New MATLAB Functions For Reading, Writing And Modifying SEG Y Files SEGY Toolbox Instructions
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MATLAB : SEGY_Toolbox
New MATLAB® functions for reading, writing, and modifying
SEG-Y files
Heather J.E. Lloyd, Kevin W. Hall and Gary F. Margrave
ABSTRACT
The SEGY_Toolbox for MATLAB® is a new set of tools that allow SEG_Y files to be
read, modified, and written. This toolbox adheres to the SEG-Y Revision 1 standard set
out in the SEG Y rev 1 Data Exchange format publication. Unlike previous SEG-Y
MATLAB® tools, this toolbox tries to be as flexible as possible when reading SEG-Y
files but very stringent when writing SEG-Y files. Currently the following tools are
available: SEGY_EditTextHeader, SEGY_StandardizeHeader, SEGY_endianSwap,
SEGY_getData, SEGY_getHeader, SEGY_getTraces, SEGY_read, SEGY_readHeader,
SEGY_readMulti, SEGY_setHeader, SEGY_write, SEGY_writeHeaders, and
SEGY_writeTraces. Please contact the authors if any further functionality is desired.
INTRODUCTION
In the past few years CREWES has developed several tools for reading and writing
SEG-Y files. Most of these tools have been based strictly on the SEG-Y Revision 1
standard, making it difficult to read in files that do not conform to this standard. There
have also been some tools that have been developed that will read in non-conforming
files, but were fragmented and difficult to use. It has also, in the past, been difficult to
use trace header values effectively. A new set of tools has been created that unites these
fragmented tools into a cohesive and user friendly tool box. The SEGY_Toolbox allows
most SEG-Y files to be read even if they do not conform to the SEG-Y standards. The
new tools are distributed as part of the CREWES Matlab toolbox, and are in the
segy/Segy_Toolbox directory. In order to use these tools, you will need to download the
toolbox, install it, and update your Matlab path.
THE OBJECTS
This toolbox has been written using an object oriented approach. This means that each
of the parts of a SEG-Y file have been made into an object. These objects are difficult to
interrogate by themselves, so a series of tools have been made to help the users get the
information that they need. There are five main objects that are used in the tool box: the
BinaryHeader object, TextHeader object, and the Trace object, which consists of two
other objects: the TraceHeader object and the TraceData object.
Trace
Main Contents:
• TraceHeader Object
• TraceData Object
A Trace object contains two other object, a TraceHeader and a TraceData object. The
Trace object is primarily used to keep these two objects together. To get the trace header
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information out of a Trace object use SEGY_getHeader. To get the trace data out of a
Trace object use SEGY_getData.
TraceHeader
Main Contents:
• Header Information
• Definition Information
A TraceHeader object stores the trace header information. This object can exist by
itself or as part of a Trace object. It uses a Definition file to format the header
information into useful data. The information contained in each trace header is specific
to the trace it belongs with. To get a piece of header information use the
SEGY_getHeader tool. To change a header value, use the SEGY_setHeader tool. These
tools will return a value for every trace that was read into Matlab.
TraceData
Main Contents:
• Trace Data
A TraceData object contains the traces of the SEG-Y file. It is found in a Traces
object. To get the data into a usable Matlab matrix use the tool SEGY_getData.
BinaryHeader
Main Contents:
• Header Information
• Definition Information
A BinaryHeader object stores the binary header information. The information
contained in this object is usually about the whole SEG-Y file. It uses a Definition file to
format the header information into useful data. To get a piece of header information use
the SEGY_getHeader tool. To change a header value, use the SEGY_setHeader tool.
TextHeader
Main Contents:
• Text header Information
A TextHeader object stores the text header information. This is ASCII or EBCDIC
formatted text, which is used to communicate information to the User and not the
computer. To get the text use the SEGY_getHeader tool. To save changes made to the
header use the SEGY_setHeader tool.
THE TOOLS
SEGY_read
Syntax: [traces,texthead,binaryhead,extendedhead]=SEGY_read(filein)
• Inputs:
filein = the name of the SEG-Y file that is to be read in (optional).
• Outputs:
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traces = a TraceHeader object. To get the trace header values use
SEGY_getHeader.
texthead = a TextHeader object. To get the character array use
SEGY_getHeader.
binaryhead = a BinaryHeader object. To get the binary header values use
SEGY_getHeader.
extendedhead = a cell array of TextHeader Objects. To get the character
array use SEGY_getHeader.
SEGY_read is a tool that reads an entire SEG-Y file into Matlab objects. This tool
automatically determines whether the text header is ASCII or EBCDIC format, uses
comma delimited files to decode the binary header and the trace header, uses a graphical
user interface that allows the user to verify the format of trace data (Figure 1) and uses a
dialog box to identify any extended headers. For trace headers and binary headers that do
not follow the SEG-Y revision 1 standards some additional steps are required, please see
the non-standard SEG-Y files section in this paper.
FIG. 1. This figure shows the trace format graphical user interface. The four top plots are
histograms of the data when decoded using different file formats. The lower window is a plot of
the trace using the selected format indicated by the radio button (in this case the upper left
option).
SEGY_readHeader
Syntax: [tracehead,texthead,binaryhead,extendedhead]=SEGY_read(filein)
• Inputs:
filein = the name of the SEG-Y file that is to be read in.
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•
Outputs:
texthead = a TextHeader object. To get the character array use
SEGY_getHeader.
binaryhead = a BinaryHeader object. To get the binary header values use
SEGY_getHeader.
tracehead = a TraceHeader object. To get the trace header values use
SEGY_getHeader.
extendedhead = a cell array of TextHeader Objects. To get the character
array use SEGY_getHeader.
SEGY_readHeader is a tool that will only read in the text header, binary header,
extended headers and the trace headers. SEGY_getTraces will load the trace data of
selected traces.
SEGY_readMulti
Syntax: [traces, texthead,binaryhead,extendedhead]=SEGY_readMulti({})
[traces, texthead,binaryhead,extendedhead]=SEGY_readMulti({file1,file2,file3})
[traces, texthead,binaryhead,extendedhead]=SEGY_readMulti({path})
• Inputs:
file = a cell array containing the names of the SEG-Y files that are to be
read in. file can also be the path to a directory containing *.sgy
files. If file is set to {} then a prompt will ask the user to select
files.
• Outputs:
texthead = a TextHeader object. To get the character array use
SEGY_getHeader.
binaryhead = a BinaryHeader object. To get the binary header values use
SEGY_getHeader.
traces = a Trace object.
To get the trace header values use
SEGY_getHeader.
To get the trace data values use
SEGY_getData.
extendedhead = a cell array of TextHeader Objects. To get the character
array use SEGY_getHeader.
SEGY_readMulti is a tool that will read in several files, or a directory of files and
combine them. For this to work, the job id, a number that indicates that the files belong
to the same survey, must be the same for all files. If the job id is the same then this
program assumes that the file contains the same amount of extended headers in the first
file as the subsequent files. This program also assumes that if the trace format code, a
value stored in the binary header, is the same for two files then they contain the same
format of data. For example, both files would have IBM floating point data, rather than
one having IBM type data and the other has IEEE type data.
SEGY_getTraces
Syntax: traces = SEGY_getTraces(tracehead)
traces = SEGY_getTraces(tracehead,word,value);
traces = SEGY_getTraces(tracehead,word1,value1,word2,value2,word2,value3,...);
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•
Input:
tracehead = can be either a TraceHeader object which can be created by
SEGY_readHeaders or a filename.
word = a name defined in the definitions file, These values are given in
Table 1.
value = can be either a scalar value or a numerical array
•
Output:
traces = a Trace object.
To get the traceheader values use
SEGY_getHeader.
To get the trace data values use
SEGY_getData.
SEGY_getTraces is a function that allows the user to selectively read some traces
from the file. This is done by comparing value to the traceheader data. The input
parameter word must be one of the names defined in the definitions file. To follow the
guidelines set out in the SEG-Y revision 1 standard, the word must any of the ones listed
in Table 1. If a word is not among the ones listed in Table 1, an error will occur. It is
possible to search using multiple words. An example would be:
traces = SEGY_getTraces(tracehead,'gx',1:50,'cdpx',60);
which would search for traces that have a group (receiver) x coordinate in the range 1:50
and a common depth point x coordinate of 60.
Table 1. This table contains the name of the variables used in the definitions file for the trace
headers along with a description of what each variable is. These variables are based on the SEGY revision 1 Standard.
Name
tracl
tracr
fldr
tracf
ep
cdp
cdpt
trid
nvs
nhs
duse
offset
gelev
selev
sdepth
gdel
sdel
Description
Trace sequence number within line
Trace sequence number within SEGY file
Original field record number
Trace number within the original field record
Energy source point number
Ensemble number
Trace number within the ensemble
Trace identification code
Number of vertically summed traces yielding this trace
Number of horizontally stacked traces yielding this trace
Data use
Distance from center of the source point to the center of the receiver group
Receiver group elevation
Surface elevation at source
Source depth below surface
Datum elevation at receiver group
Datum elevation at source
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swdep
gwdep
scalel
scalco
sx
sy
gx
gy
counit
wevel
swevel
sut
gut
sstat
gstat
tstat
laga
lagb
delrt
muts
mute
ns
dt
gain
igc
igi
corr
sfs
sfe
slen
styp
stas
stae
tatype
afilf
afils
nofilf
nofils
lcf
hcf
lcs
hcs
6
Water depth at source
Water depth at group
Scalar to be applied to all elevations and depths
Scalar to be applied to all coordinates
Source coordinate X
Source coordinate Y
Group coordinate X
Group coordinate Y
Coordinate units
Weathering velocity
Subweathering velocity
Uphole time at source in milliseconds
Uphole time at group in milliseconds
Source static correction in milliseconds
Group static correction in milliseconds
Total static applied in milliseconds
Lag time A
Lag Time B
Delay recording time
Mute start time in milliseconds
Mute end time in milliseconds
Number of samples in this trace
Sample interval in microseconds for this trace
Gain type of field instruments
Instrument gain constant (dB)
Instrument early or initial gain (dB)
Correlated
Sweep frequency at start (Hz)
Sweep frequency at end (Hz)
Sweep length in milliseconds
Sweep type
Sweep trace taper length at start in milliseconds
Sweep trace taper length at end in milliseconds
Taper type
Alias filter frequency (Hz)
Alias filter slope (dB/octave)
Notch filter frequency (Hz)
Notch filter slope (dB/octave)
Low-cut frequency (Hz)
High-cut frequency (Hz)
Low-cut slope (dB/octave)
High-cut slope (dB/octave)
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year
day
hour
minute
sec
timbas
trwf
grnors
grnofr
grnlof
gaps
otrav
Year data recorded
Day of year
Hour of day
Minute of hour
Second of minute
Time basis code
Trace weighting factor
Geophone group number of roll switch position one
Geophone group number of trace number one within original field record
Geophone group number of last trace within original field record
Gap size
Over travel associated with taper at beginning or end of line
X coordinate of ensemble (CDP) position of this trace (scalar in Trace
Header bytes 71-72 (scalco) applies). The coordinate reference system should
cdpx
be identified through an extended header Location Data stanza (see section
D-1).
Y coordinate of ensemble (CDP) position of this trace (scalar in bytes Trace
Header 71-72 (scalco) applies). The coordinate reference system should be
cdpy
identified through an extended header Location Data stanza (see section D1).
For 3-D poststack data, this field should be used for the in-line number. If
one in-line per SEG Y file is being recorded, this value should be the same
iline
for all traces in the file and the same value will be recorded in bytes 32053208 (lino) of the Binary File Header.
For 3-D poststack data, this field should be used for the cross-line number.
xline
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.
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
sp
nearest to the ensemble (CDP) location for a particular trace. If this is not the
case, there should be a comment in the Textual File Header explaining what
the shotpoint number actually refers to.
Scalar to be applied to the shotpoint number in Trace Header bytes 197-200
to give the real value. If positive, scalar is used as a multiplier; if negative as
scalsp
a divisor; if zero the shotpoint number is not scaled (i.e. it is an integer. A
typical value will be -10, allowing shotpoint numbers with one decimal digit
to the right of the decimal point).
tval
Trace value measurement unit
tconstm Transduction Constant mantissa
tconste Transduction Constant exponent
tunit
Transduction Units
devtr
Device/Trace Identifier
scalt
Scalar to be applied to times
stypeo Source Type/Orientation
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sedm
sede
smmtm
smmte
smmtu
fbpicks
scalfb
Source Energy Direction with respect to the source orientation mantissa?
Source Energy Direction with respect to the source orientation exponent?
Source Measurement mantissa
Source Measurement exponent
Source Measurement Unit
First Break Picks ( This is not part of the SEG-Y Revision 1 Standard but has
been added as a CREWES Standard)
Scaling Factor for first breaks ( This is not part of the SEG-Y Revision 1
Standard but has been added as a CREWES Standard)
SEGY_getHeader
Syntax:
hdr = SEGY_getHeader(obj,word,scalevals)
Examples: hdr = SEGY_getHeader(Texthead,'header')
val = SEGY_getHeader(Binaryhead,word)
val = SEGY_getHeader(Tracehead,word)
val = SEGY_getHeader(Trace,word)
•
•
If obj is a TextHeader object, a 40 x 80 char array will be returned;
If obj is a BinaryHeader object, word must be a name from the definitions
file. SEG-Y revision 1 standard binary header words can be found in Table2.
The value returned would be the value that was associated with the variable
name.
• If obj is a TraceHeader object or a Trace object, word it must be a name from
the trace header definitions file. SEG-Y revision 1 trace header standard
words can be found in Table 1. This function will return one value associated
with this word per trace.
For Trace, TraceHeader, or BinaryHeader objects, the values will be automatically
scaled by the scale factor in the header, if it is desired to not have these values scaled,
then scalevals should be set to zero. An example of this is seen below:
val=SEGY_getHeader(tracehead,'fbpicks',0);
To allow the values to be scaled enter either of the following:
val=SEGY_getHeader(tracehead,'fbpicks',1);
or
val=SEGY_getHeader(tracehead,'fbpicks');
SEGY_getHeader is a tool that will return the header or header values. For a text
header object this tool will return the 40 x 80 character text header. For a binary header,
trace header, or trace, object this tool requires the header variable name that is desired,
such as ‘cdp’ or ‘gx’ for trace headers. This tool will then return the values associated
with that variable name, for a binary header object there will only be one value returned
whereas for a trace or trace header object there will be an array of values where one value
belongs to each trace.
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TABLE 2. This table contains the name of the variables used in the Binary Header definitions file
along with a description of what each variable is. These variables are based on the SEG-Y
revision 1 Standard.
Name
jobid
lino
reno
ntrpr
nart
hdt
dto
hns
nso
format
fold
tsort
vscode
hsfs
hsfe
hslen
hstyp
schn
hstas
hstae
htatyp
hcorr
bgrcv
rcvm
mfeet
polyt
vpol
rev
flen
netfh
unasn1-84
Description
Job identification number
Line number
Reel number
Number of data traces per record
Number of auxiliary traces per record
Sample interval in microseconds
Sample interval of field data in microseconds
Number of samples per data trace
Number of samples recorded in field data per data trace
Data sample format code
Ensemble fold
Trace sorting code
Vertical sum code
Sweep frequency at start (Hz)
Sweep frequency at end (Hz)
Sweep length (ms)
Sweep type code
Trace number of sweep channel
Sweep trace taper length (ms)
Sweep trace taper length (ms)
Taper type
Correlated data traces
Binary gain recovered
Amplitude recovery method
Measurement system
Impulse signal polarity
Vibratory polarity code
SEGY Format Revision Number
Fixed length trace flag
Number Extended Textual File Headers
Unassigned values 1 through 84
SEGY_setHeader
Syntax:
obj=SEGY_setHeader(obj,word,value,scalevals)
Examples: Texthead=SEGY_setHeader(Texthead,'header',header)
Binaryhead=SEGY_setHeader(Binaryhead,word,value)
Tracehead=SEGY_setHeader(Tracehead,word,value)
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Trace=SEGY_setHeader(Trace,word,value)
•
•
•
For a TextHeader object, word must be 'header' and value must be a 40 x 80
character array.
For a BinaryHeader object, word must be a name from the definitions file. The
binary header words based on the SEG-Y revision 1 standard can be found in
Table 2. Value must be a single value that belonging to the word.
For a TraceHeader object or a Trace object, word must be a name from the
definitions file. SEG-Y revision 1 standard trace header words can be found in
Table 1. Value must be a numerical array where the values are described by the
word, and the length of value must be the same as the number of traces.
For Trace, TraceHeader, or BinaryHeader objects, the values will be automatically
scaled by the scale factor in the header, if it is desired to not have these values scaled then
scalevals should be zero. An example of this is seen below.
tracehead=SEGY_setHeader(tracehead,'fbpicks',values,0);
To allow the values to be scaled enter either of the following:
tracehead=SEGY_setHeader(tracehead,'fbpicks',values,1);
or
tracehead=SEGY_setHeader(tracehead,'fbpicks',values,);
SEGY_setHeader is a tool that will set the header or header values. For a text header
object this tool will change the 40 x 80 character text header to the user specified header.
This header must be of the same dimensions. For a binary header, trace header or trace
object, this tool will set the header variable name specified by the user to the values that
was specified. It is essential that for trace or trace header objects there is one value for
each trace.
SEGY_getData
Syntax:
•
•
data = SEGY_getData(trace);
Input
trace = a Trace object containing trace data
Output
data = the trace data in the object. This data is organized as follows
[number of samples x number of traces]
SEGY_getData will return the trace data that is stored in a trace object. This is
different from SEGY_getTraces as this data is already in Matlab format and is stored in
an object. The data can then be manipulated, but if traces are added or subtracted then
the trace header values must also be changed to accommodate the change in traces.
SEGY_EditTextHeader
Syntax:
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texthdr = SEGY_EditTextHeader(texthead);
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•
•
texthdr = SEGY_EditTextHeader();
Inputs:
texthead can either be left blank or be a TextHeader object
Outputs:
texthdr is a TextHeader object that contains the updated header if the input
was a TextHeader object, else texthdr is a 40 x 80 character array
of the updated information.
SEGY_EditTextHeader is a tool that allows the user to modify the text header. If the
text header is in an un-editable format, this tool allows the user to flip between ASCII
and EBCDIC formatting.
SEGY_write
Syntax:
•
SEGY_write(fileout,traces,texthead,binaryhead,extendedhead)
SEGY_write(fileout,traces,texthead,binaryhead)
Inputs:
traces = can be a Trace object or a cell array where:
{1,1}= is a matrix of traceheader information. This is a (240 x
number of traces) of type uint8.
{1,2}= is a matrix of traces. This is (tracelength x number of
traces) of 4-byte floats.
fileout = the name of the new sgy file should end in .sgy
texthead = can be a 40 x 80 char array or a TextHeader object.
binaryhead = can be a numerical array that satisfies SEG-Y Revision 1 or
a BinaryHeader object.
extendedhead = is optional. This should be either a 40 x 80 char array or a
TextHeader object. If multiple extended headers are required
extendedhead can be a cell array containing multiple char arrays or
multiple TextHeader objects.
SEGY_write is a tool for writing a new SEG-Y file. It strictly adheres to the SEG-Y
Revision 1 standards. The text header is written in ASCII format, and trace data are
written in big-endian IEEE floating-point format. The binary header and trace headers
are written using the definitions in the Revision 1 standard, however, CREWES has set
the last two unassigned variables to first break picks (bytes 233-236) and a scale factor
for the first break picks (bytes 237-240). If using nonstandard definitions it is essential
that the definitions file contains a SEGY_Rev1_EQ column in the spreadsheet, with a list
of the Revision 1 standard equivalents. Please see the non-standard SEG-Y files section
for more information.
SEGY_writeHeaders
Syntax:SEGY_writeHeaders(fileout,texthead,binaryhead,extendedhead,permission)
SEGY_writeHeaders(fileout,texthead,binaryhead)
•
Inputs:
fileout = the name of the new sgy file should end in .sgy
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texthead = can be a 40 x 80 char array or a TextHeader object.
binaryhead = can be a numerical array that satisfies SEG-Y Revision 1 or
a BinaryHeader object.
extendedhead = is optional. This should be either a 40 x 80 char array or a
TextHeader object. If multiple extended headers are required then
extendedhead can be a cell array containing multiple char arrays or
multiple TextHeader objects.
permission = if you want to rewrite the headers (not recommended) then
use permission='r+' If you want to overwrite an old file or create a
new one use permission='w+'. 'w+' is the default.
SEGY_writeHeaders is a tool that will only write the text header, binary header, and
any extended headers. This is to allow traces to be written in a loop in a subsequent step.
To write the traces use SEGY_writeTraces. Like SEGY_write, this tool also adheres to
the SEG-Y Revision 1 standards.
SEGY_writeTraces
Syntax: SEGY_writeTraces(fileout,traces,numcurtraces,numcurshots)
SEGY_writeTraces(fileout,{tracehead, tracedata},numcurtraces,numcurshots)
•
Inputs:
fileout = the name of the new sgy file should end in .sgy
traces = can be a Trace object or a cell array where:
{1,1}= is a matrix of traceheader information. This is a (240 x
number of traces) of type uint8.
{1,2}= is a matrix of traces. This is a (tracelength x number of
traces) of 4-byte floats.
numcurtraces = this numeric value states the number of traces that have
been written to the file already. This is required for SEG-Y
Revision 1 Standards.
numcurshots = this is a numeric value that depicts the number of shots that
are currently in the file. This is optional but can be helpful when
retrieving data.
SEGY_writeTraces is a tool that will write trace headers and traces to a file. This tool
appends new traces to the end of the file. This function requires the number of traces that
are already in the file to be specified. The headers must be written to the file first and
then the traces may be appended.
To write the headers to the file use
SEGY_writeHeaders first.
NON-STANDARD SEG-Y FILES
When attempting to read SEG-Y files it is rare that they will conform to the SEG-Y
Revision 1 Standard. This toolbox can read in these files, however, a few more steps are
required. If the trace headers or binary header do not conform to the SEG-Y Revision 1
standard, then a new comma delimited file with definitions needs to be created. This file
can either be generated using SEGY_StandardizeHeader, or any spreadsheet program. If
a spreadsheet program is being used there must be the column headings: Name, startByte,
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endByte, Type, SEGY_Rev1_EQ, and Description. Name is a variable name used to
describe the value, startByte is where the value begins and the endByte is where the value
ends, Type is the number format such as int16, and Description is a description of the
variable. SEGY_Rev1_EQ is the SEG-Y Revision 1 equivalent; this should be one of the
variable names defined in the Table 1 or Table 2.
An example of the
SEGY_StandardizeHeader interface can be seen in Figure 2. Once this file is created, the
non standard file can be read in by adding ‘bindefinitions’, ’bindefinitionfilename.csv’
for binary headers or ‘trcdefinitions’, ’tracedefinitionfilename.csv’. For binary header
definitions using the tool SEGY_read it would look like this:
[txth, binh, trc, exth]=SEGY_read(filein, ‘bindefinitions’,’nameofdefinitionsfile.csv’)
This tool box uses bytes 3224 and 3225 in the file to determine whether or not the file
is big endian or little endian, these bytes, in the binary header, are reserved to identify the
format of the data traces. If these bytes are not reserved for the format of the data traces
the values in the file will not be decoded correctly.
If this occurs, use
SEGY_endianSwap to correct for this. If a text header is read in incorrectly use
SEGY_EditTextHeader to decode appropriately. Extended Headers are assumed to be 40
x 80 character arrays in this initial release.
FIG. 2. This figure shows the SEGY_StandardizeHeader interface. The table contains the
essential definition information along with some plotting controls for the plot on the right. Several
different variables can be plotted at the same time.
CONCLUSIONS
The SEGY_Toolbox is a new set of Matlab tools for the purpose of reading,
modifying and writing SEG-Y files. As this toolbox is new, suggestions and comments
on the functionality of this toolbox will be appreciated.
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EXAMPLES
Reading a SEG-Y file into Matlab.
Some analyses are more easily done in a Matlab environment rather than a processing
environment. Using commercial processing software, processed data was saved as a
SEG-Y file and then opened using the SEGY_Toolbox in Matlab. The goal is to find the
common midpoint gather at 550 m and plot against offset.
1. Read in the SEG-Y file called prodata.sgy.
[traces,texthead,binaryhead,extendedhead]=SEGY_read(prodata.sgy);
2. Get the cmp values and offset values for all traces
cmp=SEGY_getHeader(traces,’cdp’);
offset=SEGY_getHeader(traces,’offset’);
3. Search the cmp to find the value at 550m.
ind=find(cmp==550);
4. Find the traces and offsets that belong to those values
offset550=offset(ind);
data=SEGY_getData(traces);
data550=data(ind);
5. Plot an image of the data against offset.
[traces,texthead,binaryhead,extendedhead]=SEGY_read(prodata.sgy);
dt= SEGY_getHeader(binaryhead,’hdt’); or dt=SEGY_getHeader(traces,’dt’);
time=0:dt(1):size(data550,1)*dt(1);
plotimage(data550,time,offset550);
Writing a SEG-Y file from Shot Records Created in Matlab.
In the CREWES toolbox there are tools that use finite differencing algorithms to
create shot records. It would be useful to make several shot records and write them to a
SEG-Y file.
1. Create the variables for the shot records.
velocity=1500*ones(101,151);velocity(51:end,:)=4500;
dx=10;
dtstep=(dx*(1/sqrt(2)./(max(velocity(:))))-.0001); % For stability
dt=0.002;
tmax=3;
xrec=0:dx:1500;
zrec=0*xrec;
[wavelet,twave]=ricker(dt,40);
2. Plan the seismic experiment. There will be 3 shots at 300m, 900m and 1200m.
Each shot will have a receiver every 10 meters along the line for a total of 151
receivers per shot.
14
CREWES Research Report — Volume 22 (2010)
MATLAB : SEGY_Toolbox
shotpos=[300,900,1200];
ns=length(0:dt:tmax);
numtraces=length(xrec)*3;% number of receivers * number of shots
3. Create the binaryheader and textheader using AFD_makeSEGYheaders, a
utility that will return values that can be used with the SEGY_Toolbox.
[texthead,binaryhead]=AFD_makeSEGYheaders(dt,ns,numtraces,1,3);
4. Write the binaryheader and textheader to
SEGY_writeHeaders.
fileout=’seismicexp1.sgy’;
SEGY_writeHeaders(fileout,texthead,binaryhead);
a
SEG-Y
file
using
5. Loop over the number of shots to create the synthetic finite differenced shot
records, create traceheaders using AFD_makeSEGYtraceheaders and write the
traces and the traceheaders to the SEG-Y file;
for k=1:length(shotpos)
snap1=zeros(101,151);snap1(1,(shotpos(k)/dx)+1)=1; snap2=snap1;
[seismogram,seis,t]=afd_shotrec(dx,dtstep,dt,tmax,velocity,snap1,snap2,...
xrec,zrec,wavelet,twave,1);
tracehead=AFD_makeSEGYtraceheaders(xrec,ones(size(xrec)),dt,...
shotpos(k),1,k);
SEGY_writeTraces(fileout,{tracehead, seismogram},length(xrec)*(k-1),k-1);
end
6. The SEG-Y file has now been created.
ACKNOWLEDGEMENTS
The authors would like to thank everyone currently or formerly in CREWES, who
have previously worked on the Matlab SEG-Y tools. A large part of the code used in the
current project was developed by Chad Hogan. Other contributors include Henry Bland
and Carla Osborne.
REFERENCES
Norris, M. W., Faichney, A. K., 2002, SEG Y rev 1 Data Exchange format, SEG Technical Standards
Committee. SEG.
Hogan, C., 2004, SEGY Matlab Tool Documentation, CREWES Matlab Toolbox
CREWES Research Report — Volume 22 (2010)
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