The SEGY-Y format

SegyMAT has been implemented using the SEG-Y revision 0 and revision 1 standards as defined by SEG¹.

SegyMAT also has support for reading and writing the format used by CWP's Seismic Unix package (the SU format), which is merely a simplified version the SEG-Y format.

A short description of the formats follows here.

Structure of a file

A SEG-Y file consists of a 3600 byte header; a number of extended textual headers; a number trace headers+data.

A 3200 byte Textual File Header, ASCII or EBCDIC formated.
A 400 byte Binary File Header
A (optional) number of 'Extended Textual File Headers', 3200 bytes long, ASCII or EBCDIC formatted.
A number of traces, separated into a 240 bytes long binary Trace Header, followed by the Trace Data, that can be formatted in a number of ways : IEEE, IBM Floating Point, 1,2 and 4 byte two's complement integers.

Structure of a SU file

A SU formatted file is just a simple version of a file, containing only trace information :

No 3200 byte textual header and no extended textual headers.
No binary header.
The data must be formatted as IEEE.
Data can be both little and big endian formatted.

What is supported in SegyMAT ?

The following parts of the SEG-Y format, revision 0 and 1, are supported

Textual file headers

The Textual 400 byte file header can be both ASCII and EBCDIC formatted, using revision 1.

Extended Textual Headers

In revision 1 a number of extended textual file headers are allowed.

Data Sample Format / Revision

The following data formats are supported :

REVISION 0 (1975):

Type	DataSampleFormat	Supported
1	4 Byte IBM Floating Point	Yes
2	4 Byte Fixed Point	No
3	2 Byte Fixed Point	No
4	4 Byte Fixed Point with Gain	No

REVISION 1 (2002)

Type	DataSampleFormat	Supported
1	4 Byte IBM Floating Point	Yes
2	4 Byte two's complement integer	Yes
3	2 Byte two's complement integer	Yes
4	4 Byte Fixed Point with Gain	No
5	4 Byte IEEE FLoating Pint	Yes
6	Not Specified
7	Not Specified
8	1 Byte Fixed Point with Gain	Yes

The type number is the number that should be used as 'dsf' (Data Sample Format), for functions like ReadSegy, WriteSegy, WriteSegyStructure.

Segy Trace Header name definition

The definition of trace header names, location in the Trace Header and precision can be listed by running

TraceHeaderDef;

which provides the folloing output:

 POS   PREC Traece Header Name
   0  int32 TraceSequenceLine
   4  int32 TraceSequenceFile
   8  int32 FieldRecord
  12  int32 TraceNumber
  16  int32 EnergySourcePoint
  20  int32 cdp
  24  int32 cdpTrace
  28  int16 TraceIdenitifactionCode
  30  int16 NSummedTraces
  32  int16 NStackedTraces
  34  int16 DataUse
  36  int32 offset
  40  int32 ReceiverGroupElevation
  44  int32 SourceSurfaceElevation
  48  int32 SourceDepth
  52  int32 ReceiverDatumElevation
  56  int32 SourceDatumElevation
  60  int32 SourceWaterDepth
  64  int32 GroupWaterDepth
  68  int16 ElevationScalar
  70  int16 SourceGroupScalar
  72  int32 SourceX
  76  int32 SourceY
  80  int32 GroupX
  84  int32 GroupY
  88  int16 CoordinateUnits
  90  int16 WeatheringVelocity
  92  int16 SubWeatheringVelocity
  94  int16 SourceUpholeTime
  96  int16 GroupUpholeTime
  98  int16 SourceStaticCorrection
 100  int16 GroupStaticCorrection
 102  int16 TotalStaticApplied
 104  int16 LagTimeA
 106  int16 LagTimeB
 108  int16 DelayRecordingTime
 110  int16 MuteTimeStart
 112  int16 MuteTimeEND
 114 uint16 ns
 116 uint16 dt
 118  int16 GainType
 120  int16 InstrumentGainConstant
 122  int16 InstrumentInitialGain
 124  int16 Correlated
 126  int16 SweepFrequenceStart
 128  int16 SweepFrequenceEnd
 130  int16 SweepLength
 132  int16 SweepType
 134  int16 SweepTraceTaperLengthStart
 136  int16 SweepTraceTaperLengthEnd
 138  int16 TaperType
 140  int16 AliasFilterFrequency
 142  int16 AliasFilterSlope
 144  int16 NotchFilterFrequency
 146  int16 NotchFilterSlope
 148  int16 LowCutFrequency
 150  int16 HighCutFrequency
 152  int16 LowCutSlope
 154  int16 HighCutSlope
 156  int16 YearDataRecorded
 158  int16 DayOfYear
 160  int16 HourOfDay
 162  int16 MinuteOfHour
 164  int16 SecondOfMinute
 166  int16 TimeBaseCode
 168  int16 TraceWeightningFactor
 170  int16 GeophoneGroupNumberRoll1
 172  int16 GeophoneGroupNumberFirstTraceOrigField
 174  int16 GeophoneGroupNumberLastTraceOrigField
 176  int16 GapSize
 178  int16 OverTravel
 180  int32 cdpX
 184  int32 cdpY
 188  int32 Inline3D
 192  int32 Crossline3D
 196  int32 ShotPoint
 200  int16 ShotPointScalar
 202  int16 TraceValueMeasurementUnit
 204  int32 TransductionConstantMantissa
 208  int16 TransductionConstantPower
 210  int16 TransductionUnit
 212  int16 TraceIdentifier
 214  int16 ScalarTraceHeader
 216  int16 SourceType
 218  int32 SourceEnergyDirectionMantissa
 222  int16 SourceEnergyDirectionExponent
 224   in32 SourceMeasurementMantissa
 228  int16 SourceMeasurementExponent
 230  int16 SourceMeasurementUnit
 232  int32 UnassignedInt1
 236  int32 UnassignedInt2

¹. The Society of Exploration Geophysicists ↩

The SEG-Y format