PSN-L Email List Message

Subject: larger seismometer mass
From: S-T Morrissey sean@...........
Date: Fri, 15 Oct 1999 11:57:25 -0500 (CDT)


OK everyone, before I get more mail on the subject:

Re largest commercial mass of seis: there are larger:

An oversight on my part, as I was only thinking of long period
seismometers in the context of the Lehman questions.
I suggested the S5100 series because almost 400 three-component
sets were made, mostly for the WWNSS.(Worldwide Network of Standard
Seismographs) of the late '50s. 

To be technically correct (as asked!), the LP-Z is 11.201kg,
and the LP horizontals are 10.678kg. A number of PSNers have
these seismometers, some in operation, like at Merediths' station.

Larger masses (much) were in the 1-second Big Benioff short
period seismometers of the WWNSS. Also, even larger masses were used
in individual experiments, but not in large quantities.

I have had considerable experience with these big Benioff seismometers
with 100 kg net masses; in fact I am still operating one of the 
Geotech prototype verticals (c1956) in our vault at SLM. I have had
to relocate several sets, like from the old FLO (Florissant, MO),
station (I called a safe moving company) and from ADK (Adak, AK). 
Even when the masses are removed, the 3/4" thick steel frame leaves 
over 200 pounds to deal with.  The spring for the vertical weighs 
over 20 pounds.

Fortunately, Geotech realized that when high power output( to drive
a 2 hz galvanometer to a photographic magnification of 100k or more)
was not necessary, just the moving coil portion  of the variable 
reluctance transducer had enough mass at about 14 kg. So they built 
the "baby" Benioff, of which I still have two sets in service with 
telemetry. The transducer is hugh, with 8 windings, and an inductance
of 5 henries, which has to be accounted for in the transfer function,
since the inductive pole is at 4.5hz. But for all its size and complexity,
the output is only 135V*sec/meter, or about half the open-circuit
output of an L4-C (5500ohm coil) . With electronic amplification, 
this is the main consideration.

Regards,
Sean-Thomas

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