Overview

This section discusses the contents of the SAC data file, describes the binary and alphanumeric formats of this file, and documents the SAC header in detail.

Since version 100.0, SAC can handle binary data files in either endian (byte order), so big-endian systems (Sun Solaris, Mac PPC) can read SAC data files written on little-endian systems (Linux, MAC i686, Cygwin) and vice versa.

Each signal is stored on disk in a separate SAC data file. These files contain a fixed length header section followed by one or two data sections. The header contains floating point, integer, logical, and character fields. Evenly spaced data files have only one data section which contains the dependent variable. Unevenly spaced data and spectral data files contain two data sections. For unevenly spaced data, the first data section contains the dependent variable and the second contains the independent variable. For spectral files the first component is either the amplitude or the real component and the second component is either the phase or imaginary component.

SAC Binary Format

This is the format that you will use most often. It is used in SAC itself(READ and WRITE commands) and in the subroutine library (RSAC1, RSAC2, WSAC1, WSAC2, WSAC0.) These are binary (unformatted) files so that they can be quickly read from disk into memory. The header is 158 32-bit words in length, followed by the data section(s). In order to rapidly read only a small section of a data file (see the CUT command), these files also have a physical record length of 512 bytes (128 32-bit words) and are opened for direct-access. There is no physical record structure. This format is shown schematically in the following figure.

Structure of SAC Binary Data File

SAC Binary Header

The following table shows the contents and layout of the SAC binary data file header. The W and T columns give the beginning word and header data type for the header variables named on that line. These header variables and data types are described later in this section. If the name is INTERNAL then that variable is internal to SAC and not normally of interest to the user. If the name is UNUSED then that variable is not currently being used. For any given file, some of these variables will not have meaningful values. These are refered to as "undefined variables" for that file. For each data type, a special value signifies this undefined state. They are listed in a table at the end of this section.

Header Data Types

This table lists the header types and their definitions. The third column lists the special value used to signify that a particular header variable is undefined in a particular file.

Header Variables

KEVNM is 16 characters (4 words) long.

All other K fields are 8 characters (2 words) long.

SAC Alphanumeric Format

This file is essentially the alphanumeric equivalent of the SAC binary data file. The header section is stored on the first 30 cards. This is followed by one or two data sections. The data is in 5G15.7 format. The following table shows the card number, formats and names of the variables on the header section cards.

SAC Alphanumeric Data File Example

The header section and first five lines of the data section of a sample SAC alphanumeric data file is shown below. You can reproduce this file (with the entire data section) on your system by executing the following commands:

SAC> FUNCGEN SEISMOGRAM
SAC> LH
SAC> WRITE ALPHA TEMP1

You can then convert this alphanumeric file to a binary one and read it into SAC with the following commands:

SAC> CONVERT FROM ALPHA TEMP1 TO SAC TEMP2
SAC> READ TEMP2
SAC> LH

Alternatively, you can accomplish this without using CONVERT, but just using READ and WRITE:

SAC> READ ALPHA TEMP1
SAC> WRITE SAC TEMP2
SAC> READ TEMP2
SAC> LH

This little test shows the equivalence of the alphanumeric and binary file formats.:

     0.01000000      -1.569280       1.520640      -12345.00      -12345.00
       9.459999       19.45000       0.000000       10.47000       2.000000
      -12345.00       20.00000      -12345.00      -12345.00      -12345.00
      -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
       17.78000      -12345.00      -12345.00      -12345.00      -12345.00
      -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
      -12345.00       87.99997      -120.0000      -12345.00      -12345.00
       47.99997      -125.0000      -12345.00      -12345.00      -12345.00
       123.4560      -12345.00      -12345.00      -12345.00      -12345.00
      -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
       4461.052      0.2719055       185.2047       40.18595      -12345.00
      -12345.00    -0.09854718       0.000000       0.000000      -12345.00
      -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
      -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
      1981        88        10        38        14
         0         6         0         0      1000
    -12345    -12345    -12345    -12345    -12345
         1        50         9    -12345    -12345
    -12345    -12345        42    -12345    -12345
    -12345    -12345    -12345    -12345    -12345
    -12345    -12345    -12345    -12345    -12345
         1         1         1         1         0
CDV      K8108838
-12345  HOLE    IPD0
XYZ     -12345  KT1
-12345  -12345  -12345
-12345  -12345  -12345
-12345  -12345  ABKD
USER0   -12345  Q
-12345  -12345  -12345
    -0.09728001    -0.09728001    -0.09856002    -0.09856002    -0.09728001
    -0.09600000    -0.09472002    -0.09344001    -0.09344001    -0.09344001
    -0.09344001    -0.09344001    -0.09472002    -0.09472002    -0.09344001
    -0.09344001    -0.09216000    -0.09216000    -0.09216000    -0.09216000
    -0.09088002    -0.09088002    -0.09216000    -0.09344001    -0.09472002
    -0.09472002    -0.09472002    -0.09472002    -0.09472002    -0.09472002
    -0.09344001    -0.09344001    -0.09216000    -0.09088002    -0.09088002
    -0.09216000    -0.09216000    -0.09216000    -0.09344001    -0.09472002
    ....

SAC Header Variables

This table lists the header variables, their types, and descriptions. They are grouped by category: required fields, time fields, phase picks, instrument parameters, station parameters, event parameters, misc. The header types are defined in the second table.

Phase Picks

Instrument Fields

Station Fields

Event Fields

Miscellaneous Fields

Enumerated Header Field Values

The enumerated header field values are stored in the header as integers. Their names and values are given in the table below.

LATEST REVISION

June 2011 (Version 101.5)