[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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