PSN-L Email List Message

Subject: A/D bits, LP filters
From: sean@...........
Date: Sat, 13 Jan 2001 03:00:41 -0600 (CST)

Regarding the amplitude of seismic waves and digitizer range:

I must disagree with Karl when he says 8 bits of accuracy, by which I
assume he also means resolution, is sufficient. (Accuracy more properly
describes the precision of the reference voltage that determines the value
of a bit). Modern seismic studies use the entire content of the waveform 
for source modeling, tomagraphy, path attenuation, etc, which are dependent
on having an un-clipped and un-distorted signal for spectral analysis.
A single clipped peak ruins the spectral analysis.

Earthquake waveform amplitudes vary logarithmically, and the magnitude
scales reflect this. A magnitude M 5 is 100 times more amplitude than a 
M 3 (everything else being equal). An 8-bit digitizer has a range of
256 counts, 12 bits is 4096, and 16 bits is 65536 counts (or half these
values if a bit is used for the sign (+,-).  All our newer multichannel
broadband stations use 24 bit digitizers which have a range of 16 777 216 
counts for a 40 volt p-p signal from sensors with an output of about
2000 volts/meter/second. While we infrequently (like monthly) see amplitudes
that actually exceed the 16 bit range, large quakes greater than M 7+ do
use much of the dynamic range. I have data from our stations exceeding 
2 000 000 counts from South Pacific and Eurasian events. Of course a great 
quake in the near field of the station would clip the 24 bit system, so most
IRIS stations also have a 3 component FBA (Force Balance Accelerometer).
The background noise (natural and instrumental) of our stations runs
about 200 counts at about -160 db PSD. The least count is 2.384 microvolts.

But for my digitizer here at home, I am still using the 12 bit multimeter
at 1 sample/second, with a full scale of 200 mv, and a least count of 
0.1 mv connected to the STM seis with an output of 1640 v/m/sec. 6-second
microseisms run 3 to 15 mv, and it clips on most events greater than M 7.0.

And a comment about filters. Doug is quite correct about the natural
attenuation of frequencies above a few tens of hz. For all out short
period stations (1 hz seismometers) we have a 4-pole Bessel filter in
the pre-amp (plus the usual single 30 to 50 hz low pass poles (to reduce 
amplifying 60 hz noise) that are created by by-passing the high value gain 
(feedback) Rs with a capacitor, as shown in the pre-amp schematic on my site).
After the telemetry (VCO, FM transmitter, FM Receiver, and Discriminator PLL)
another 17hz 4-pole filter provides anti-aliasing for the 20 sps digitizer.
At my CCM station, the 1 hz high gain system for the CTBT (Comprehensive 
Test Ban Treaty) uses a 24 hz low pass filter for its 40 sample/sec channels.
Although the rules for anti-aliasing are exact, the content of the seismic
signal allows some convenient variation; as Doug points out, the actual
noise is only a small percentage of the full scale signal.


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Larry Cochrane <cochrane@..............>