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