Seismic Unix WIKI

SU Data format

Here will have an explanation of how SU data is, it's headers and how they are organized in a big SU file with more than one gather:

SEG-Y Data Exchange Format, Rev. 0 and Rev. 1 and Rev. 2, with Seismic Un*x keys

SU key	Length	Bytes	Description

tracl	4	1-4	Trace sequence number within line – numbers continue to increase if additional reels are required on same line.
tracr	4	5-8	Trace sequence number within reel – each reel starts at trace number one.
fldr	4	9-12	Original field record number.
tracf	4	13-16	Trace number within the original field record.
ep	4	17-20	Energy source point number – used when more than one record occurs at the same effective surface location.
cdp	4	21-24	“Ensemble number: CDP, CMP, CRP, etc.”
cdpt	4	25-28	Trace number within the CDP ensemble – each ensemble starts with trace number one.
trid	2	29-30	“Trace identification code. -1=other, 0=unknown, 1=seismic, 2=dead, 3=dummy, 4=time-break, 5=uphole, 6=sweep, 7=timing, 8=water-break, etc.”
nvs	2	31-32	“Number of vertically summed traces yielding this trace. (1 is one trace, 2 is two summed traces, etc.)“
nhs	2	33-34	“Number of horizontally stacked traces yielding this trace. (1 is one trace, 2 is two stacked traces, etc.)“
duse	2	35-36	“Data use: 1=production, 2=test.”
offset	4	37-40	Distance from source point center to receiver group center (negative if opposite to direction in which line is shot).
gelev	4	41-44	“Receiver group elevation; elevations above sea level are positive, below sea level are negative.”
selev	4	45-48	Surface elevation at source.
sdepth	4	49-52	Source depth below surface (a positive number).
gdel	4	53-56	Datum elevation at receiver group.
sdel	4	57-60	Datum elevation at source.
swdep	4	61-64	Water depth at source.
gwdep	4	65-68	Water depth at group.
scalel	2	69-70	“Scalar to be applied to all elevations and depths in bytes 41-68 to give real value. Scaler = 1, +/- 10, … +/- 10,000. Positive=multiplier; negative=divisor.”
scalco	2	71-72	“Scalar to be applied to all coordinates in bytes 73-88 to give real value. Scaler = 1, +/- 10, … +/- 10,000. Positive=multiplier; negative=divisor.”
sx	4	73-76	Source coordinate - X.
sy	4	77-80	Source coordinate - Y.
gx	4	81-84	Group coordinate - X.
gy	4	85-88	Group coordinate - Y.
counit	2	89-90	“Coordinate units: 1=length (m or ft), 2=seconds of arc, 3=decimal degrees, 4=degrees, minutes, seconds (DMS).”
wevel	2	91-92	Weathering velocity (ft/s or m/s).
swevel	2	93-94	Subweathering velocity (ft/s or m/s).
sut	2	95-96	Uphole time at source (ms).
gut	2	97-98	Uphole time at group (ms).
sstat	2	99-100	Source static correction (ms).
gstat	2	101-102	Group static correction (ms).
tstat	2	103-104	Total static applied (ms). (Zero if no static has been applied.)
laga	2	105-106	“Lag time A (ms). Time between end of 240-byte trace ID header and time break.Positive if time break occurs after end of header, negative if time break occurs before end of header.”
lagb	2	107-108	Lag Time B (ms). Time between time break and the initiation time of the energy source. May be positive or negative.
delrt	2	109-110	Delay recording time (ms). Time between initiation time of energy source and time when recording of data samples begins. (For deep water work if data recording does not start at zero time.)
muts	2	111-112	Mute time – start (ms).
mute	2	113-114	Mute time – end (ms).
ns	2	115-116	Number of samples in this trace.
dt	2	117-118	Sample interval in µsec for this trace.
gain	2	119-120	“Gain type of field instruments: 1=fixed, 2=binary, 3=floating point, 4-N = optional use.”
igc	2	121-122	Instrument gain constant (dB).
igi	2	123-124	Instrument early or initial gain (dB).
corr	2	125-126	“Correlated: 1=no, 2=yes.”
sfs	2	127-128	Sweep frequency at start (Hz).
sfe	2	129-130	Sweep frequency at end (Hz).
slen	2	131-132	Sweep length (ms).
styp	2	133-134	“Sweep type: 1=linear, 2=parabolic, 3=exponential, 4=other.”
stas	2	135-136	Sweep trace taper length at start (ms).
stae	2	137-138	Sweep trace taper length at end (ms).
tatyp	2	139-140	“Taper type: 1=linear, 2=cosine squared, 3=other.”
afilf	2	141-142	“Alias filter frequency (Hz), if used.”
afils	2	143-144	Alias filter slope (dB/octave).
nofilf	2	145-146	“Notch filter frequency (Hz), if used.”
nofils	2	147-148	Notch filter slope (dB/octave).
lcf	2	149-150	“Low cut frequency (Hz), if used.”
hcf	2	151-152	“High cut frequency (Hz), if used.”
lcs	2	153-154	Low cut slope (dB/octave).
hcs	2	155-156	High cut slope (dB/octave).
year	2	157-158	Year data recorded.
day	2	159-160	Day of year (Julian day for GMT and UTC time basis).
hour	2	161-162	Hour of day (24 hour clock).
minute	2	163-164	Minute of hour.
sec	2	165-166	Second of minute.
timbas	2	167-168	“Time basis code: I=local, 2=GMT, 3=other, 4=UTC (Coordinated Universal Time).”
trwf	2	169-170	“Trace weighting factor – defined as 2 raised to the power (-N) volts for the least significant bit. (N = 0, 1, …. 32, 767.)“
grnors	2	171-172	Geophone group number of roll switch position one.
grnofr	2	173-174	Geophone group number of trace number one within original field record.

grnlof	2	175-176	Geophone group number of last trace within original field record.
gaps	2	177-178	Gap size (total number of groups dropped).
ofrav	2	179-180	“Overtravel associated with taper at beginning or end of line: I=down (or behind), 2=up (or ahead).”
-	60	181-240	SEG-Y Revison 0: Unassigned – for optional information.
-	-	-	-
-	-	-
-	-	-	-
d1	4	181-184	(float) sample spacing for non-seismic data
f1	4	185-188	(float) first sample location for non-seismic data
d2	4	189-192	(float) sample spacing between traces
f2	4	193-196	(float) first trace location
ungpow	4	197-200	(float) negative of power used for dynamic range compression
unscale	4	201-204	(float) reciprocal of scaling factor to normalize range
ntr	4	205-208	number of traces
mark	2	209-210	mark selected traces
shortpad	2	211-212	alignment padding
unass	28	213-240	unassigned
-	-	-	-
-	-	-	<b>SEG-Y Revison 1 locations follow</b>
-	-	-	-
-	4	181-184	X-coordinate of ensemble (CDP) position of this trace (scalar in Trace Header bytes 71-72 applies).
-	4	185-188	Y-coordinate of ensemble (CDP) position of this trace (scalar in Trace Header bytes 71-72 applies).
-	4	189-192	“For 3-D poststack data, in-line number. If one in-line per SEG-Y file is being recorded, this value should be the same for all traces.”
-	4	193-196	“For 3-D poststack data, cross-line number. This will typically be the same value as the ensemble (CDP) number in Trace Header bytes 21-24, but this does not have to be the case.”
-	4	197-200	Shotpoint number. This is probably only applicable to 2-D poststack data. Note that it is assumed that the shotpoint number refers to the source location nearest to the ensemble (CDP) location for a particular trace.
-	2	201-202	Scalar to be applied to SP number in bytes 197-200 to give the real value. Positive=multiplier; negative=divisor; zero=no scaling.
-	2	203-204	“Trace value measurement unit: -1=other, 0=unknown, 1=Pascal (Pa), 2=Volts (V), 3=milliVolts (mV), 4=Amperes (A), 5=Meters (m), 6=meters/second, 7=meters/(sec power 2), 8=Newton (N), 9=Watt (W).”
-	6	205-210	Transcuction constant – multiplicative constant used to convert the Data Trace samples to the Transduction Units (bytes 211-212).
-	2	211-212	“Transduction units: -1=other, 0=unknown, 1=Pascal (Pa), 2=Volts (V), 3=milliVolts (mV), 4=Amperes (A), 5=Meters (m), 6=meters/second, 7=meters/(sec power 2), 8=Newton (N), 9=Watt (W).”
-	2	213-214	Device/Trace identifier – unit number or id number of the device associated with the Data Trace. This allows traces to be associated across trace ensembles independent of the trace number (bytes 25-28).
-	2	215-216	“Scalar to be applied to times specified in bytes 95-114 to give true time value in ms. Scalar = 1, +/- 10, … +/- 10,000. Positive=multiplier, negative=divisor, zero=no scaling.”
-	2	217-218	“Source Type/Orientation: -1 to -n=other, 0=unknown, 1=vibrator vertical, 2=vibrator xline, 3=vibrator inline, 4=impulse vertical, 5=impulse xline, 6=impulse inline, 7=distributed impulse vertical, 8=distributed impulse xline, 9=distributed impulse inline.”
-	6	219-224	Source energy direction with respect to the source orientation. The positive orientation direction is in bytes 217-218. Energy direction is encoded in tenths of degrees (347.8 deg is encoded as 3478).
-	6	225-230	Source measurement.
-	2	231-232	“Source measurement unit: -1=other, 0=unknown, 1=Joule (J), 2=kilowatt (kW), 3=Pascal (Pa), 4=Bar (Bar), 4=Bar-meter (Bar-m), 5=Newton (N), 6=kilogram (kg).”
-	8	233-240	Unassigned – for optional information.

SEG-Y Binary Headerb, Revision 2 (2017)

The 400-byte Binary File Header record contains binary values relevant to the whole SEG-Y file. The values in the Binary File Header are defined as two-byte or four-byte, two's complement or unsigned integers, with the exception of IEEE double precision sample intervals in the optional SEG00001 trace header. Certain values in this header are crucial for the processing of the data in the file, particularly the sample interval, trace length, and format code. Revision 2.0 defines a few additional fields in the optional portion, as well as providing some clarification on the use of some existing entries.

The SEG-Y binary header table starts counting byte locations at 3201 because SEG-Y documents consider the binary header an extension of the first 3200 byte textual file header. Therefore, that is the numbering in the first column. The second column is byte length. The third column starts counting byte locations in the binary header at 1 (one) for convenience when counting these bytes as a separate file. When byte locations are stated in the Description, column 3 references are in square brackets ([ ]).

https://seg.org/Portals/0/SEG/News%20and%20Resources/Technical%20Standards/seg_y_rev2_0-mar2017.pdf SEG-Y revision 2.0 Data Exchange Format, SEG Technical Standards Committee, January 2017

Where not otherwise indicated, a value of zero indicates an unknown or unspecified value.

Bytes	Length	Bytes	Description
3201-3204	4	1-4	Job identification number.
3205-3208	4	5-8	Line number. For 3-D poststack data, this will typically contain the in-line number.
3209-3212	4	9-12	Reel number.
3213-3214	2	13-14	Number of data traces per ensemble. Mandatory for prestack data.
3215-3216	2	15-16	Number of auxiliary traces per ensemble. Mandatory for prestack data.
3217-3218	2	17-18	Sample interval. Microseconds (µs) for time data, Hertz (Hz) for frequency data, meters (m) or feet (ft) for depth data.
3219-3220	2	19-20	Sample interval of original field recording. Microseconds (µs) for time data, Hertz (Hz) for frequency data, meters (m) or feet (ft) for depth data.
3221-3222	2	21-22	Number of samples per data trace. Note: The sample interval and number of samples in the Binary File Header should be for the primary set of seismic data traces in the file.
3223-3224	2	23-24
3225-3226	2.	25-26	Data sample format code. Mandatory for all data. These formats are described in Appendix E. 1 = 4-byte IBM floating-point 2 = 4-byte, two's complement integer 3 = 2-byte, two's complement integer 4 = 4-byte fixed-point with gain (obsolete) 5 = 4-byte IEEE floating-point 6 = 8-byte IEEE floating-point 7 = 3-byte two’s complement integer 8 = 1-byte, two's complement integer 9 = 8-byte, two's complement integer 10 = 4-byte, unsigned integer 11 = 2-byte, unsigned integer 12 = 8-byte, unsigned integer 15 = 3-byte, unsigned integer 16 = 1-byte, unsigned integer
3227-3228	2	27-28	Ensemble fold — The expected number of data traces per trace ensemble (e.g., the CMP fold).
3229-3230	2	29-30	Trace sorting code (i.e., type of ensemble):–1 = Other (should be explained in a user Extended Textual File Header stanza) 0 = Unknown 1 = As recorded (no sorting) 2 = CDP ensemble 3 = Single fold continuous profile 4 = Horizontally stacked 5 = Common source point 6 = Common receiver point 7 = Common offset point 8 = Common mid-point 9 = Common conversion point
3231-3232	2	31-32	Vertical sum code: 1 = no sum,2 = two sum,…, N = M–1 sum (M = 2 to 32,767)
3233-3234	2	33-34	Sweep frequency at start (Hz).
3235-3236	2	35-36	Sweep frequency at end (Hz).
3237-3238	2	37-38	Sweep length (ms).
3239-3240	2	39-40	Sweep type code: 1 = linear 2 = parabolic 3 = exponential 4 = other
3241-3242	2	41-42	Trace number of sweep channel.
3243-3244	2	43-44	Sweep trace taper length in milliseconds at start if tapered (the taper starts at zero time and is effective for this length).
3245-3246	2	45-46	Sweep trace taper length in milliseconds at end (the ending taper starts at sweep length minus the taper length at end).
3247-3248	2	47-48	Taper type: 1 = linear 2 = cosine squared 3 = other
3249-3250	2	49-50	Correlated data traces: 1 = no 2 = yes
3251-3252	2	51-52	Binary gain recovered: 1 = yes 2 = no
3253-3254	2	53-54	Amplitude recovery method: 1 = none 2 = spherical divergence 3 = AGC 4 = other
3255-3256	2	55-56	Measurement system: If Location Data stanzas are included in the file, this entry would normally agree with the Location Data stanza. If there is a disagreement, the last Location Data stanza is the controlling authority. If units are mixed, e.g., meters on surface, feet in depth, then a Location Data stanza is mandatory. 1 = Meters 2 = Feet
3257-3258	2	57-58	Impulse signal polarity 1 = Increase in pressure or upward geophone case movement gives negative number on trace. 2 = Increase in pressure or upward geophone case movement gives positive number on trace.
3259-3260	2	59-60	Vibratory polarity code: Seismic signal lags pilot signal by:1 = 337.5° to 22.5° 2 = 22.5° to 67.5° 3 = 67.5° to 112.5° 4 = 112.5° to 157.5° 5 = 157.5° to 202.5° 6 = 202.5° to 247.5° 7 = 247.5° to 292.5° 8 = 292.5° to 337.5°
3261-3264	4	61-64	Extended number of data traces per ensemble. If nonzero, this overrides the number of data traces per ensemble in bytes 3213–3214 [13-14].
3265-3268	4	65-68	Extended number of auxiliary traces per ensemble. If nonzero, this overrides the number of auxiliary traces per ensemble in bytes 3215–3216 [15-16].
3269-3272	4	69-72	Extended number of samples per data trace. If nonzero, this overrides the number of samples per data trace in bytes 3221–3222 [21-22].
3273-3280	8	73-80	Extended sample interval, IEEE double precision (64-bit). If nonzero, this overrides the sample interval in bytes 3217–3218 [17-18] with the same units.
3281-3288	8	81-88	Extended sample interval of original field recording, IEEE double precision (64-bit). If nonzero, this overrides the sample interval of original field recording in bytes 3219–3220 [19-20] with the same units.
3289-3292	4	89-92	Extended number of samples per data trace in original recording. If nonzero, this overrides the number of samples per data trace in original recording in bytes 3223–3224 [23-24].
3293-3296	4	93-96	Extended ensemble fold. If nonzero, this overrides ensemble fold in bytes	3227–3228 [27-28].
3297-3300	4	97-100	The integer constant 1690906010 (0102030416). This is used to allow unambiguous detection of the byte ordering to expect for this SEG-Y file. For example, if this field reads as 6730598510 (0403020116) then the bytes in every Binary File Header, Trace Header and Trace Data field must be reversed as they are read, i.e., converting the endian-ness of the fields. If it reads 3362099510 (0201040316) then consecutive pairs of bytes need to be swapped in every Binary File Header, Trace Header and Trace Data field. The byte ordering of all other portions (the Extended Textual Header and Data Trailer) of the SEG-Y file is not affected by this field.
3301-3500	200	101-300	Unassigned
3501-3501	1	301-301	Major SEG-Y Format Revision Number. This is an 8-bit unsigned value. Thus for SEG-Y Revision 2.0, as defined in this document, this will be recorded as 0216. This field is mandatory for all versions of SEG-Y, although a value of zero indicates “traditional” SEG-Y conforming to the 1975 standard.
3502-3502	1	302-302	Minor SEG-Y Format Revision Number. This is an 8-bit unsigned value with a radix point between the first and second bytes. Thus for SEG-Y Revision 2.0, as defined in this document, this will be recorded as 0016. This field is mandatory for all versions of SEG-Y.
3503-3504	2	303-304	Fixed length trace flag. A value of one indicates that all traces in this SEG-Y file are guaranteed to have the same sample interval, number of trace header blocks and trace samples, as specified in Binary File Header bytes 3217–3218 [17-18] or 3281–3288 [81-88], 3517–3518 [317-318], and 3221–3222 [21-22] or 3289–3292 [89-92]. A value of zero indicates that the length of the traces in the file may vary and the number of samples in bytes 115–116 of the Standard SEG-Y Trace Header and, if present, bytes 137–140 of SEG-Y Trace Header Extension 1 must be examined to determine the actual length of each trace. This field is mandatory for all versions of SEG-Y, although a value of zero indicates “traditional” SEGY conforming to the 1975 standard. Irrespective of this flag, it is strongly recommended that corect values for the number of samples per trace and sample interval appear in the appropriate trace Trace Header locations.
3505-3506	2	305-306	Number of 3200-byte, Extended Textual File Header records following the Binary Header. If bytes 3521–3528 [321-328] are nonzero, that field overrides this one. A value of zero indicates there are no Extended Textual File Header records (i.e., this file has no Extended Textual File Header(s)). A value of -1 indicates that there are a variable number of Extended Textual File Header records and the end of the Extended Textual File Header is denoted by an ¹⁾ stanza in the final record (Section 6.2). A positive value indicates that there are exactly that many Extended Textual File Header records.Note that, although the exact number of Extended Textual File Header records may be a useful piece of information, it will not always be known at the time the Binary Header is written and it is not mandatory that a positive value be recorded here or in bytes 3521–3528 [321-328]. It is however recommended to record the number of records if possible as this makes reading more effective and supports direct access to traces on disk files. In the event that this number exceeds 32767, set this field to –1 and bytes 3521–3528 [321-328] to 3600+3200*(number of Extended Textual File Header records). Add a further 128 if a SEG-Y Tape Label is present.
3507-3510	4	307-310	Maximum number of additional 240 byte trace headers. A value of zero indicates there are no additional 240 byte trace headers. The actual number for a given trace may be supplied in bytes 157–158 of SEG-Y Trace Header Extension 1.
3511-3512	2	311-312	Time basis code: 1 = Local 2 = GMT (Greenwich Mean Time) 3 = Other, should be explained in a user defined stanza in the Extended Textual File Header 4 = UTC (Coordinated Universal Time) 5 = GPS (Global Positioning System Time)
3513-3520	8	313-320	Number of traces in this file or stream. (64-bit unsigned integer value) If zero, all bytes in the file or stream are part of this SEG-Y dataset.
3521-3528	8	321-328	Byte offset of first trace relative to start of file or stream if known, otherwise zero. (64-bit unsigned integer value) This byte count will include the initial 3600 bytes of the Textual and this Binary File Header plus the Extended Textual Header if present. When nonzero, this field overrides the byte offset implied by any nonnegative number of Extended Textual Header records present in bytes 3505–3506 [305-306].
3529-3532	4	329-332	Number of 3200-byte data trailer stanza records following the last trace (4 byte signed integer). A value of 0 indicates there are no trailer records. A value of -1 indicates an undefined number of trailer records (0 or more) following the data. It is, however, recommended to record
3533-3600	68	333-400	Unassigned

¹⁾

SEG: EndText