Thread: Fwd: Help to pick p & s phases, get instrument gain ...

Started: 2011-07-29 15:35:31
Last activity: 2011-07-29 15:35:31
Topics: SAC Help
Sheila Peacock
2011-07-29 15:35:31
Dear Eric,

You need a lot more help and time than I can devote, but here is a start.

I hope that other members of this list can point you to web resources
(perhaps some that they have written themselves).

Regards,
Sheila Peacock.

-------- Original Message --------
Subject: [SAC-HELP] Help to pick p & s phases, get instrument gain ...
Date: Thu, 28 Jul 2011 14:44:14 -0700 (PDT)
From: ndoh ndikum <ndikumeric<at>yahoo.com>
Reply-To: ndoh ndikum <ndikumeric<at>yahoo.com>
To: chad<at>iris.washington.edu <chad<at>iris.washington.edu>
CC: sac-help<at>iris.washington.edu <sac-help<at>iris.washington.edu>



good day,

I am currently working with some sac waveforms, wishing to locate the events
in order to futher carry out inversions but I am faced with the following problems:


- I have succeeded to pick the p and s phases in some of the data files
but can't for others. could I be helped with the p and s phases of the accompanying
attached sac data files.

## Have you used a set of traveltime tables to predict the expected arrival
times of your P and S phases, given the origin time of the event? This is
also useful to distinguish the phases from other phases that might arrive
at nearly the same time.

## For S phases, it is useful to use the horizontal components if your
seismometers are three-component instruments.


- could I also be helped with some basic lecture notes on phase identification and picking.


- how can I instrument gain of these sac files in particular and any other
sac file I come across in general?


- how do know if the have the same instrument response?

## Information about instrument responses should be provided by the
operator of the seismic network. It is not usually encoded in SAC files,
although if your SAC files came from SEED files you might be able to
extract instrument responses by running "rdseed" and answering "yes"
to "extract responses"; the SAC "transfer" command with type "evalresp"
can then be used to remove the instrument response.

## If the seismometers are made by Guralp you can find the instrument
response if you can find out the serial numbers of the seismometer and
the digitiser. You then send a simple email to "caldoc<at>guralp.com" with
the serial number in the subject line. You will receive a Microsoft Word
document containing the response information. Beware that the document
for digitiser responses cannot be read in OpenOffice, only in Microsoft
Word proper. Furthermore, the poles and zeros are given in terms of
velocity (or sometimes acceleration) in Hz not radians, and have to be
multiplied by 2pi and an extra zero at (0.0, 0.0) added to make them
into displacement in radians suitable for use in SAC. Also you have to
find out whether the seismometer is connected "differentially" to the
digitiser, in which case the "2x" shown in the data sheet has to be
applied.


- what is the procedure to choose the appropriate filter windows to use for
a specific data type? what tips are available?

## There are no hard and fast rules other than not to filter too severely
i.e. not use a narrow passband or steep slopes on the filter, for picking.
The CTBTO IDC uses 0.8-4.5 Hz for picking, but this is not appropriate
for large events (too high; arrivals from large events shouldn't need
bandpass filtering), or small events (too low)). If there is very high-frequency
or very low-frequency noise (frequency far away from the signal
frequency) then a suitable lowpass or highpass filter will be seen to
remove it. Several of your seismograms benefit from a lowpass filter.

## You should always make a note of the filter applied along with your
pick time, so that you can go back and check it.

## If the earthquake is a long way from your network (i.e. much farther
than the diameter of your network) then comparing seismograms from
all the stations in the network, lined up on the expected P-wave arrival
time at each station, should help you identify phases.


I will be grateful for your help!

Eric.
---------------------------------------------------
NDIKUM ERIC NDOH,
C/O C.M.F.I. YAOUNDE,
P.O. Box 6090,
CAMEROON,
WEST AFRICA .

TEL:(237) 75 17 91 66



10:30:00 v.01697673