PSN-L Email List Message

Subject: Re: Water/Laser geophone instrumentation
From: Charles R Patton charles.r.patton@........
Date: Tue, 11 Jun 2013 09:42:10 -0700

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VBB-BB Tiltmeter info
5/5/2001 11:45 PM
from S-T Morrissey
This was the project he was working on just before he died.

In addition a Chris  email references the NSF (National Science 
Foundation) paper:
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Re: Morrissey Balance-beam Tiltmeter
Date:Thu, 25 Dec 2008 22:40:21 EST
This contains info that the project was finished.  I have a copy of the 
NSF proposal (not the final report).  It includes data on proposed 
sensitivity, notes on balance beam construction and proposed transducer 
methods and problems.   I will email the 1.05MB file to interested 
folks. I don't have the final report mentioned by Chris.

The abstract mentioned above is:
********start quote*******
Abstract submitted for 2000 fall AGU meeting:

Updated 10 December, 2000

A Beam-Balance Broadband Tiltmeter That is Insensitive to Horizontal 

Sean-Thomas Morrissey (Dept. of Earth and Atmospheric Sciences, St.Louis 
University, 3507 Laclede; St. Louis MO, 63103; 314 977 3129; 

A new broadband seismic instrument has been developed that is sensitive 
only to "pure" tilt and not to horizontal acceleration.

A new beam-balance tiltmeter has been developed that does not respond to 
horizontal ground movement. Since all horizontal seismometers are also 
tiltmeters they are sensitive to tilt, especially at longer periods, and 
seismic data can be compromised in the period range of 20 to 3000 
seconds. The most important noise source in the horizontal data is 
tilting of the pier, mostly due to barometric loading. But all 
tiltmeters, up to now, are also seismometers, (ie. they are sensitive to 
horizontal translation), so can not be used to separate tilt noise from 
seismic signals.

Our new beam balance tiltmeter does not respond to horizontal 
acceleration because the masses at each end of the horizontal beam are 
suspended through the exact center of mass. This system is inherently 
unstable, so broadband feedback is used to control it. With appropriate 
feedback, the beam remains relatively horizontal when the base is 
tilted, and the output is the relative motion at the displacement 
detector. There is no rotation or output when the base is translated 
horizontally along the axis of the beam.

The beam-balance tiltmeter is designed with separated lead masses 
mounted in an aluminum bar that is suspended exactly at the center of 
mass of the horizontal beam with a new low torque hinge flexure. The 
center of mass is trimmed by a unique vertical mass centering adjustment 
above the flexures. Displacement transducers and compact force feedback 
coils with rare-earth magnets are placed at both ends of the beam.

Three sensors have been assembled as "proof of concept" prototypes. One 
instrument is operating at station CCMO, near Saint Louis University, 
and data from it are being digitized. Another is on a table and can be 
used to demonstrate that tilt can be separated from horizontal 
acceleration by simply sliding it horizontally. The static or DC tilt 
sensitivity of the prototype is about 120 millivolts per microradian, 
and the resolution is better than 0.1 nanoradian. Initial comparisons 
using data generated by large quakes on the horizontal components of 
nearby broadband seismic stations (18 km distant) show that the response 
to horizontal acceleration is reduced by a factor greater than 1000 
while maintaining the equalivant tilt sensitivity of the seismometer.

Ideally, the noise recorded in the tiltmeter output will exactly emulate 
the tilt noise from the seismometer, at least in the flat portions of 
their broadband velocity response. The success of this new instrument 
has a tremendous implications for broadband stations in all regions of 
the world where tilting from barometric, thermal, hydrologic, etc., 
effects may limit the usefulness of the horizontal data.
Charles R. Patton

On 6/11/2013 8:19 AM, Ted Channel wrote:
> Hi All,  Chris your rain gutter idea is very interesting too.  Is 
> there a pict illustrating the version of the Cascades used?
> Here is a different question...   Picture a perfectly balanced beam, 
> on a center pivot, just like a see-saw, with two kids of similar mass.
> Would this beam tend to remain fixed during an earthquake?    I can 
> visualize, how a diving board with a person on the end would respond, 
> with the mass moving differently from the earth.   But I don't know if 
> the see-saw, would tend to remain fixed/level as the earth isolated 
> during an event.
> cheers, Ted


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