PSN-L Email List Message

Subject: Re: Channel, 1,2 and 3?
From: Brett Nordgren Brett3mr@.............
Date: Sun, 23 Mar 2008 07:40:03 -0400


No particular problem; I think what you are seeing can be 
explained.  First, there's always the possibility that the E/W component of 
the quake motion was much smaller than its N/S motion.  However that's not 
what I believe you're seeing.

Of the six motions that these sensors might respond to, Channel 1 responds 
well to both E/W translation and to E/W tilt/rotation oscillations.  The 
rotations might better be described as rotation about a N/S axis.

Channel 2 responds only to vertical translation.

Channel 3 responds well to N/S tilt oscillation, that is, rotation about an 
E/W axis, and has almost no response to N/S translation.

Then how do we explain the Channel 3 sensitivity to you walking and to the 
backhoe but not to the quakes?  I think it's a matter of 
wavelength/frequency.  The local disturbances have much more high-frequency 
content.  In Rayleigh waves, the size of the tilt oscillation component 
(rotation) is inversely related to wavelength.  So low-frequency, long 
wavelength telesisms, have very little tilt compared with translation, 
while local high-frequency noises have a relatively much larger 
tilt/rotation component.  (Also, I would expect that Channel 3 would 
respond much more strongly to local disturbances located to the North or 
South, compared with ones directly to the East or West.)

The thing to note is that long period does not by itself make for a good 
sensor, you also need to have sensitivity to the motions you are hoping to 
measure.  The nearly balanced pendulum is not very sensitive to 
translation, only to rotation/tilt.


At 06:24 PM 3/22/2008 -0600, you wrote:
>Hi Folks,  I just posted three data files, of the 6.2M ANDREANOF ISLANDS, 
>ALEUTIAN IS. 080322.212412 as ch1, ch2 and ch3
>Channel 1 is what I call a tiltmeter, just a normal pendulum with a 2 
>second period.  It can only swing East and West.  It works well
>Channel 2 is a vertical Spring Slinky II, also has a 2 second period.
>Channel 3 is my newest on called a Nearly Balanced, a normal pendulum, 
>with a 20 second period, It can only swing North and South.
>Channel 1 and 2 recorded the event well.
>Channel 3 did not record it at all.   I know it is working, as all three 
>are recording the road construction, all three show the same large backhoe 
>at the same moment in time.
>Channel 3 is also sensitive to me moving around the upstairs.
>I posted all three and filtered Channel 1 and 2, using Low pass 1, and 
>High pass .05..   These filters did nothing on channel 3 so I posted it 
>Perhaps someone can look at all three, to see if I have a problem.   I 
>don't think it is over damped, if anything it is under.   Could this be 
>the direction the channel is facing?
>Thanks, Ted


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