PSN-L Email List Message

Subject: Re: Integrating in WinQuake
From: RSparks rsparks@..........
Date: Mon, 23 Nov 2009 15:04:02 -0800

Thanks to all who replied to my query.

Bob, you were exactly right.  As you might remember, I am using a 
tweaked version of Amaseis.  One of the tweaks is the ability to 
generate a PSN file customized to my location with automatic naming of 
the file.  All I needed to do was to change my hard coded description of 
my seismometer from velocity to acceleration.  Then WinQuake recognized 
the file as describing an acceleration device and allowed double 
integration.  Thanks for the advice.

Chris, the link through John Lahr's PSN site to Prof Wielandt's 
materials no longer works.  I did find what is apparently his new site 
at this address 
 but did not 
take time to search for his graphs that you mention.

There seems to be an ongoing difference of opinion of what constitutes a 
velocity detector or an acceleration detector.  To my mind, if a 
seismometer automatically returns to a zero position, then it must be 
recording acceleration.  All vertical seismometers do this because they 
all are measuring against gravity, so they would all be acceleration 
devices.  Excepting the sensitivity to tilt, all horizontal seismometers 
also return to a zero position so they could also be called acceleration 
sensitive devices.  But maybe this description oversimplifies the situation.

Some sensors (capacitive and optical for example) clearly record 
displacement (neither acceleration or velocity).  On the other hand, 
with magnetic/coil devices, velocity is always observed when electrical 
output is observed so magnetic/coil devices are velocity detectors.  
Following the formula's given by Geoffrey, if there is displacement, 
there must have been velocity, and if velocity, there must have been 
acceleration at some time in the past. 

All this said, the best we can do with a seismometer is to detect motion 
against a floating mass which in turn moves as a result of the action of 
detection and carried over velocity from past accelerations.  A 
displacement sensor would record the relative distance from a some zero 
point at the instant of data read, the velocity sensor would record the 
relative velocity at the instant of data read,  and acceleration would 
be a calculated number found by using the data from any two velocity 
data points and any three displacement data points.

Finally, if two seismometers, identical except for detectors, were 
placed side by side, they would both plot the identical earthquake wave 
form, assuming that the frequency characteristics were the same.  
However, when it is recognized that displacement position is not time 
sensitive but velocity is, the builder can expect dramatic frequency 
response differences between displacement detectors and velocity 
detectors .  Simply put, distance is distance, but velocity is the 
distance divided by the time needed to travel between two points.   As a 
result, for velocity detectors, the longer the wave length, the less 
energy for each instant resulting in decreased voltage (and current) 
detected at each instant (for any defined magnetic field).

It is easy to see why there might be differences of as to type of 
detector in use and how to best put the resulting data to use.



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