mbsegygrid
Section:  MB-System 5.0  (1)
Updated:  12 October 2013 
Index


 
NAME

mbsegygrid - Generate time vs. trace number grids of seismic data from segy files.

 
VERSION

Version 5.0

 
SYNOPSIS

mbsegygrid -Isegyfile -Ogridfile
[-Ashotscale/timescale -Bmaxvalue/window -Ddecimatex/decimatey
-Gmode/gain[/window]
-Smode[/start/end[/schan/echan]] -Tsweep[/delay]
-Wmode/start/end -H -V]";

 
DESCRIPTION

MBsegygrid generates grids of seismic data from segy files.
The program works by inserting trace data into a grid in
which the y-axis is some measure of trace number, range, or distance
along a profile, and the y-axis is time. The output files are GMT netCDF
format grid files.

By default or with use of the -A option, the x-dimension of
the grid is determined by the number of traces specified
by the -S option and any decimation specified with option -D.
If the -R option is used, then the profile will represent a line between
specified start and end positions divided into binning cells according to a specified
cell size, and the x-dimension of the grid will be the distance along the
line (in meters) divided by the cell size and any decimation specified with
option -D.

The y-dimension of the grid is determined by the sample spacing, any decimation
specified with option -D, and the time sweep specified by the  -T
option.

If the -S option is not specified, then all of the traces in the segy file
will be gridded. If the time sweep and delay (if any) are not specified using
the -T option, then the sweep and delay will be set so that all trace samples
are incorporated into the grid.

A simple time-varying gain can be applied to the traces before gridding using
the -G option. The start time for the gain may be either the start
of the trace or the seafloor arrival time.

Simple time-domain low-pass filtering can be applied to the traces before
gridding using the -F option.

Regions of the grid without data are indicated in the output by NaN values.

 
AUTHORSHIP

David W. Caress (caress@mbari.org)




  Monterey Bay Aquarium Research Institute



Dale N. Chayes (dale@ldeo.columbia.edu)




  Lamont-Doherty Earth Observatory

 
OPTIONS


-A


shotscale/timescale



This option causes the x-axis to be rescaled from shot number to distance in meters
and the y-axis to be rescaled from time in seconds to depth in meters.
The shotscale value represents the shot spacing in meters and the
timescale value is the scaling from time to depth (typically 750 m/s
for the water column).
-B


maxvalue[/window]



This option causes the application of automatic gain control (AGC). At each sample, a time interval
window seconds long centered on the sample is considered. The sample value is multiplied
by a scaling factor calculated as maxvalue divided by the maximum original value in the
time interval. This approach can emphasize coherent arrivals regardless of amplitude level. If
window is not specified, then the window is taken to be the trace length in time so that
the entire trace is normalized using the same scale factor, and all traces has the same maximum
value of maxvalue.
-D


decimatex/decimatey



Sets the decimation of traces (decimatex) and samples (decimatey) used in
generating the output grid. The downsampling of the data is actually accomplished by averaging
the values within each grid bin rather than omitting traces or samples.
-F


mode/window



Sets the application of low-pass filtering to the trace data. The filtering is applied
by convolving a filter function of length window seconds with the trace. Currently the only
available filter is a cosine taper, set using mode = 1.
-G


mode/gain[/window]



Sets the application of gain to the trace data. The mode value determines how
the gain is applied. If mode = 0, no gain is applied.If mode = 1,
the gain is applied starting from the beginning of the trace. If  mode = 2,
the gain is applied starting at the seafloor (assuming that a seafloor pick has
been defined in the segy data file). The gain value sets the gain applied
as follows. If G = gain and t is the time in seconds since the start of
gain application, then each sample is multiplied by a factor of (1 + (G * t)).
The parameter window, if given, sets the length of the time interval
in seconds that gain is applied. When the gain is windowed, trace values after
the end of the gain window are zeroed.
-H


This "help" flag cause the program to print out a description
of its operation and then exit immediately.
-I


segyfile



Sets the filename of the input segy seismic data file to be gridded.
-O


gridfile



Sets the filename for the output GMT netCDF format grid.
-R


cellsize/startlon/endlon/startlat/endlat



If the -R option is used, then the profile will represent a line between
start and end positions specified using longitude and latitude values in
decimal degrees. This line segment is divided into binning cells according to
cellsize in meters, and the x-dimension of the grid will be the distance along the
line (in meters) divided by the cell size and any decimation specified with
option -D. Effectively, the location of each trace will be projected onto the line
segment and the segment added to the appropriate cell. Where cells have multiple traces,
the traces will be averaged.
-S


mode[/start/end[/schan/echan]]



This option sets the range of traces that are gridded, and thus determines
the x-dimension of the output grid (also impacted by any decimation
specified with -D). If mode = 0, then start and end
refer to shot numbers. This typically is useful for subbottom data or seismic
data in shot gather form. If mode = 1, then start and end
refer to CMP (or RP or CDP) numbers. This typically is useful for seismic
reflection data in stacked or CMP gather forms. If the data are multichannel
seismic reflection or seismic refraction in either shot or CMP gathers, the
start and end of the channels selected for gridding is set using the optional schan
and echan, respectively. The x-dimension of the output grid is determined by
(end - start + 1) * (echan - schan + 1) / decimatex.
-T


sweep[/delay]



Sets the time range of seismic data to be gridded. The sweep specifies
the number of seconds of data comprising the y-axis of the grid. The number of
samples follows from the sample interval of the data, and any decimation
applied using -D. The optional delay value sets the sweep
start time, again in seconds.
The y-dimension of the output grid is determined by
sweep / sampleinterval / decimatey, where the sampleinterval is
a parameter read with the seismic data.
-V


Normally, mbsegygrid prints out information regarding its
controlling parameters during execution; the
-V option causes the program to also print out statements
indicating its progress.
-W


mode/start/end



This option can be used to limit the data being gridded to a particular time
window in various ways. This option does not impact the definition of the overall
grid bounds, but does restrict the data gridded to samples within particular times
of interest. If mode = 1, then start and end are simply start and end
times of good data in seconds. If mode = 2, then start and end are relative to
the time of the bottom return. In this case start is often negative so that the
grid shows data above the seafloor, and then down into the subsurface. Finally, if mode = 3,
then start and end are relative to the time corresponding to the sonar
depth.


 
EXAMPLES

Suppose that we have a Reson 7k format file (format 88) called 20040722_152111.s7k
that contains subbottom profiler data. Further suppose that we
have used mbextractsegy to extract the subbottom profiler data
into a segy format file called 20040722_152111.s7k.segy. MBextractsegy
also generates a "sinf" file containing statistics about the data in the segy
file. In this case, the contents of the sinf file 20040722_152111.s7k.segy.sinf are:

        SEGY Data File:      20040722_152111.s7k.segy



        File Header Info:


          Channels:                          1


          Auxiliary Channels:                0


          Sample Interval (usec):           64


          Number of Samples in Trace:     8330


          Trace length (sec):         0.533120


          Data Format:                IEEE 32 bit integer


          CDP Fold:                          0



        Data Totals:


          Number of Traces:               2527


          Min Max Delta:


            Shot number:                    56     2582     2527


            Shot trace:                      1        1        1


            RP number:                      56     2582     2527


            RP trace:                        1        1        1


            Delay (sec):              0.000000 0.000000 0.000000


            Range (m):                0.000000 0.000000 0.000000


            Receiver Elevation (m):   -224.030000 -2.860000 -221.170000


            Source Elevation (m):     -224.030000 -2.860000 -221.170000


            Source Depth (m):         2.860000 224.030000 -221.170000


            Receiver Water Depth (m): 51.510000 487.670000 -436.160000


            Source Water Depth (m):   51.510000 487.670000 -436.160000



        Navigation Totals:



          Start of Data:


            Time:  07 22 2004 15:20:37.029000  JD204


            Lon: -121.8573     Lat:   36.7755



          End of Data:


            Time:  07 22 2004 15:44:15.438000  JD204


            Lon: -121.8572     Lat:   36.7952



        Limits:


          Minimum Longitude:    -121.8574   Maximum Longitude:    -121.8572


          Minimum Latitude:       36.7755   Maximum Latitude:       36.7952


In order to generate a time vs. trace number grid of all traces in the
segy file, with a column for each trace and a row for each time sample,
one can simply invoke mbsegygrid without specifying a range of
desired traces or the desired time sweep. The program automatically sizes
the grid to include all traces and samples, taking into account trace start
delays if necessary. The root of the output grid filename is specified with
the -O option; if "SubbottomGrid" is specified then the output
grid will be named SubbottomGrid.grd. The command is:

        mbsegygrid -I 20040722_152111.s7k.segy                  -O SubbottomGrid




and the output to the terminal looks like:


        MBsegygrid Parameters:


        Input segy file:         20040722_152111.s7k.segy


        Output fileroot:         SubbottomGrid


        Input Parameters:


             trace mode:         1


             trace start:        56


             trace end:          2582


             channel start:      1


             channel end:        1


             trace decimation:   1


             time sweep:         0.533120 seconds


             time delay:         0.000000 seconds


             sample interval:    0.000064 seconds


             sample decimation:  1


             window mode:        0


             window start:       0.000000 seconds


             window end:         0.000000 seconds


             gain mode:          0


             gain:               0.000000


        Output Parameters:


             grid filename:      SubbottomGrid.grd


             x grid dimension:   2527


             y grid dimension:   8331


             grid xmin:          55.500000


             grid xmax:          2582.500000


             grid ymin:          -0.533152


             grid ymax:          0.000032


             NaN values used to flag regions with no data


        PROCESS read:0 position:0 rp:56 channel:1 2004/204 15:20:37.029 samples:8330 interval:64 usec minmax: 0.000000 2425.784912


        PROCESS read:25 position:25 rp:81 channel:1 2004/204 15:20:51.068 samples:8330 interval:64 usec minmax: 0.000000 3832.087402


        PROCESS read:50 position:50 rp:106 channel:1 2004/204 15:21:05.106 samples:8330 interval:64 usec minmax: 0.000000 1590.149658


        ....................


        PROCESS read:2475 position:2475 rp:2531 channel:1 2004/204 15:43:46.800 samples:8330 interval:64 usec minmax: 0.000000 991.466064


        PROCESS read:2500 position:2500 rp:2556 channel:1 2004/204 15:44:00.838 samples:8330 interval:64 usec minmax: 0.000000 838.286926


        PROCESS read:2525 position:2525 rp:2581 channel:1 2004/204 15:44:14.877 samples:8330 interval:64 usec minmax: 0.000000 985.870667



        --------------



        Plot generation shellscript <SubbottomGrid.grd.cmd> created.



        Instructions:


          Execute <SubbottomGrid.grd.cmd> to generate Postscript plot <SubbottomGrid.grd.ps>.


          Executing <SubbottomGrid.grd.cmd> also invokes gv to view the plot on the screen.



        --------------


The size of the the output grid can be checked using the GMT program
grdinfo. The results of the command:

        grdinfo SubbottomGrid.grd




look like:

        SubbottomGrid.grd: Title: Seismic Grid from 20040722_152111.s7k.segy


        SubbottomGrid.grd: Command:


        SubbottomGrid.grd: Remark:


                Projection: SeismicProfile


                Grid created by MBsegygrid


                MB-system Version 5.0.4


                Run by <caress> on <hess> at <Thu Sep 23 16:24:39 2004>


        SubbottomGrid.grd: Pixel node registration used


        SubbottomGrid.grd: grdfile format # 0


        SubbottomGrid.grd: x_min: 55.5 x_max: 2582.5 x_inc: 1 units: Trace Number nx: 2527


        SubbottomGrid.grd: y_min: -0.533152 y_max: 3.2e-05 y_inc: 6.4e-05 units: Time (seconds) ny: 8331


        SubbottomGrid.grd: z_min: 0 z_max: 2623.34 units: Trace Signal


        SubbottomGrid.grd: scale_factor: 1 add_offset: 0


MBsegygrid also uses the macro mbm_grdplot to generate a shellscript
called SubbottomGrid.grd.cmd that, when executed, will use GMT commands
to generate a Postscript plot of the grid.

In order to generate a grid of a subset of the subbottom data (for instance,
shots 1500 - 2000 and the time sweep from 0.25 to 0.35 seconds), use the -S
and -T options:

        mbsegygrid -I 20040722_152111.s7k.segy                  -O SubbottomGrid2               -S0/1500/2000 -T0.1/0.25




This time the grdinfo output looks like:

        SubbottomGrid2.grd: Title: Seismic Grid from 20040722_152111.s7k.segy


        SubbottomGrid2.grd: Command:


        SubbottomGrid2.grd: Remark:


                Projection: SeismicProfile


                Grid created by MBsegygrid


                MB-system Version 5.0.4


                Run by <caress> on <hess> at <Thu Sep 23 16:40:34 2004>


        SubbottomGrid2.grd: Pixel node registration used


        SubbottomGrid2.grd: grdfile format # 0


        SubbottomGrid2.grd: x_min: 1499.5 x_max: 2000.5 x_inc: 1 units: Trace Number nx: 501


        SubbottomGrid2.grd: y_min: -0.35 y_max: -0.249968 y_inc: 6.4e-05 units: Time (seconds) ny: 1563


        SubbottomGrid2.grd: z_min: 0 z_max: 558.42 units: Trace Signal


        SubbottomGrid2.grd: scale_factor: 1 add_offset: 0


 
SEE ALSO

mbsystem(1), mbm_grdplot(1), mbmosaic(1), mbm_grid(1)

 
BUGS

This tool isn't close to being complete...



 
Index

NAME

VERSION

SYNOPSIS

DESCRIPTION

AUTHORSHIP

OPTIONS

EXAMPLES

SEE ALSO

BUGS

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