Using SAC

SAC Data File Format

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. Through SAC version 101.6a, 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.

All reals in the SAC header and data sections are single-precision (32 bits, 4 bytes). As discussed in the section on precision in TUTORIAL, although having single-precision for the data is sufficient, having only single-precision for time and distance variables in the header is no longer good enough to handle many modern data sets. SAC version 102.0 solves this problem. The header is unchanged, but a "footer" is added with 22 header variables in double-precision. Version 102.0 can handle "old" SAC data files with no footer or files with a footer, so there is complete compatibility. SAC v102.0 distinguishes between "old" and "new" files by the header variable NVHDR: if NVHDR = 6, the SAC file is in "old" format, if NVHDR = 7, it is in "new" format with a footer. See see the section on Floating-Point Precision in SAC in TUTORIAL for a more detailed discussion.

SAC Binary Format for NVHDR = 6

The binary format is the most common one use. It is is the default format in the READ and WRITE commands and in the subroutine library (RSAC1, RSAC2, WSAC1, WSAC2, WSAC0.) These are binary (unformatted) files.

Structure of SAC Binary Data File for NVHDR = 6

Header Section First Data Section Second Data Section (if present)
start word: 0 start word: 158 start word: 158+NPTS
word length: 158 word length: NPTS word length: NPTS
see table
  • dependent variable
  • amplitude
  • real component
  • independent variable unevenly spaced
  • phase
  • imaginary component

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.

Type Definition Undefined Description
F Floating -12345.0 Single precision.
N Integer -12345 Name begins with an "N".
I Enumerated -12345 Name begins with an "I". Has a limited set of integer values. Each value is given a specific name. Each value represents a specific condition. Subroutines use the equivalent alphanumeric name.
L Logical FALSE Name begins with an "L". Value is either TRUE or FALSE.
K Alphanumeric "-12345.." Name begins with a "K". Either 8 or 16 characters long.
A Auxilary   Not really in the header. Derived from other header fields.

Header Variables

Word Type NAMES o o o o
0 F DELTA DEPMIN DEPMAX UNUSED ODELTA
5 F B E O A INTERNAL
10 F T0 T1 T2 T3 T4
15 F T5 T6 T7 T8 T9
20 F F RESP0 RESP1 RESP2 RESP3
25 F RESP4 RESP5 RESP6 RESP7 RESP8
30 F RESP9 STLA STLO STEL STDP
35 F EVLA EVLO EVEL EVDP MAG
40 F USER0 USER1 USER2 USER3 USER4
45 F USER5 USER6 USER7 USER8 USER9
50 F DIST AZ BAZ GCARC INTERNAL
55 F INTERNAL DEPMEN CMPAZ CMPINC XMINIMUM
60 F XMAXIMUM YMINIMUM YMAXIMUM UNUSED UNUSED
65 F UNUSED UNUSED UNUSED UNUSED UNUSED
70 I NZYEAR NZJDAY NZHOUR NZMIN NZSEC
75 I NZMSEC NVHDR NORID NEVID NPTS
80 I INTERNAL NWFID NXSIZE NYSIZE UNUSED
85 I IFTYPE IDEP IZTYPE UNUSED IINST
90 I ISTREG IEVREG IEVTYP IQUAL ISYNTH
95 I IMAGTYP IMAGSRC IBODY UNUSED UNUSED
100 I UNUSED UNUSED UNUSED UNUSED UNUSED
105 L LEVEN LPSPOL LOVROK LCALDA UNUSED
110 K KSTNM KEVNM*      
116 K KHOLE KO KA    
122 K KT0 KT1 KT2    
128 K KT3 KT4 KT5    
134 K KT6 KT7 KT8    
140 K KT9 KF KUSER0    
146 K KUSER1 KUSER2 KCMPNM    
152 K KNETWK KDATRD KINST    

KEVNM is 16 characters (4 words, 16 bytes) long.

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

Structure of SAC Binary Data File for NVHD = 7

The only difference between a SAC binary data file with NVHDR = 6 and one with NVHDR = 7 is the addition of a "footer" following the data section(s). The footer is comprised of 22 double-precision (64-bit) floating point variables: DELTA, B, E, O, A, T0 ... T9, F, EVLO, EVLA, STLO, STLA, SB, and SDELTA. The names are the same as in the header; SAC distinguishes between the single- and double-precision variables in internal calculations by their location in the file.

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 80-character lines. For both NVHDR = 6 and NVHDR = 7, this is followed by one or two data sections. The data are in 5G15.7 format. The following table shows the line number, formats and names of the variables on the header section lines.

LINE FORMAT NAMES o o o o
01 (5G15.7) DELTA DEPMIN DEPMAX UNUSED ODELTA
02 (5G15.7) B E O A INTERNAL
03 (5G15.7) T0 T1 T2 T3 T4
04 (5G15.7) T5 T6 T7 T8 T9
05 (5G15.7) F RESP0 RESP1 RESP2 RESP3
06 (5G15.7) RESP4 RESP5 RESP6 RESP7 RESP8
07 (5G15.7) RESP9 STLA STLO STEL STDP
08 (5G15.7) EVLA EVLO EVEL EVDP MAG
09 (5G15.7) USER0 USER1 USER2 USER3 USER4
10 (5G15.7) USER5 USER6 USER7 USER8 USER9
11 (5G15.7) DIST AZ BAZ GCARC SB
12 (5G15.7) SDELTA DEPMEN CMPAZ CMPINC XMINIMUM
13 (5G15.7) XMAXIMUM YMINIMUM YMAXIMUM ADJTM UNUSED
14 (5G15.7) UNUSED UNUSED UNUSED UNUSED UNUSED
15 (5I10) NZYEAR NZJDAY NZHOUR NZMIN NZSEC
16 (5I10) NZMSEC NVHDR NORID NEVID NPTS
17 (5I10) NSNPTS NWFID NXSIZE NYSIZE UNUSED
18 (5I10) IFTYPE IDEP IZTYPE UNUSED IINST
19 (5I10) ISTREG IEVREG IEVTYP IQUAL ISYNTH
20 (5I10) IMAGTYP IMAGSRC IBODY UNUSED UNUSED
21 (5I10) UNUSED UNUSED UNUSED UNUSED UNUSED
22 (5I10) LEVEN LPSPOL LOVROK LCALDA UNUSED
23 (A8,A16) KSTNM KEVNM      
24 (3A8) KHOLE KO KA    
25 (3A8) KT0 KT1 KT2    
26 (3A8) KT3 KT4 KT5    
27 (3A8) KT6 KT7 KT8    
28 (3A8) KT9 KF KUSER0    
29 (3A8) KUSER1 KUSER2 KCMPNM    
30 (3A8) KNETWK KDATRD KINST    

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> CH NVDHR 7
SAC> WRITE ALPHA TEMP1

You can read this alphanumeric file directly into SAC with the following commands:

SAC> READ TEMP1
SAC> LH

SAC knows the difference between an alphanumeric and binary SAC files and should be able to read them directly. You can specify an ALPHA flag to the READ command for backwards compatibility:

SAC> READ ALPHA TEMP1
SAC> LH

Listed next are the first lines of TEMP1 -- header plus first 8 lines of data and ending with the 22 lines of the footer (one line per variable).:

    0.01000000      -1.569280       1.520640      -12345.00      -12345.00
      9.459999       19.45000      -41.43000       10.46400      -12345.00
     -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
     -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
     -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
     -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
     -12345.00       48.00000      -120.0000      -12345.00      -12345.00
      48.00000      -125.0000      -12345.00       15.00000      -12345.00
     -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
     -12345.00      -12345.00      -12345.00      -12345.00      -12345.00
      373.0627       88.14721       271.8528       3.357465      -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         7         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  -12345  -12345
-12345  -12345  -12345
-12345  -12345  -12345
-12345  -12345  -12345
-12345  -12345  -12345
-12345  -12345  -12345
-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
   ....
0.0099999997764825821
9.4599990844726562
19.449998861178756
-41.430000305175781
10.46399974822998
-12345
-12345
-12345
-12345
-12345
-12345
-12345
-12345
-12345
-12345
-12345
-125
48
-120
48
-12345
-1234

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.

Name Type Description
NPTS N Number of points per data component. [required]
NVHDR N Header version number. Current value is the integer 7. Older version data (NVHDR < 6) are automatically updated when read into sac. In SAC v102.0, one can still use NVHDR = 6. To have double-precision capability, NVHDR = 7 [required]
B F Beginning value of the independent variable. [required]
E F Ending value of the independent variable. [required]
IFTYPE I
Type of file [required]:
  • ITIME {Time series file}
  • IRLIM {Spectral file---real and imaginary}
  • IAMPH {Spectral file---amplitude and phase}
  • IXY {General x versus y data}
  • IXYZ {General XYZ (3-D) file}
LEVEN L TRUE if data is evenly spaced. [required]
DELTA F Increment between evenly spaced samples (nominal value). [required]
ODELTA F Observed increment if different from nominal value.
IDEP I
Type of dependent variable:
  • IUNKN (Unknown)
  • IDISP (Displacement in nm)
  • IVEL (Velocity in nm/sec)
  • IACC (Acceleration in nm/sec/sec)
  • NOTE Others use SI units for IDISP, IVEL, IACC.
  • IVOLTS (Velocity in volts)
DEPMIN F Minimum value of dependent variable.
DEPMAX F Maximum value of dependent variable.
DEPMEN F Mean value of dependent variable.
NZYEAR N GMT year corresponding to reference (zero) time in file.
NZJDAY N GMT Day of the Year.
NZHOUR N GMT hour.
NZMIN N GMT minute.
NZSEC N GMT second.
NZMSEC N GMT millisecond.
NZDTTM N GMT date-time array. Six element array equivalenced to NZYEAR, NZJDAY, NZHOUR, NZMIN, NZSEC, and NZMSEC.
KZDATE A Alphanumeric form of GMT reference date. Derived from NZYEAR and NZJDAY.
KZTIME A Alphanumeric form of GMT reference time. Derived from NZHOUR, NZMIN, NZSEC, and NZMSEC.
IZTYPE I
Reference time equivalence:
  • IUNKN (Unknown)
  • IB (Begin time)
  • IDAY (Midnight of refernece GMT day)
  • IO (Event origin time)
  • IA (First arrival time)
  • ITn (User defined time pick n, n=0,9)
O F Event origin time (seconds relative to reference time.)
KO A Event origin time identification.
NSNPTS I Number of points of original time series file (Stored NPTS).
SB F Begin value of orignal time series file (Stored B).
SDELTA F Time increment in original time series file (Stored DELTA).

Phase Picks

Name Type Description
A F First arrival time (seconds relative to reference time.)
KA K First arrival time identification.
F F Fini or end of event time (seconds relative to reference time.)
KF A Fini identification.
Tn F User defined time picks or markers, n = 0 - 9 (seconds relative to reference time).
KT{n} K A User defined time pick identifications, n = 0 - 9.

Instrument Fields

Name Type Description
KINST K Generic name of recording instrument.
IINST I Type of recording instrument. [not currently used]
RESPn F Instrument response parameters, n=0,9. [not currently used]

Station Fields

Name Type Description
KNETWK K Name of seismic network.
KSTNM K Station name.
ISTREG I Station geographic region. [not currently used]
STLA F Station latitude (degrees, north positive)
STLO F Station longitude (degrees, east positive).
STEL F Station elevation above sea level (meters). [not currently used]
STDP F Station depth below surface (meters). [not currently used]
CMPAZ F Component azimuth (degrees clockwise from north).
CMPINC F Component incident angle (degrees from upward vertical; SEED/MINISEED uses dip: degrees from horizontal down).
KCMPNM K Channel name. SEED volumes use three character names, and the third is the component/orientation. For horizontals, the current trend is to use 1 and 2 instead of N and E.
KSTCMP A Station component. Derived from KSTNM, CMPAZ, and CMPINC.
LPSPOL L TRUE if station components have a positive polarity (left-hand rule).

Event Fields

Name Type Description
KEVNM K Event name.
IEVREG I Event geographic region. [not currently used]
EVLA F Event latitude (degrees, north positive).
EVLO F Event longitude (degrees, east positive).
EVEL F Event elevation (meters). [not currently used]
EVDP F Event depth below surface (kilommeters -- previously meters)
MAG F Event magnitude.
IMAGTYP I
Magnitude type:
  • IMB (Bodywave Magnitude)
  • IMS (Surfacewave Magnitude)
  • IML (Local Magnitude)
  • IMW (Moment Magnitude)
  • IMD (Duration Magnitude)
  • IMX (User Defined Magnitude)
IMAGSRC I
Source of magnitude information:
  • INEIC (National Earthquake Information Center)
  • IPDE (Preliminary Determination of Epicenter)
  • IISC (Internation Seismological Centre)
  • IREB (Reviewed Event Bulletin)
  • IUSGS (US Geological Survey)
  • IBRK (UC Berkeley)
  • ICALTECH (California Institute of Technology)
  • ILLNL (Lawrence Livermore National Laboratory)
  • IEVLOC (Event Location (computer program) )
  • IJSOP (Joint Seismic Observation Program)
  • IUSER (The individual using SAC2000)
  • IUNKNOWN (unknown)
IEVTYP I
Type of event:
  • IUNKN (Unknown)
  • INUCL (Nuclear event)
  • IPREN (Nuclear pre-shot event)
  • IPOSTN (Nuclear post-shot event)
  • IQUAKE (Earthquake)
  • IPREQ (Foreshock)
  • IPOSTQ (Aftershock)
  • ICHEM (Chemical explosion)
  • IQB (Quarry or mine blast confirmed by quarry)
  • IQB1 (Quarry/mine blast with designed shot info-ripple fired)
  • IQB2 (Quarry/mine blast with observed shot info-ripple fired)
  • IQBX (Quarry or mine blast - single shot)
  • IQMT (Quarry/mining-induced events: tremors and rockbursts)
  • IEQ (Earthquake)
  • IEQ1 (Earthquakes in a swarm or aftershock sequence)
  • IEQ2 (Felt earthquake)
  • IME (Marine explosion)
  • IEX (Other explosion)
  • INU (Nuclear explosion)
  • INC (Nuclear cavity collapse)
  • IO (Other source of known origin)
  • IL (Local event of unknown origin)
  • IR (Regional event of unknown origin)
  • IT (Teleseismic event of unknown origin)
  • IU (Undetermined or conflicting information)
  • IOTHER (Other)
NEVID N Event ID (CSS 3.0)
NORID N Origin ID (CSS 3.0)
NWFID N Waveform ID (CSS 3.0)
KHOLE K Nuclear: hole identifier; Other: location identifier (LOCID).
DIST F Station to event distance (km).
AZ F Event to station azimuth (degrees).
BAZ F Station to event azimuth (degrees).
GCARC F Station to event great circle arc length (degrees).
IBODY I
Body / Spheroid definition used in Distance Calculations
  • undef / -12345 Sac Historical Spheroid version
    • a: 6378160.0 m
    • f: 0.00335293
    • Important Note: For consistency with previous versions, it is best to leave the IBODY as undefined. This is not WGS-84, see IEARTH
  • ISUN / 98 Sun Meftah2015 Rozelot2001
    • a: 696000000.0 m
    • f: 8.189e-6
  • IMERCURY / 99 Mercury IAU2000 IAU-WGCCRE
    • a: 2439700.0 m
    • f: 0.0
  • IVENUS / 100 Venus IAU2000 IAU-WGCCRE
    • a: 6051800.0 m
    • f: 0.0
  • IEARTH / 101 WGS-84, EPSG:4325
    • a: 6378137.0 m
    • f: 1.0 / 298.257223563 ( = 0.0033528106647474805)
  • IMOON / 102 Moon IAU2000 IAU-WGCCRE
    • a: 1737400.0 m
    • f: 0.0
  • IMARS / 103 Mars IAU2000 IAU-WGCCRE
    • a: 3396190.0 m
    • f: 1.0 / 169.89444722361179 ( = 0.005886007555525457)

Miscellaneous Fields

Name Type Description
LCALDA L TRUE if DIST, AZ, BAZ, and GCARC are to be calculated from station and event coordinates.
IQUAL I
Quality of data [not currently used]:
  • IGOOD (Good data)
  • IGLCH (Glitches)
  • IDROP (Dropouts)
  • ILOWSN (Low signal to noise ratio)
  • IOTHER (Other)
ISYNTH I
Synthetic data flag [not currently used]:
  • IRLDTA (Real data)
  • ????? (Flags for various synthetic seismogram codes)
KDATRD K Date data was read onto computer.
USER{n} F User defined variable storage area, n = 0,9.
KUSER{n} K User defined variable storage area, n = 0,2.
LOVROK L TRUE if it is okay to overwrite this file on disk.
NXSIZE N Spectral Length (Spectral files only)
NYSIZE N Spectral Width (Spectral files only)
XMINIMUM F Minimum value of X (Spectral files only)
XMAXIMUM F Maximum value of X (Spectral files only)
YMINIMUM F Minimum value of Y (Spectral files only)
YMAXIMUM F Maximum value of Y (Spectral files only)

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.

Name ID
itime 01
irlim 02
iamph 03
ixy 04
iunkn 05
idisp 06
ivel 07
iacc 08
ib 09
iday 10
io 11
ia 12
it0 13
it1 14
it2 15
it3 16
it4 17
it5 18
it6 19
it7 20
it8 21
it9 22
iradnv 23
itannv 24
iradev 25
itanev 26
inorth 27
ieast 28
ihorza 29
idown 30
iup 31
illlbb 32
iwwsn1 33
iwwsn2 34
ihglp 35
isro 36
inucl 37
ipren 38
ipostn 39
iquake 40
ipreq 41
ipostq 42
ichem 43
iother 44
igood 45
iglch 46
idrop 47
ilowsn 48
irldta 49
ivolts 50
imb 52
ims 53
iml 54
imw 55
imd 56
imx 57
ineic 58
ipdeq 59
ipdew 60
ipde 61
iisc 62
ireb 63
iusgs 64
ibrk 65
icaltech 66
illnl 67
ievloc 68
ijsop 69
iuser 70
iunknown 71
iqb 72
iqb1 73
iqb2 74
iqbx 75
iqmt 76
ieq 77
ieq1 78
ieq2 79
ime 80
iex 81
inu 82
inc 83
io_ 84
il 85
ir 86
it 87
iu 88
ieq3 89
ieq0 90
iex0 91
iqc 92
iqb0 93
igey 94
ilit 95
imet 96
iodor 97
isun 98
imercury 99
ivenus 100
iearth 101
imoon 102
imars 103

REFERENCES

[IAU2000] https://link.springer.com/article/10.1023/A:1013939327465

Seidelmann, P., V. Abalakin, M. Bursa, M. Davies, C. De Bergh, J. Lieske, J. Oberst, J. Simon, E. Standish, P. Stooke, et al. (2002), Report of the IAU/IAG working group on cartographic coordinates and rotational elements of the planets and satellites: 2000, Celestial Mechanics and Dynamical Astronomy, 82 (1), 83–111.

[IAU-WGCCRE] https://astrogeology.usgs.gov/groups/IAU-WGCCRE

[Meftah2015] https://link.springer.com/article/10.1007%2Fs11207-015-0655-6

Meftah, M., A. Irbah, A. Hauchecorne, T. Corbard, S. Turck-Chi`eze, J.-F. Hochedez, P. Boumier, A. Cheva- lier, S. Dewitte, S. Mekaoui, et al. (2015), On the determination and constancy of the solar oblateness, Solar Physics, 290(3), 673–687.

  • 8.189e-6 (Flattening)

[Rozelot2001] https://link.springer.com/article/10.1023/A:1005238718479

Rozelot, J. P., Godier, S., & Lefebvre, S. (2001). On the theory of the oblateness of the Sun. Solar Physics, 198(2), 223-240.

  • 8.33e-6 (Flattening)
  • 6.50e-6 - 10.20e-6 (Flattening Range)

LATEST REVISION

Version 102.0