segy — GMT 6.3.0 documentation

# segy

Plot a SEGY file in 2-D

## Synopsis

gmt segy SEGYfile -J parameters -R region -D deviation -F[color] -W [ -C clip ] [ -E error ] [ -I ] [ -L nsamp ] [ -M ntrace ] [ -N ] [ -Q<mode><value> ] [ -S header ] [ -T filename ] [ -U[stamp] ] [ -V[level] ] [ -X[a|c|f|r][xshift] ] [ -Y[a|c|f|r][yshift] ] [ -p flags ] [ -t transp ] [ --PAR=value ]

## Description

Reads a native (IEEE) format SEGY file and produces a plot of the seismic data. The imagemask operator is used so that the seismic data are plotted as a 1-bit deep bitmap in a single (user-specified) color or gray shade, with a transparent background. The bitmap resolution is taken from the current GMT defaults. The seismic traces may be plotted at their true locations using information in the trace headers (in which case order of the traces in the file is not significant). Standard GMT geometry routines are used so that in principle any map projection may be used, however it is likely that the geographic projections will lead to unexpected results. Beware also that some parameters have non-standard meanings.

Note that the order of operations before the seismic data are plotted is deviation*[clip]([bias]+[normalize](sample value)). Deviation determines how far in the plot coordinates a [normalized][biased][clipped] sample value of 1 plots from the trace location.

The SEGY file should be a disk image of the tape format (i.e., 3200 byte text header, which is ignored, 400 byte binary reel header, and 240 byte header for each trace) with samples as native real*4 (IEEE real on all the platforms to which I have access).

## Required Arguments

SEGYfile

  Seismic SEGY data set to be imaged.

-Jparameters

  Specify the projection. (See full description) (See cookbook summary) (See projections table).

-Rwest/east/south/north[/zmin/zmax][+r][+uunit]

> Specify the region of interest.

> The region may be specified in one of several ways:

> 1. -Rwest/east/south/north[+uunit]. This is the standard way to specify geographic regions when using map projections where meridians and parallels are rectilinear. The coordinates may be specified in decimal degrees or in [±]dd:mm[:ss.xxx][W|E|S|N] format. Optionally, append +uunit to specify a region in projected units (e.g., UTM meters) where west/east/south/north are Cartesian projected coordinates compatible with the chosen projection (-J) and unit is an allowable distance unit; we inversely project to determine the actual rectangular geographic region.

> 2. -Rwest/south/east/north+r. This form is useful for map projections that are oblique, making meridians and parallels poor choices for map boundaries. Here, we instead specify the lower left corner and upper right corner geographic coordinates, followed by the modifier +r. This form guarantees a rectangular map even though lines of equal longitude and latitude are not straight lines.

> 3. -Rg or -Rd. These forms can be used to quickly specify the global domain (0/360 for -Rg and -180/+180 for -Rd in longitude, with -90/+90 in latitude).

> 4. -Rcode1,code2,…[+e|r|Rincs]. This indirectly supplies the region by consulting the DCW (Digital Chart of the World) database and derives the bounding regions for one or more countries given by the codes. Simply append one or more comma-separated countries using the two-character ISO 3166-1 alpha-2 convention. To select a state within a country (if available), append .state, e.g, US.TX for Texas. To specify a whole continent, prepend = to any of the continent codes AF (Africa), AN (Antarctica), AS (Asia), EU (Europe), OC (Oceania), NA (North America), or SA (South America). The following modifiers can be appended:

>    > - +r to adjust the region boundaries to be multiples of the steps indicated by inc, xinc/yinc, or winc/einc/sinc/ninc [default is no adjustment]. For example, -RFR+r1 will select the national bounding box of France rounded to nearest integer degree.

>    > - +R to extend the region outward by adding the amounts specified by inc, xinc/yinc, or winc/einc/sinc/ninc [default is no extension].

>    > - +e to adjust the region boundaries to be multiples of the steps indicated by inc, xinc/yinc, or winc/einc/sinc/ninc, while ensuring that the bounding box extends by at least 0.25 times the increment [default is no adjustment].

> 5. -Rjustifylon0/lat0/nx/ny, where justify is a 2-character combination of L|C|R (for left, center, or right) and T|M|B (for top, middle, or bottom) (e.g., BL for lower left). The two character code justify indicates which point on a rectangular region region the lon0/lat0 coordinates refer to and the grid dimensions nx and ny are used with grid spacings given via -I to create the corresponding region. This method can be used when creating grids. For example, -RCM25/25/50/50 specifies a 50x50 grid centered on 25,25.

> 6. -Rgridfile. This will copy the domain settings found for the grid in specified file. Note that depending on the nature of the calling module, this mechanism will also set grid spacing and possibly the grid registration (see Grid registration: The -r option).

> 7. -Ra[uto] or -Re[xact]. Under modern mode, and for plotting modules only, you can automatically determine the region from the data used. You can either get the exact area using -Re [Default if no -R is given] or a slightly larger area sensibly rounded outwards to the next multiple of increments that depend on the data range using -Ra.

-Ddeviation

  gives the deviation in X units of the plot for 1.0 on the scaled trace.

-F[color]

  Fill trace (variable area, defaults to filling positive). Specify the color with which the imagemask is filled.

-W

  Draw wiggle trace.

You must specify at least one of -W and -F.

## Optional Arguments

-A

  Flip the default byte-swap state (default assumes data have a bigendian byte-order).

-Cclip

  Sample value at which to clip data (clipping is applied to both positive and negative values).

-Eerror

  Allow error difference between requested and actual trace locations when using -T option.

-I

  Fill negative rather than positive excursions.

-L

  Override number of samples per trace in reel header (program attempts to determine number of samples from each trace header if possible to allow for variable length traces).

-M

  Override number of traces specified in reel header. Program detects end of file (relatively) gracefully, but this parameter limits number of traces that the program attempts to read.

-N

  Normalize trace by dividing by rms amplitude over full trace length.

-Q<mode><value>
  Can be used to change 5 different settings depending on mode:

    -Qbbias to bias scaled traces (-Qb-0.1 subtracts 0.1 from values).

    -Qidpi sets the dots-per-inch resolution of the image [300].

    -Quredvel to apply reduction velocity (negative value removes reduction already present).

    -Qxmult to multiply trace locations by mult.

    -Qydy to override sample interval in SEGY reel header.

-Sheader

  Read trace locations from trace headers: header is either c for CDP, o for offset, or bnum to read a long starting at byte num in the header (first byte corresponds to num = 0). Default has location given by trace number.

-Tfilename

  Plot only traces whose location corresponds to a list given in filename. Order in which traces are listed is not significant - the entire space is checked for each trace.

-U[label|+c][+jjust][+odx[/dy]]

  Draw GMT time stamp logo on plot. (See full description) (See cookbook information).

-V[level]

  Select verbosity level [w]. (See full description) (See cookbook information).

-X[a|c|f|r][xshift]

  Shift plot origin. (See full description) (See cookbook information).

-Y[a|c|f|r][yshift]

  Shift plot origin. (See full description) (See cookbook information).

-Z

  Do not plot traces with zero rms amplitude.

-p[x|y|z]azim[/elev[/zlevel]][+wlon0/lat0[/z0]][+vx0/y0] (more …)

  Select perspective view.

-ttransp[/transp2] (more …)

  Set transparency level(s) in percent.

-^ or just -

  Print a short message about the syntax of the command, then exit (NOTE: on Windows just use -).

-+ or just +

  Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exit.

-? or no arguments

  Print a complete usage (help) message, including the explanation of all options, then exit.

--PAR=value

  Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

## Examples

Note: Since many GMT plot examples are very short (i.e., one module call between the gmt begin and gmt end commands), we will often present them using the quick modern mode GMT Modern Mode One-line Commands syntax, which simplifies such short scripts.

To plot the SEGY file wa1.segy with normalized traces plotted at true offset locations, clipped at ±3 and with wiggle trace and positive variable area shading in black, use

>     gmt segy wa1.segy -JX5i/-5i -R0/100/0/10 -D1 -C3 -N -So -W -Fblack -pdf segy

To plot the SEGY file wa1.segy with traces plotted at true cdp*0.1, clipped at ±3, with bias -1 and negative variable area shaded red, use

>     gmt segy wa1.segy -JX5i/-5i -R0/100/0/10 -D1 -C3 -Sc -Qx0.1 -Fred -Qb-1 -I -pdf segy

## See Also

gmt, segyz, segy2grd