PSN-L Email List Message

Subject: drum recorders
From: S-T Morrissey sean@...........
Date: Sun, 18 Jun 2000 13:45:53 -0500 (CDT)


You mention that many seismic observatories still operate visible
drum recorders rather than count on a computer display.

Its not that there isn't a plethora of good display software available,
like HELIEM, that try to make the display actually look like a drum
record in nearly real time. At some institutions the data is even 
available as a screensaver.

But there are several advantages to the analog recording, which is 
usually of 24 hours duration on 11" x 36" sheets of paper. 

a: The computer display depends on a chain of software, and while it
indicates a problem if it goes flat, this doesn't necessarily mean that
the station is down. We have drum monitors at two IRIS stations at state
park visitor centers, and although they may not know what the wiggles
mean, they jump on the phone when they stop. (Like two weeks ago at CCM:
they called about some new construction and to ask me to mark the buried
fiber optic cable, which I could easily do the next day, since it was 
marked with magnets; however, they called the next AM that the drum went 
dead, and they found that the contractors hadn't waited and had put a 
backhoe through the cable, stretching it in two, which will now require 
replacing about 100 meters of cable to repair it).

b: The analog drum is an excellent "wazzup" monitor of activity; anyone
can glance at it and know something has happened. One has to "look at"
a computer display, often needing to key in display parameters. But live
drum recorders make great station monitors; even when everything is 
digitized, you don't have to manipulate a display to be aware that there 
is an event or a problem. At SLU we maintain several SP (60 mm/min) 
and LP (15 mm/min) pen/ink drums. The one in the building lobby is 
connected to the 600 second VBB, and does show the constant meander of 
microseisms (even though they are reduced by 1/100th in the broadband 
signal by a TT notch filter). And NEIS has a dozen or so drums monitoring 
key corners of the world, as well as HELIEM displays; but everyone first 
looks at the drum records; one can take in the whole earth's activity at 
a glance from the hallway.

c: Television loves the drum displays and records. We have even had them
take a past record and wrap it around an unused drum to tape the event.
When the Iben Browning nonsense was happening (the "prediction" of
a major New Madrid quake), they were fighting for space in front of
the drums to get a live shot of the event that never occurred.

d: Although the computer display can be printed, the drum records make
excellent display items; at the State Park visitor center at CCM, they 
tack up the "quake of the week", etc, and mark up the record with timing 
and phase info. So if the live drum is quiet, the visitors can look at 
past activity while they wait for their cave tour (in Onondaga cave:
the CCM sensors are in isolated Cathedral cave, 3km west of it).

I have an old "Portacorder" here that I have recycled with a hot stylus
on FAX paper; at 2mm trace spacing, the 2.5 day record is crowded, 
but I could see the quake this AM from the kitchen over the coffee
mug, and then power up the PC monitor to see wazzup. I wish I had 
the time to put together some ideas for a PSN monitor drum recorder.


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