Based on Digital Tape Standards published by the Society of Exploration Geophysicists (SEG)
The SEG-Y format is one of several tape standards developed by the Society of Exploration Geophysicists (SEG). It is the most common format used for seismic data in the exploration and production industry. However, it was created in 1973 and many different 'modernized' flavors exist.
SEG-Y was designed for storing a single line of seismic data on IBM 9-track tapes attached to IBM mainframe computers. Most of the variations in modern SEG-Y varieties result from trying to overcome these limitations.
Some of the features of SEG-Y which are outdated today include:
The official standard SEG-Y consists of the following components:
As mentioned earlier there are many variations of the standard.
The EBCDIC reel header is equivalent to 40 IBM punch-cards (EBCDIC? punchcards? Welcome to the 70's, man!). The official layout of these 80-character cards is the EBCDIC equivalent of the following:
12345678901234567890123456789012345678901234567890123456789012345678901234567890 C 1 CLIENT COMPANY CREW NO C 2 LINE AREA MAP ID C 3 REEL NO DAY-START OF REEL YEAR OBSERVER C 4 INSTRUMENT: MFG MODEL SERIAL NO C 5 DATA TRACES/RECORD AUXILIARY TRACES/RECORD CDP FOLD C 6 SAMPLE INTERVAL SAMPLES/TRACE BITS/IN BYTES/SAMPLE C 7 RECORDING FORMAT FORMAT THIS REEL MEASUREMENT SYSTEM C 8 SAMPLE CODE: FLOATING PT FIXED PT FIXED PT-GAIN CORRELATED C 9 GAIN TYPE: FIXED BINARY FLOATING POINT OTHER C10 FILTERS: ALIAS HZ NOTCH HZ BAND - HZ SLOPE - DB/OCT C11 SOURCE: TYPE NUMBER/POINT POINT INTERVAL C12 PATTERN: LENGTH WIDTH C13 SWEEP: START HZ END HZ LENGTH MS CHANNEL NO TYPE C14 TAPER: START LENGTH MS END LENGTH MS TYPE C15 SPREAD: OFFSET MAX DISTANCE GROUP INTERVAL C16 GEOPHONES: PER GROUP SPACING FREQUENCY MFG MODEL C17 PATTERN: LENGTH WIDTH C18 TRACES SORTED BY: RECORD CDP OTHER C19 AMPLITUDE RECOVERY: NONE SPHERICAL DIV AGC OTHER C20 MAP PROJECTION ZONE ID COORDINATE UNITS C21 PROCESSING: C22 PROCESSING: C23 C24 C25 C26 C27 C28 C29 C30 C31 C32 C33 C34 C35 C36 C37 C38 C39 C40 END EBCDIC
The blank spaces in the cards are fill-in-the-blanks. For example, the client's name is intended to go in the space after 'CLIENT' in the first card. Multiple-choice entries like 'SAMPLE CODE' in card 8 are intended to have the appropriate choice (such as 'FLOATING PT') marked with an 'X'.
Cards 21 through 40 are intended for general descriptions such as the data set's processing history.
The binary reel header contains much information about the data. Much of this information is optional, that is, the entire header is not required to be valid. In fact, none of it is required to be valid, although some fields are strongly recommended.
The 400 bytes contain 2-byte and 4-byte integers in the following layout:
Bytes Description
001 - 004 Job identification number.
005 - 008 * Line number.
009 - 012 * Reel number.
013 - 014 * Number of data traces per record.
015 - 016 * Number of auxiliary traces per record.
017 - 018 * Sample interval of this reel's data in microseconds.
019 - 020 Sample interval of original field recording in microseconds.
021 - 022 * Number of samples per trace for this reel's data.
023 - 024 Number of samples per trace in original field recording.
025 - 026 * Data sample format code:
1 = 32-bit IBM floating point
2 = 32-bit fixed-point (integer)
3 = 16-bit fixed-point (integer)
4 = 32-bit fixed-point with gain code (integer)
027 - 028 * CDP fold (expected number of data traces per ensemble).
029 - 030 Trace sorting code:
1 = as recorded
2 = CDP ensemble
3 = single fold continuous profile
4 = horizontally stacked
031 - 032 Vertical sum code (1 = no sum, 2 = two sum, ...)
033 - 034 Sweep frequency at start in Hertz.
035 - 036 Sweep frequency at end in Hertz.
037 - 038 Sweep length in milliseconds.
039 - 040 Sweep type code:
1 = linear
2 = parabolic
3 = exponential
4 = other
041 - 042 Trace number of sweep channel.
043 - 044 Sweep trace taper length at start in milliseconds.
045 - 046 Sweep trace taper length at end in milliseconds.
047 - 048 Taper type code:
1 = linear
2 = cosine squared
3 = other
049 - 050 Correlated data traces (1 = no, 2 = yes).
051 - 052 Binary gain recovered (1 = yes, 2 = no).
053 - 054 Amplitude recovery method code:
1 = one
2 = spherical divergence
3 = AGC
4 = other
055 - 056 * Measurement system (1 = meters, 2 = feet).
057 - 058 Impulse signal polarity (increase in pressure or upward
geophone case movement gives 1=negative or 2=positive number).
059 - 060 Vibratory polarity code (seismic lags pilot signal by):
1 = 337.5 to 22.5 degrees
2 = 22.5 to 67.5 degrees
3 = 67.5 to 112.5 degrees
4 = 112.5 to 157.5 degrees
5 = 157.5 to 202.5 degrees
6 = 202.5 to 247.5 degrees
7 = 247.5 to 292.5 degrees
8 = 292.5 to 337.5 degrees
061 - 400 Unassigned (for optional information).
* strongly recommended
The 240-byte binary trace header consists of 2-byte and 4-byte integers in the following layout:
Bytes Description
001 - 004 * Trace sequence number within line.
005 - 008 Trace sequence number within reel.
009 - 012 * Original field record number.
013 - 016 * Trace sequence number within original field record.
017 - 020 Energy source point number.
021 - 024 CDP ensemble number.
025 - 028 Trace sequence number within CDP ensemble.
029 - 030 * Trace identification code:
1 = seismic data
2 = dead
3 = dummy
4 = time break
5 = uphole
6 = sweep
7 = timing
8 = water break
9+ = optional use
031 - 032 Number of vertically summed traces yielding this trace.
033 - 034 Number of horizontally stacked traced yielding this trace.
035 - 036 Data use (1 = production, 2 = test).
037 - 040 Distance from source point to receiver group.
041 - 044 Receiver group elevation.
045 - 048 Surface elevation at source.
049 - 052 Source depth below surface.
053 - 056 Datum elevation at receiver group.
057 - 060 Datum elevation at source.
061 - 064 Water depth at source.
065 - 068 Water depth at receiver group.
069 - 070 Scalar for elevations and depths (+ = multiplier, - = divisor).
071 - 072 Scalar for coordinates (+ = multiplier, - = divisor).
073 - 076 X source coordinate.
077 - 080 Y source coordinate.
081 - 084 X receiver group coordinate.
085 - 088 Y receiver group coordinate.
089 - 090 Coordinate units (1 = length in meters or feet, 2 = arc seconds).
091 - 092 Weathering velocity.
093 - 094 Subweathering velocity.
095 - 096 Uphole time at source.
097 - 098 Uphole time at receiver group.
099 - 100 Source static correction.
101 - 102 Receiver group static correction.
103 - 104 Total static applied.
105 - 106 Lag time between end of header and time break in milliseconds.
107 - 108 Lag time between time break and shot in milliseconds.
109 - 110 Lag time beteen shot and recording start in milliseconds.
111 - 112 Start of mute time.
113 - 114 End of mute time.
115 - 116 * Number of samples in this trace.
117 - 118 * Sample interval of this trace in microseconds.
119 - 120 Field instrument gain type code:
1 = fixed
2 = binary
3 = floating point
4+ = optional use
121 - 122 Instrument gain constant.
123 - 124 Intrument early gain in decibels.
125 - 126 Correlated (1 = no, 2 = yes).
127 - 128 Sweep frequency at start.
129 - 130 Sweep fequency at end.
131 - 132 Sweep length in milliseconds.
133 - 134 Sweep type code:
1 = linear
2 = parabolic
3 = exponential
4 = other
135 - 136 Sweep taper trace length at start in milliseconds.
137 - 138 Sweep taper trace length at end in milliseconds.
139 - 140 Taper type code:
1 = linear
2 = cosine squared
3 = other
141 - 142 Alias filter frequency.
143 - 144 Alias filter slope.
145 - 146 Notch filter frequency.
147 - 148 Notch filter slope.
149 - 150 Low cut frequency.
151 - 152 High cut frequency.
153 - 154 Low cut slope.
155 - 156 High cut slope.
157 - 158 Year data recorded.
159 - 160 Day of year.
161 - 162 Hour of day (24-hour clock).
163 - 164 Minute of hour.
165 - 166 Second of minute.
167 - 168 Time basis (1 = local, 2 = GMT, 3 = other).
169 - 170 Trace weighting factor for fixed-point format data.
171 - 172 Geophone group number of roll switch position one.
173 - 174 Geophone group number of first trace of original field record.
175 - 176 Geophone group number of last trace of original field record.
177 - 178 Gap size (total number of groups dropped).
179 - 180 Overtravel associated with taper (1 = down/behind, 2 = up/ahead).
181 - 240 Unassigned (for optional information).
* strongly recommended
Seismic data is acquired by generating a loud sound at one location and recording the resulting rumblings at another location.
The source or shot which generates the sound is typically an explosion or vibration at the Earth's surface (land or sea). Each shot is recorded by many receivers. Generally a line of shots is fired. If one line is recorded, the data is a 2D survey, and if more than one line is recorded, the data is a 3D survey.
The object of recording is to infer geological subsurface structure from the strength (amplitude) of the recorded signal at different times in the recording.
A trace begins life as the recording from one receiver. The recording is sampled at some discrete interval, typically around 4 milliseconds, and lasts for some duration, typically 4 or more seconds. After the initial recording, the traces are processed in any number of ways. This processing usually changes the absolute amplitudes such that amplitude units are irrelevant, and only relative amplitudes are significant. Also the trace may reflect a logical ordering different from the original (shot,receiver) pair.
But in the end, seismic data is almost always stored as a sequence of traces, each trace consisting of amplitude samples for one location (physical or logical).
Many variations of SEG-Y exist, most created to overcome SEG-Y's limitations.
The EBCDIC reel header is usually completely ignored, and when it is used, it may or may not follow the standard template, and it may even be in ASCII format.
The binary reel header is almost completely ignored. None of the fields should be assumed to be correct, although the number of samples per trace and the sample rate usually are. Often, programs that use the SEG-Y format will read values from the binary header by default but allow the user to override the header values.
The trace header contains important information but not always in the locations specified by the standard. To adapt SEG-Y for 3D surveys, a line number field is often added somewhere in the trace header. Programs that use values from the SEG-Y trace header usually allow the user to specify the byte location and length of the values.
The trace data is most often in 32-bit IBM floating point format. Occasionally 32-bit IEEE floating-point format is used.
Although the standard applies only to tapes, SEG-Y has been adapted for storing surveys on disk as well. Disk files have no record marks or file marks, so traditional methods of reading from tapes don't work with files. There are several SEG-Y disk adaptations: a binary file with 3200-byte and 400-byte headers followed by traces, a binary file with just traces, and Fortran sequential-access files which have 3200-byte and 400-byte headers followed by traces but with Fortran record marks separating them. A similar and common format is a flat file of trace data with no reel or trace headers.