In a message dated 04/12/2007, tchannel1@............ writes:

I am using Larry's board, AmaSeis and  Winquake.   I just made a new coil and 
magnet assembly, using 4000  turns of 38 ga. and a stack of three ring 
magnets, each .250  x.750"
This replace a coil of 2000 turns and two magnets  .250 x.750"   Larry's 
board in set for the high gain, but with the  pot turn all the way down to minimum.
 
My question is what is the best approach to  setting the Amp. gain?   This 
new coil arrangement is more powerful  the the last one.  Using AmaSeis the 
helicorder gain in 1 or 2, more than  that is too much.   I know this gain 
adjustment is just for viewing,  and 1 or 2 looks okay.
 
I could move the jumper on Larry's board to the  lower gain level, or I could 
leave it on the higher gain.
 
I am just looking for some advise as to setting  the gain.  
The best one I heard, was to use a quiet night  and set the gain to show none 
or just a little back ground  noise.

Hi Ted,
 
    First, check the gain and measure the noise when  the magnet is away from 
the sensor. Then turn the gain up to maximum and check  again. Use the 'Show 
Data Values' in AmaSeis. It can be informative to record  the trace for an 
hour or more to see if you are getting any noise  pickup.
 
    The two essential factors that you DON'T mention  are the period of the 
sensor and the resolution of your ADC! Assuming that you  are using a 12 bit 
ADC and that you can sense the 6 second microseisms, you want  to be able to 
actually measure seismic signals of less than 1/20 the  average 2 micron 
displacement - minimum ~0.5 micron. 
    Set the gain initially to give a background of  at least +/-20 total 
counts, then look at the short period and long period  filtered counts. Increase 
the gain to give at least +/-10 filtered counts in the  seismic periods of 
interest. 
    Then start recording quakes and see whether your  gain setting a) records 
small quakes OK and b) doesn't saturate the trace  on most of the large 
quakes. 
    If you can sense out to 20 seconds, you can run the  low pass display 
filter at 8 to 10 seconds and pick out the surface waves.  You can then use 
filters to hunt for the P and S waves if they are not  obvious on the raw extracted 
trace. There is a long period display filter  in AmaSeis to compensate for 
sort periods, but it does need some counts to do  the arithmetic! 
    
    Regards,
 
    Chris Chapman



   





In a message dated 04/12/2007, tchannel1@............ writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
I am using Larry's board, AmaSeis and=20
  Winquake.   I just made a new coil and magnet assembly, using 40=
00=20
  turns of 38 ga. and a stack of three ring magnets, each .250=20
  x.750"
  
This replace a coil of 2000 turns and two=
 magnets=20
  .250 x.750"   Larry's board in set for the high gain, but with t=
he=20
  pot turn all the way down to minimum.
  
 
  
My question is what is the best approach=20=
to=20
  setting the Amp. gain?   This new coil arrangement is more power=
ful=20
  the the last one.  Using AmaSeis the helicorder gain in 1 or 2, more=20=
than=20
  that is too much.   I know this gain adjustment is just for view=
ing,=20
  and 1 or 2 looks okay.
  
 
  
I could move the jumper on Larry's board=20=
to the=20
  lower gain level, or I could leave it on the higher gain.
  
 
  
I am just looking for some advise as to s=
etting=20
  the gain.  
  
The best one I heard, was to use a quiet=20=
night=20
  and set the gain to show none or just a little back ground=20
  noise.
Hi Ted,
 
    First, check the gain and measure the noise whe=
n=20
the magnet is away from the sensor. Then turn the gain up to maximum and che=
ck=20
again. Use the 'Show Data Values' in AmaSeis. It can be informative to recor=
d=20
the trace for an hour or more to see if you are getting any noise=20
pickup.
 
    The two essential factors that you DON'T mentio=
n=20
are the period of the sensor and the resolution of your ADC! Assuming that y=
ou=20
are using a 12 bit ADC and that you can sense the 6 second microseisms, you=20=
want=20
to be able to actually measure seismic signals of less than 1/20 the=20
average 2 micron displacement - minimum ~0.5 micron. 
    Set the gain initially to give a backgroun=
d of=20
at least +/-20 total counts, then look at the short period and long period=20
filtered counts. Increase the gain to give at least +/-10 filtered counts in=
 the=20
seismic periods of interest. 
    Then start recording quakes and see whether you=
r=20
gain setting a) records small quakes OK and b) doesn't saturate the tra=
ce=20
on most of the large quakes. 
    If you can sense out to 20 seconds, you can run=
 the=20
low pass display filter at 8 to 10 seconds and pick out the surface wav=
es.=20
You can then use filters to hunt for the P and S waves if they are not=20
obvious on the raw extracted trace. There is a long period display filt=
er=20
in AmaSeis to compensate for sort periods, but it does need some counts to d=
o=20
the arithmetic! 
    
    Regards,
 
    Chris Chapman