In a message dated 2008/03/05, tchannel1@............ writes:

> Hi All,   Perhaps someone can explain the different in recorded data, 
> between a (let's say a Lehman) at 2 second period and the same sensor at 20 
> seconds.
> I have compared data from two such sensors.   Here is what I have found, but 
> I am sure I am understanding the TRUE difference.
> I get smaller EQ's on the shorter periods,  my guess is about 1/3 more 
> events.   Mainly on the closer EQ's or smaller EQ's for the 2 second machine.
> That is not to say the 20 second period machine, does not pick these up, 
> just not as many, if they are weak.   

Hi Ted, 

       You have a wide band incoming signal with instrumental, environmental 
and seismic nose on it as well as the signal you want to see. The not-so-easy 
bit is sorting these out. This also involves the amplifier + filter settings. 
       Extending the seismometer period does increase the sensitivity. You 
could expect to see 20 second signals at ~1/100 the amplitude on the 2 second 
sensor.

       Try wedging the mass and look to see what noise, if any, is recorded. 
You need to know what the electronic noise level / period is like. 

       Try comparing signals from the 2 second sensor with the 20 second 
sensor low pass filtered to 2 seconds?  

       Obvious differences are in the microseism noise which will be present 
on the 20 second sensor and the wider bandwidth allowing increased 1/f noise.

       While the 2 second sensor may 'see' signals during the reception of 
surface waves, this is unlikely to have a similar 'envelope' - outline wave 
shape - to the surface wave signal. You are selecting out the high frequency 
components of inherently noisy transient signals.

       What you see does depend on how you filter your signal. 
       You may be used to thinking in terms of sine wave signals. Seismic 
signals are pulsed waves several seconds long, usually with a wide range of 
periods.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/03/05, tchannel1@............ writes:



Hi All,   Perhaps som=
eone can explain the different in recorded data, between a (let's say a Lehm=
an) at 2 second period and the same sensor at 20 seconds.

I have compared data from two such sensors.   Here is what I have=
 found, but I am sure I am understanding the TRUE difference.

I get smaller EQ's on the shorter periods,  my guess is about 1/3 more=
 events.   Mainly on the closer EQ's or smaller EQ's for the 2 sec=
ond machine.

That is not to say the 20 second period machine, does not pick these up, ju=
st not as many, if they are weak.   




Hi Ted, 



       You have a wide band incoming signal wi=
th instrumental, environmental and seismic nose on it as well as the signal=20=
you want to see. The not-so-easy bit is sorting these out. This also involve=
s the amplifier + filter settings. 

       Extending the seismometer period does i=
ncrease the sensitivity. You could expect to see 20 second signals at ~1/100=
 the amplitude on the 2 second sensor.



       Try wedging the mass and look to see wh=
at noise, if any, is recorded. You need to know what the electronic noise le=
vel / period is like. 



       Try comparing signals from the 2 second=
 sensor with the 20 second sensor low pass filtered to 2 seconds?  



       Obvious differences are in the microsei=
sm noise which will be present on the 20 second sensor and the wider bandwid=
th allowing increased 1/f noise.



       While the 2 second sensor may 'see' sig=
nals during the reception of surface waves, this is unlikely to have a simil=
ar 'envelope' - outline wave shape - to the surface wave signal. You are sel=
ecting out the high frequency components of inherently noisy transient signa=
ls.



       What you see does depend on how you fil=
ter your signal. 

       You may be used to thinking in terms of=
 sine wave signals. Seismic signals are pulsed waves several seconds long, u=
sually with a wide range of periods.



       Regards,



       Chris Chapman