Unfiltered AmaSeis record of 3/24/2007
Unfiltered AmaSeis record of 3/24/2007
Same time interval with the band pass filter set to 10s to 25s and the gain
increased by a factor of ten.
The high frequency P arrival is clear in both records, but the surface waves are emphasized by the filter and gain settings. These records show a combination of the waves from two different earthquakes that took place within two minutes of each other.
Another example from Alan Jones comparing unfiltered (left) with filtered
(right) helicorder images.
Any seismic signal can be decomposed into an infinite set of sine waves, each
with a specific
amplitude and frequency. If all of the sine waves are added up, the original
seismogram will be
produced. Often the signal of interest has a low frequency and is recorded with
a low amplitude.
When viewed on the original seismogram, this small signal is buried in
higher-frequency, higher-
amplitude noise. In this case, if just the high frequencies are reduced in
amplitude by a "low-pass
filter," then the gain can be increased to boost the low frequencies enough to
become visible.
AmaSeis has two filters, 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 0.05 Hz, it's easier to think in terms of period -- in this
case 1/0.05 = 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 theoretically 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
There is a interactive applet that demonstrates filtering of a signal that
consists primarily of just two frequencies. The band-pass filter bands can be
adjusted to "bring out" each of the frequencies by itself. In the case of
seismology, the lower frequency would be so small that it would be hard to see
prior to filtering out the higher frequency and raising the gain. See:
http://www.chem.uoa.gr/applets/AppletFourAnal/Appl_FourAnal2.html
Finally, here's an example in which a regional earthquake is not visible prior to applying a high-pass filter.