In a message dated 31/12/2006, jerdell<at>cox.net writes:
Greetings,
I am trying to understand the band pass filters and have a few questions.
1. John Lahr stated that;
“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.”
You interpret this as six/2 = max frequency = 3Hz You tend not to get
waveforms accurately represented until you get several samples over the cycle,
say five.
So if a sample is taken every .17 seconds (6 times per second) and you
divide that in half you get .33seconds.
0.17 x 2 sec = 0.34 sec ~= 3 Hz
Does this mean that if the period is less than 3 seconds the AS-1 will not
record it?
The AS-1 response is from about 0.34 sec up to about 15 secs
I am used to thinking about sound and light and find it hard to believe that
it could take so long for one complete cycle. Does .33 Hz mean that only
33/100 of a wave passes a fixed point per second? I find all this amazing. I
just never thought about seismic waves like this before. I find it much easier
to think about period than Hz.
Seismic waves have high velocities compared to sound in air, but they have
long periods. The quake generates a wide spectrum of P and S waves, but waves
with frequencies above about 2 Hz are selectively abosrbed as they pass
through the earth. When the P and S waves reach the Earth's surface, they
generate Rayleigh and Love surface waves, which spread out radially, but have much
longer periods.
Are the following two statements true?
A "high-pass filter" allows waves with periods lower than a set corner
period to pass unchanged, while attenuating higher periods.
Correct.
A "low-pass filter" allows waves with periods that are higher than a set
corner period to pass unchanged, while attenuating lower periods
Correct.
2. What are the normal periods for P waves? S waves? L waves?
1 sec downwards, 2 sec downwards and typically 10 to 30 secs for very
distant events. Local events may have P & S frequency components over 10 Hz
(0.1Sec). I suggest that you download and read
_http://psn.quake.net/info/analysis.pdf_ (http://psn.quake.net/info/analysis.pdf) for more information
3. Is it best to just not use filters at all till you want to study a
recording in more detail?
With Amaseis, the waveform should all be recorded. The filter settings just
determine how you process the data for display. If you set the lowpass less
than 2 to 3 Hz, your are likely to miss some local P waves.
4. Considering my location in Arizona, how do you think I set my filters?
Just after the earthquake on 12/26/2004 a “sound” recording of the
earthquake was released. The recording was made by a recording device located on the
bottom of the Indian Ocean. The lowest sound we can hear is about 20 Hz. Does
that mean that the recording wasn’t real time? It must have been speeded up.
The quake would have been greatly 'speeded up', possibly over 1000
times. You simply convert the data file into a .wav file. You have several
playback speed options.
Hope that this helps,
Chris Chapman
Greetings,
I am trying to understand the band pass filters and have a few questions.
1. John Lahr stated that;
“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.”
You interpret this as six/2 = max frequency = 3Hz You tend not to get
waveforms accurately represented until you get several samples over the cycle,
say five.
So if a sample is taken every .17 seconds (6 times per second) and you
divide that in half you get .33seconds.
0.17 x 2 sec = 0.34 sec ~= 3 Hz
Does this mean that if the period is less than 3 seconds the AS-1 will not
record it?
The AS-1 response is from about 0.34 sec up to about 15 secs
I am used to thinking about sound and light and find it hard to believe that
it could take so long for one complete cycle. Does .33 Hz mean that only
33/100 of a wave passes a fixed point per second? I find all this amazing. I
just never thought about seismic waves like this before. I find it much easier
to think about period than Hz.
Seismic waves have high velocities compared to sound in air, but they have
long periods. The quake generates a wide spectrum of P and S waves, but waves
with frequencies above about 2 Hz are selectively abosrbed as they pass
through the earth. When the P and S waves reach the Earth's surface, they
generate Rayleigh and Love surface waves, which spread out radially, but have much
longer periods.
Are the following two statements true?
A "high-pass filter" allows waves with periods lower than a set corner
period to pass unchanged, while attenuating higher periods.
Correct.
A "low-pass filter" allows waves with periods that are higher than a set
corner period to pass unchanged, while attenuating lower periods
Correct.
2. What are the normal periods for P waves? S waves? L waves?
1 sec downwards, 2 sec downwards and typically 10 to 30 secs for very
distant events. Local events may have P & S frequency components over 10 Hz
(0.1Sec). I suggest that you download and read
_http://psn.quake.net/info/analysis.pdf_ (http://psn.quake.net/info/analysis.pdf) for more information
3. Is it best to just not use filters at all till you want to study a
recording in more detail?
With Amaseis, the waveform should all be recorded. The filter settings just
determine how you process the data for display. If you set the lowpass less
than 2 to 3 Hz, your are likely to miss some local P waves.
4. Considering my location in Arizona, how do you think I set my filters?
Just after the earthquake on 12/26/2004 a “sound” recording of the
earthquake was released. The recording was made by a recording device located on the
bottom of the Indian Ocean. The lowest sound we can hear is about 20 Hz. Does
that mean that the recording wasn’t real time? It must have been speeded up.
The quake would have been greatly 'speeded up', possibly over 1000
times. You simply convert the data file into a .wav file. You have several
playback speed options.
Hope that this helps,
Chris Chapman