I purchased a CME 4111 (previously called
a CME 2123) Russian MET seismometer back in the spring of 2002. The unit
had an advertised pass band of 60 seconds to 50 hz. I have experienced
some problems with the instrument over the time that I have owned it. However,
these problems have all been overcome and have not detracted from the overall
usefulness of the instrument, and my pleasure with it. I particularly
like the fact that it is a plug and play instrument and does not require a mass
lock. The instrument also required the use of a differential receiver
which Larry Cochrane was kind enough to put together.
While checking with other folks, including
some professional seismologists, I found they had reviewed the MET instruments
and that there are some problems in maintaining the accurate calibration
required of professional instruments. I have had to adjust the advertised
sensitivity numbers to bring the instrument more in line with its current sensitivity.
However for the overall study of seismic waves on all three channels, and
seismology in general, it’s a great instrument, and is virtually
maintenance free.
Overall, I have been impressed with the
instrument. It is great as it does not require the periodic leveling adjustments
of Lehman type instruments. I dug a hole in the front yard, and set a 12
inch square, 2 inch thick cement block in the ground. I covered the instrument
with a plastic bag and covered that with dirt. The instrument is
currently resting on the cement block doing its thing. It is an extremely
sensitive instrument and I like it. I periodically have compared the wave
forms received with nearby professional instruments and believe the instrument
does a good job.
Bob Hancock
Tucson, AZ
From:
psn-l-request@.............. [mailto:psn-l-request@............... On Behalf Of Dave Nelson
Sent: Thursday, February 24, 2005
00:53
To: PSN-L@..............
Subject: Microseism filter
I have recently added a Russian ASMET -1V MET broadband
seismometer to my setup. As I live near the coast the microseism level was
dominating the noise backgrond to the extent that earthquakes of interest were
being lost in the " noise". The microseism spectrum is very distinct
and I felt its elimination would not seriously effect the
earthquake detection capability .
I have developed a simple filter based on switched
capacitor technology that virtualy eliminates the microseism background. The
filter has no critical componants or capacitors and is tuned by a
clock frequency. The Q is 0.5 which gives roughly an octave bandwidth centered
at 0.015 hz or 6.6 seconds . Since the center frequncy is clock dependant it
can be shifted arbitarily but 6.6 seconds seems to work very well. The filter
is placed directly ahead of the A/D at the highest voltage swing level in
the system to avoid adding noise. Just two wires -- in and out (plus
power of course).
The results are very good, the harmonic like signal from
microseism is gone and the effective signal to noise ratio of eathquakes is
significantly better. The spectrum has a deep null at 6.6 seconds instead
of a dominant peak .
I know this kind of spectrum shaping can be easily done in
software but that is usually beyond the scope of an amateur or computer
dum- dum like me. The whole thing is two IC's and a 15 hz clock
generator. ( 100 times the center frequency)
The Russian seismometer is truely amazing. Virtually plug
and play -- four wires ,no adjustments or critical setup. The respose is
advertised to 20 seconds but seems to respond well to over 100 seconds on the
low end and too well to be useful ,given the urban background noise
,at the high frequency end. I have another swiched capacitor low pass filter
switchable to 5, 2.5 and 1.25 hz. to control the high end cutoff.
My setup also includes 3 axes of short period based on
HS-10-1 1 hz seismometers that I found in a surplus store. I am using
Amaseis and experimenting with Seismowin for analysis and display.
I am a retired aerospace engineer who spent too much of my
career in management and I am having a ball doing the seismology thing.Good to
be back doing some development and putting the old brain to work. I intend to
do some seismometer development as my next project -- I have some ideas for
some different ,but not outrageous, concepts which may ( or may not )
work.
I would be glad to correspond with anyone interested in or
commenting on what I am doing.
Dave Nelson ( California
not New Zealand)