Thread: Re: Calculating the CONSTANT to use to remove instument response

Started: 2009-04-09 23:28:34
Last activity: 2009-04-09 23:28:34
Topics: SAC Help

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

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:
. 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.


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>

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.


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
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