Thread: Re: Filtering Options in AmaSeis

Started: 2006-10-01 13:50:24
Last activity: 2006-10-01 13:50:24
Topics: IRIS EPO
Michael Breed
2006-10-01 13:50:24
I have a question for everyone. I'm new to seismology and have just
gotten our AS-1 running nearly to my satisfaction. Can anyone tell me
why the background vibration for a number of stations this morning
picked up suddenly? At around 2:00-3:00 UTC, it picked up for 4-5 hours
prior to the Kuril Islands' 6.2 event. I saw this on SUNY Binghamton,
Iowa, and several other locales. I've seen this a number of times.
Thanks.

Michael Breed
Chenango Valley High School Science Dept.
1160 Chenango Street
Binghamton, NY 13901
(607) 779-4743 x4248
John or Jan Lahr <JohnJan<at>lahr.org> 10/01/06 3:02 AM >>>
The Kuril Islands earthquake provides an opportunity to see how
filter settings can effect the appearance of the helicorder record in
AmaSeis. Two filters are involved, a high-pass filter and a low-pass
filter.

A "high-pass filter" allows frequencies higher than a set corner
frequency to pass unchanged, while attenuating lower frequencies.

A "low-pass filter" allows frequencies that are lower than a set
corner frequency to pass unchanged, while attenuating higher
frequencies.

When both filters are set, the filter is called a "band pass" filter.

The corner frequencies can be adjusted by specifying either the
frequency or the period of the corner. The period is just 1 divided
by the frequency. For low frequencies, such as .05 Hz, it's easier
to think in terms of period -- in this case 20 seconds period.

Since the AS-1 samples the amplitude of the seismic signal about 6
times per second, clearly high frequencies will not be well
represented. The highest frequency that it's possible to record
faithfully is one half of the sample rate. (For more details on this,
look up Nyquist Frequency,
http://en.wikipedia.org/wiki/Nyquist_frequency)

Therefore, the low pass corner setting for AmaSeis should never be
set to a frequency higher than 3 Hz.

In order to enhance the surface waves, which have a lot of energy in
the range of .05 Hz (20 seconds period), I find it useful to set the
helicorder filters to pass the band from 12s to 25s. In other words,
the low-pass filter corner is set to 12 seconds and the high-pass
filter corner is set to 25 seconds.

Try these settings on today's record to see the effect on your
station. Keep in mind that the filter settings do not change the
data that are stored on disk, but simply change the way they are
displayed.

To see what a broad-band research station looks like unfiltered,
low-pass filtered, and high-pass filtered, click on this image to
toggle between the three
views: http://www.jclahr.com/science/psn/cor/index.html

Cheers,
John

Universal Time (UTC): 30 Sep 2006 17:50:25
Region: KURIL
ISLANDS
Geographic coordinates: 46.423N, 153.003E
Magnitude: 6.3 Mb
Depth: 22 km



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