblies are
positioned as shown in the instructions and that the magnet and coil do not
touch
when the boom (and magnet) are moved up and down. When you are ready to
connect to the computer and set up the software (see below), carefully place
the
clear plastic cover over the seismometer to decrease air currents on the
spring
which cause significant noise.
3.
To test the instrument, be sure that the output is approximately
"centered" (see
below), walk up close, or jump near to the seismometer and you should see a
small noise pulse on the screen. Gain levels for reasonably quiet sites
(concrete
slab or floor installation) should be about 20 to 60 (see 24 hour seismogram
record below for example of relatively small noise level. At this level,
one should
record the following events (sometimes the signal will be visible but no
distinct
arrivals will be distinguished and amplitudes will be so small that measuring
amplitudes for magnitude calculation will be impossible; sometimes very clear
arrivals and amplitude will be visible):
Local earthquakes (0 to 5 degrees distance) for magnitude ~3.5+
Regional earthquakes (5 to 20 degrees distance) for magnitude ~4.5+
Distant earthquakes (20 to 90 degrees distance) for magnitude ~6.0+
Very distant earthquakes (90 to 180 degrees distance) for magnitude ~6.5+
(these "rules of thumb" will depend on the site and gain level, your
location, for example, western US or eastern US, and the nature of the
earthquake and the path that the waves have traveled from the earthquake
location to your station)
My experience is that a very "visible" earthquake signal is
recorded, on the average, of
about once a week. About half of these can be used to calculate a
magnitude (mb or MS)
and about a fourth are at distances of less than 105 degrees and have S
arrivals that are
distinct enough (usually after filtering with a 0.0001 to 0.2 Hz bandpass
filter in
AmaSeis; the S arrival is often best seen by the lower frequency arrival) to
pick an S-P
time (using the arrival picking tool in AmaSeis) and estimate the distance to
the event
using the S-P and travel time curve matching tool in AmaSeis.
for information on downloading and installing AmaSeis and for more information
on recording and using the AS-1/AmaSeis seismic data):
1. Help file for AmaSeis:
AmaSeis has a fairly complete help file that includes instructions for setting
up the
software, options that are available for analyzing and displaying data, and a
tutorial. To
obtain help information, click on the Help icon (menu) at the top of the
screen, and then
on Help topics. Select Contents. Double-click on one of the topics
and then on one of
the question marks that appears before the name of a sub-topic, then use the
double-arrow
controls (right or left arrows at the top of the Help dialog box) to find
information related
to that topic. To investigate a specific topic, feature or setting,
select Index from the Help
topics dialog box, and double-click on the appropriate entry.
2. Settings For AmaSeis:
Settings Menu
Helicorder
Lines per hour: usually set to 1 (gives a 24 hour record on screen which
is useful for
monitoring because when you observe the screen once per day, you will know
immediately if an earthquake has occurred; to go back to look at earlier data,
use the
scroll bar on the right); can be set to as high as 20 (60 in the latest
release of AmaSeis) so
that the cursor moves quickly across the screen and the details of the
waveform can be
seen; this setting is useful for laboratory testing and demonstrations and
"make your own
earthquake" (stomp test, etc.) activities; for lines per hour greater
than 1, temporarily set
the decimate factor to 1.
Gain: amplification of the signal by the software; set to 1 for set up
and initial centering
of the trace on the screen, then to a higher number depending on the noise
level; in
reasonably quiet installations, a gain of 50 or more is possible.
Low pass filter cutoff: low pass filter option for the trace on the
helicorder screen
display; set at 3 Hz; if high frequency noise is visible on the screen, try a
lower cutoff
such as 1 or 0.5Hz to see if the noise is reduced; earthquake signals will
generally not be
greatly affected by this cutoff frequency for this seismometer.
Days to retain record: this value controls how far back in time one can
scroll (if you are
scrolling back more than a few days, move the scroll bar slider with the mouse
instead of
using the up or down arrows or clicking on the bar above or below the slider)
to view the
seismic data on the screen; set to 365 to allow viewing data back as far as
one year; the
value can be set larger, but if there are old data that you would like to
view, they usually
would have been saved as individual events (extracted and saved as .sac files;
so that data
archive doesn't get too large) and then opened under the File menu.
Decimate factor: Set to 10, normally; this setting will provide a fairly
accurate view of
the data on the screen but allow rapid scrolling; a decimate factor of 10
plots only every
10th point; some distortion is caused by this choice, so for large numbers of
lines per
hour, "make your own earthquake" experiments, and whenever you want
to see the most
accurate view of a seismogram on the screen, set the decimate factor to 1.
The decimate
factor does not affect the plotting of an extracted seismogram which is always
plotted
with all of the available points.
Settings
Show data values
This dialog box and display allows one to center the cursor so that it tracks
approximately
on the "zero line" (blue line) that is defined for each hour.
To center the trace, it is
convenient to start with a low gain setting (such as 1). Turn the black
knob on the small
black interface box that comes with the AS-1 seismometer that between the
seismometer
and the computer in the electric circuit. The knob adjusts the cursor
position on the
screen. It is very sensitive, so make small adjustments and determine
which way to turn
the knob in order to cause the cursor to move upwards. As you adjust the
knob, the
cursor should get closer to the zero line and the display numbers should be
close to zero.
When you have succeeded in getting the numbers close to zero, turn the gain up
and
repeat, eventually setting the gain to the highest level that is consistent
with the
background noise (you should se relatively continuous, small, about +/- 1 to 2
mm,
movements on the screen), and making the displayed values close to zero.
To adjust the
knob when the gain is set to a high value (greater than about 10), it is
convenient to use a
pencil and very, very lightly tap the knob from the side to cause it to move.
THIS
CENTERING PROCESS CAN TAKE CONSIDERABLE TIME ... PROCEED
SYSTEMATICALLY AND PATIENTLY.
3.
Archiving data:
Recording the AS-1 seismic data with the AmaSeis software generates about 1 MB
of
data per day. If you computer has insufficient disk space to archive
data for many
months or years, you can copy the files onto a zip disk or to a CDR.
Alternatively, you
can sort through the recorded data and delete data for days in which there was
no
significant signal.
(Image is available from Larry Braile in the original MS Word file.)
Example of screen image. Gain setting at 60 (relatively quiet site in
basement, on
concrete slab). Note noise level. The event at about 7 hours and
10 minutes is a
magnitude 6.7 earthquake near the north island of New Zealand. Prominent
P wave
arrivals and surface waves (longer period wave train) are visible. The
light, straight
lines are "zero lines" that mark the approximate location of the
data for each hour of the
day. The "wiggly lines" are the output of the seismometer and
display both background
"noise" and seismic signals generated by earthquakes or other
identifiable sources. The
noise level is fairly low for this record. Noise levels are typically a
little higher during
the winter months, during periods of high wind, and when large storms are
located near
the coast producing significant microseisms.