PSN-L Email List Message

Subject: boom angle of horiz seis
From: S-T Morrissey sean@...........
Date: Tue, 15 Feb 2000 13:59:41 -0600 (CST)


You ask about the proper angle of the boom of a horizontal seismometer.
This has to do with the restoring force that makes it swing back to
center. This force is a portion of gravity, namely  g*sine(i), where
i is the angle with respect to the horizontal, measured in radians. In the 
case of a SG, the boom is vertical, but horizontal in the Lehman design.
There it is generally assumed that the boom support is adjusted so the boom
is parallel to the base, so the base angle adjusts the period. IF the
boom is exactly horizontal, it will experience NO gravitational restoring
force, so it will swing back and forth aimlessly. If the mass end is
lowered slightly, it will swing in a shallow curve in the gravitational
field, with the minimum being at the bottom or center of the swing. 

Here is a repeat of the calculation of the actual numbers you can expect:

SO ... Lets consider some formulae of interest for the horizontal pendulum:
(assuming that the restoring force by the hinges and/or pivot are minimal):

The natural period: Tn = 2*pi*sqrt(L/(g*sine i))
where L is the boom length in cm, g=980cm/sec^2, i is the angle
that the boom makes wrt the horizontal, (if i is measured in radians,
(360 degrees = 2*pi radian, or 1 radian = 57.3 degrees), and i is small,
sine i = i). For example, a 40 cm boom hanging vertically (mass at the bottom)
as a simple tick-tock pendulum ( an SG design at an angle of 90 degrees) 
has a period of 1.3 seconds. (a one second clock pendulum is 24.8 cm). 
But the pendulum supports or hinges can be arranged in a "garden gate" 
configuration as is the Lehman and most long-period horizontals. When 
tilted horizontally to about 4 degrees, the period is 5 seconds. At
about a 1 degree ((2*pi/360) radian) angle, it is 10 seconds, and at 
about 0.23 deg. it is 20 seconds. However, if we increase the boom length 
by times 4 to 160cm, (an impractical 60 inches), we also get a period of 
20 seconds.  So the period is changing with the square root of the boom
length as well as the inverse of the square root of the angle the boom
makes with the horizontal. In general, a practical boom length is 10"
to 15", with a baseplate of 15" to 24" long and about half as wide at
the leveling end (for a horizontal; leveling for a vertical, as shown
above, is nowhere as critical, and 4" to 8" widths are workable).

It is important to note that the size of the mass determines nothing
of the period or sensitivity to tilting. Any reasonable size will work;
larger is better for overcomming any torque of the hinges or flexures,
to the point where the mass/boom structure begins to distort any part
of the suspension.  (THe size of the mass IS a factor in a VBB fedback 
system). The total mass of the boom should be less than 10% of the main
mass, which includes the sensor coils.

The tilt sensitivity of a seismometer is therefore a function of the
square of the operating period Tn.

		(where z is the displacement, and phi is the tilt)
		z = (g * Tn^2 / 4 * pi^2) * phi 
		z = (g * Tn^2 / 8 * pi^2) * phi^2) (vertical)

Note the vertical sensor responds to the SQUARE of the tilt. BUT .. Since 
the angle is always small and less than 1, the square of a small angle
(measured in radians) is smaller than the original number. So conversely
the horizontal is MORE sensitive to tilt of the base (at a right angle
to the boom) by the square of the tilt angle.

SO what does this mean in comparing the tilt noise of a vertical
compared with a horizontal of the same period. Suppose the seis is
in a corner of the garage or basement. Then suppose that when you
walk up to the site you deflect the floor by 1 micron (10^-6 meter)
when you are 1 meter away (Or your neighbor parks his Humvee 100
meters away and deflects the neighborhood by 0.1 millimeter.) In both
cases the tilt is delta(L)/L or 10^-6 radian.  So if your horizontal
seis has a period of 10 seconds, the mass will offset 24.8 microns.
This is a large number; the 6-second microseisms run about 2 to 4 microns.
HOWever: if you have a vertical seis, the displacement from a 10^-6 tilt
is 10^-6th of the horizontal. Conversely, it takes a floor deflection
if 1mm at 1 meter distance to get the same 24.8 microns movement on a

So when you push the operating period from 10 to 20 seconds, the tilt
sensitivity increases by 4. Even a modest period increase demands a good
site for the instrument. The WWNSS (worldwide network of standard 
seismographs) originally tried to operate the long period sensors at
30 seconds, but so many were always at the stops that they backed
off to 15 seconds as the standard.

You can test the tilt sensitivity using your leveling screw, which
I guess is something like 40 threads/inch. In the above formula, "phi"
is the angle in radians, so if you turn the screw one turn and if the base
support width is 10 inches, the tilt is 1/40" divided by 10", or 0.0025
radians. Using the formula above for a horizontal sensor:
The displacement then is 0.062cm times the square of the period. If Tn
is 10 seconds, it is 6.2cm; if Tn is 100 seconds, one turn of th 40 tpi
screw will try to move the boom 620 cm. Even if 1/100 turn can be used,
the displacement is still quite large. THis is why VBB instruments have
a feature that allows a shorter period to be switched in for setup at
installation, and a motor-driven lever of 100:1 to level the sensor in
operation. A typical tilt noise level for the 360-second STS-1 is 
equivalent to about 6 nanoradians. For practical operation of a home-made 
VBB, an operating period of 20 to 40 seconds would be preferred.



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