[SAC-HELP] Calculating the CONSTANT to use to remove instument response
Chad
chad at iris.washington.edu
Thu Apr 9 16:28:34 PDT 2009
Hello Melaku,
I am forwarding this to the sac-help list in the hopes that it may
garner the attention of more expertise.
Your calculation of CONSTANT looks generally correct with the
exception of the 2*pi, it should not be included.
A standard gain STS-2 should be nominally 1500 Volts/meters/second,
which needs to be scaled if you want nanometers. I have no idea what
the digitizer gain for a RefTek 130 is, but lets assume your number is
correct for now.
CONSTANT = A0 * SensorGain * Digitizer Gain
CONSTANT = 5.92e+07 * 1500 / 1.589e-06 = 5.588e+16 (so it looks like
2*pi is not in there after all).
That CONSTANT results in units of meters. So scale it by a factor of
1e9 for nanometers = 5.588e25
Hopefully someone more knowledgeable will correct any mistakes I've
made.
On a slight related note, the poles and zeros you are using are the
"truncated"/"simplified" STS-2 response and not the STS-2 nominal
responses (you can access the nominal responses for each of 3
generations of STS-2 here: http://www.iris.edu/NRL/sensors/streckeisen/streckeisen_sts2_sensors.html)
. This is not a critical problem unless you are working with high
frequency data, I have heard that it's not important below 35 Hz.
Chad
On Apr 9, 2009, at 3:11 PM, Melaku Ayenew wrote:
> Hello Chad,
> Thank you for your response
> I am just starting to learn sac for my research, I was trying to
> remove STS-2 broadband instrument and convolute the Wood-Anderson
> response. This is the STS2.pz file I came up with would you check it
> for me please
>
> ZEROS 2 (rad/sec)
> 0.000 0.000
> 0.000 0.000
>
> POLES 5 (rad/sec)
>
> -0.03701 0.03701
> -0.03701 -0.03701
> -251.3 0.0000
> -131.0 467.30
> -131.0 -467.30
>
> CONSTANT 5.5884E+16
>
> This is how I calculate the the Constant
> CONSTANT=A0 X SensorGain X Digitizer Gain X 2*pi
> Where A0 is normalization factor =5.92 E+07
> The digitizer is REF TEC 130 data logger with bit weight
> 1.589E-06volts which I figure the digitizer gain would be 1/1.589
> E-06 ( I am not sure about this step)
>
> And from SAC>transfer from polezero subtype STS2.pz to WA
>
> when I plot this I got a waveform with amplitude in the order of
> 10E-4 nm (transfer returns values in nm)
>
> I really appreciate your help.
> Thank you in advance.!!
>
>
>
> Melaku Bogale
> New Mexico State University
> Department of Physics
> Las Cruces NM 88003-8001
>
>
>
>
> On Wed, Apr 8, 2009 at 10:13 PM, Chad Trabant <chad at iris.washington.edu
> > wrote:
>
> Hello Melaku Bogale,
>
> You need to know the total sensitivity of the digitizer, this is the
> value which relates digital counts to ground units. The poles and
> zeros only represent the sensor. The CONSTANT in the SAC poles and
> zeros file should be the total sensitivity multiplied by the
> normalization factor for the poles and zeroes. The value for
> CONSTANT depends on the units desired also.
>
> Chad
> IRIS DMC
>
>
> On Apr 8, 2009, at 3:50 PM, Melaku Ayenew wrote:
>
> Hi All,
> I am trying to remove instrument responses from STS_2, CMG3T. CMG_ES
> broadband instruments. I got the poles and zeros from IRIS Pascal
> Instrumentation. But I have difficulty figuring out what constant I
> should use in the *.pz file that has to read by the SAC TRANSFER
> command. For example, I was trying to synthesis a WA seismogram by
> removing the STS_2 response and convoulating the WA response. I used
> a constant of 1 in this calcultion. The synthesis seismogrm looks
> good but the displacemnt values don't make sense (they are several
> hunderd meters). I understand it is because of the CONSTANT=1 I put.
> My Question is How can I calculate the correct CONSTANT that I need
> to put with the zeros and poles in the SAC *.pz file. The IRIS
> PASCAL instrumentation page gives the zeros and poles and a
> normalization factor.
> I greatly appreciate your help in this matter.
>
> Melaku Bogale
> New Mexico State University
> Department of Physics
> Las Cruces NM 88003-8001
> _______________________________________________
> sac-help mailing list
> sac-help at iris.washington.edu
> http://www.iris.washington.edu/mailman/listinfo/sac-help
>
>
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