SAC Command Reference Manual
Using SAC Commands: A - Z Functional Subprocesses: SPE SSS SAC/IRIS

WRITE

SUMMARY

Writes data in memory to disk.

SYNTAX

WRITE {options} {namingoptions}

where options are one or more of the following:

SAC|ALPHA|XDR
COMMIT|ROLLBACK|RECALLTRACE
DIR OFF|CURRENT|name
KSTCMP

These options MUST preceed any element in the namingoptions:

OVER
APPEND text
PREPEND text
DELETE text
CHANGE text1 text2
filelist

Only one of these naming options is allowed at a time.

INPUT

no arguments:

Use previous format and previous write filelist.
SAC:

Write in SAC binary data file format.
ALPHA:

Write in SAC alphanumeric data file format.
SEGY:

Write file formatted according to the IRIS/PASSCAL form of the SEGY format. This format allows one waveform per file. Note only evenly-spaced, time-series files will be written in SEGY.

Since SAC stores the waveform as a series of real*4 numbers, and SEGY stores the waveform as a series of long integers, the datapoints from SAC are normalized to the maxmimum allowable integer. The scale field in SEGY is determined to be the factor which will restore the waveform as close as possible to that of the original SAC file, when read with the READ SEGY command.

The following SAC header fields are saved as the following SEGY header fields

SAC SEGY
KZDATE year, day, hour, minute, second, and m_secs are .
KZTIME set to BEGIN time corrected by KZDATE and KZTIME.
BEGIN trigyear, trigday, trighour, trigminute, trigsecond,
ORIGIN and trigmills are ORIGIN corrected by KZDATE & TIME.
NPTS sampleLength and/or num_samps
DELTA deltaSample and/or samp_rate
DEPMAZ max, corrected by SEGY's scale.
DEPMIN min, corrected by SEGY's scale.
DIST sourceToRecDist
STLA recLatOrY (written as latitude in degrees)
STLO recLongOrX (written as longitude in degrees)
EVLA sourceLatOrY (written as latitude in degrees)
EVLO sourceLongOrX (written as longitude in degrees) lats and lons are multiplied by 3600 to correct units
STEL recElevation
EVEL sourceSurfaceElevation
EVDP sourceDepth
KSTNM station_name
KCMPNM channel_name
KEVNM event_number (only if KEVNM is numeric and < 1e09)

The following SEGY fields are hardwired

SEGY Value
elevationScale 1
coordScale 1
coordUnits 2
gainType 1
gainConst 1
data_form 1
XDR:

Write in SAC binary xdr format. This format is used for the moving binary data files to/from a different architecture, such as a pc running LINUX.
COMMIT:

Commits headers and waveforms in SAC memory -- removing any previous versions of headers or waveforms from RAM -- prior to writing files. COMMIT is the default.
ROLLBACK:

reverts to the last committed version of the header and waveform before writing files.
RECALLTRACE:
  • reverts to the last committed version of the waveform,
  • reverts to the last committed version of those header variables closely linked to the waveform,
  • commits those header variables which are loosely linked to the waveform. (See RECALLTRACE for a list of which header variables are committed, and which are rolled back.)
DIR OFF:

Turn directory option off. When off, writes to current directory.
DIR CURRENT:

Turn directory option on and set name of write directory to the "current directory" (e.g. the directory from which you started SAC.)
DIR name:

Turn directory option on and set name of write directory to name. Write all filenames to the directory called name. This may be a relative or absolute directory name.
KSTCMP:

Use the KSTNM and KCMPNM header variables to define a file name for each data file in memory. The names generated will be checked for uniqueness, and will have sequencing digits added as necessary to avoid name clashes.
OVER:

Use current read filelist as write filelist. Overwrite files on disk with data in memory.
APPEND text:

Write filelist is created by appending text to each name in the current read filelist.
PREPEND text:

Write filelist is created by prepending text to each name in the current read filelist.
DELETE text:

Write filelist is created by deleting the first occurrence of text in each name in the current read filelist.
CHANGE text1 text2:
 

Write filelist is created by changing the first occurrence of text1 in each name in the current read filelist to text2.
filelist:

Write filelist is set to filelist. This list may contain simple filenames, relative pathnames, or full pathnames. It may not contain wildcards.

DEFAULT VALUES

WRITE SAC COMMIT

DESCRIPTION

This command allows you, at any point in the processing of data, to save the results on disk. Several disk file formats are available. More will be added as needed. Each file in memory is written without being cut or desampled.

Most of the time, you will want to use to the SAC data file format. This is a compact binary file format which is fast to read and write. It contains a large header record and one or two data records. See the Users Manual for details on the physical format. The alphanumeric data file format is an ASCII equivalent of the SAC data file format. It takes up much more room on disk and is much slower to read and write. It is useful if you wish to look at the content of the file using a text editor or wish to transfer data to a different kind of computer.

You can directly specify the names of the files to write or you can indirectly specify them by having SAC modify the names of files that are currently in memory. The OVER options sets the write file list to the read file list. It is used to overwrite the last set of disk files read with the data that is currently in memory. The APPEND, PREPEND, DELETE, or CHANGE options create a write file list by modifying each of the names in the read file list in the requested way. This is very useful in macros where you are automatically processing large numbers of data files and need to keep trace of the output files in a consistent manner. The write file list is output when any of these four options is selected. This lets you see the names that were actually used.

EXAMPLES

To filter a set of data files and then save the results in a new set of data files:

SAC> READ D1 D2 D3
SAC> LOWPASS BUTTER NPOLES 4
SAC> WRITE F1 F2 F3

This could have also been done using the CHANGE option:

SAC> READ D1 D2 D3
SAC> LOWPASS BUTTER NPOLES 4
SAC> WRITE CHANGE D F
s:  F1 F2 F3

Notice that SAC output the write file list in this case. To replace the original data on disk with the filtered data the third line in the above example would be:

SAC> WRITE OVER

Note: for examples of the behavior of COMMIT, ROLLBACK, and RECALLTRACE, see the commands of the same name.

ERROR MESSAGES

  • 1301: No data files read in.
  • 1311: No list of filenames to write.
  • 1312: Bad number of files in write file list:
    • the number of files in the write file list must be the same as the number in the data file list (the number read into memory).
  • 1303: Overwrite flag is not on for file
    • header variable LOVROK is .FALSE.
    • this provides some protection for valuable data.

SEE COMMANDS

READ, COMMIT, ROLLBACK, RECALLTRACE

ACKNOWLEDGEMENTS

Our thanks to Steve Roecker or RPI for providing SAC2SEGY which served as our starting point.

LATEST REVISION

Oct. 27, 1998 (Version 0.58)
Using SAC Commands: A - Z Functional Subprocesses: SPE SSS SAC/IRIS