segy2grd - Converting SEGY data to a GMT grid
segy2grd segyfile -Ggrdfile -Iincrement -Rregion [ -A[n|z] ] [ -Dxname/yname/zname/scale/offset/title/remark ] [ -L[nsamp] ] [ -M[ntraces] ] [ -Nnodata ] [ -Q<mode><value> ] [ -S[header] ] [ -V[level] ] [ -bibinary ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated arguments.
segy2grd reads an IEEE SEGY file and creates a binary grid file. Either a simple mapping (equivalent to xyz2grd -Z) or a more complicated averaging where a particular grid cell includes values from more than one sample in the SEGY file can be done. segy2grd will report if some of the nodes are not filled in with data. Such unconstrained nodes are set to a value specified by the user [Default is NaN]. Nodes with more than one value will be set to the average value.
segyfile is an IEEE floating point SEGY file. Traces are all assumed to start at 0 time/depth.
- grdfile is the name of the binary output grid file.
- x_inc [and optionally y_inc] is the grid spacing. Append m to indicate minutes or s to indicate seconds.
- west, east, south, and north specify the region of interest, and you may specify them in decimal degrees or in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left and upper right map coordinates are given instead of w/e/s/n. The two shorthands -Rg and -Rd stand for global domain (0/360 and -180/+180 in longitude respectively, with -90/+90 in latitude). Alternatively for grid creation, give Rcodelon/lat/nx/ny, where code 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. This indicates which point on a rectangular region the lon/lat coordinate refers to, and the grid dimensions nx and ny with grid spacings via -I is used to create the corresponding region. Alternatively, specify the name of an existing grid file and the -R settings (and grid spacing, if applicable) are copied from the grid. Using -Runit expects projected (Cartesian) coordinates compatible with chosen -J and we inversely project to determine actual rectangular geographic region. For perspective view (-p), optionally append /zmin/zmax. In case of perspective view (-p), a z-range (zmin, zmax) can be appended to indicate the third dimension. This needs to be done only when using the -Jz option, not when using only the -p option. In the latter case a perspective view of the plane is plotted, with no third dimension.
- Add up multiple values that belong to the same node (same as -Az). Append n to simply count the number of data points that were assigned to each node. [Default (no -A option) will calculate mean value]. Not used for simple mapping.
- Give values for xname, yname, zname (give the names of those variables and in square bracket their units, e.g., “distance [km]”), scale (to multiply grid values after read [normally 1]), offset (to add to grid after scaling [normally 0]), invalid (a value to represent missing data [NaN]), title (anything you like), and remark (anything you like). To leave some of these values untouched, leave field blank. Empty fields in the end may be skipped. Alternatively, to allow “/” to be part of one of the values, use any non-alphanumeric character (and not the equal sign) as separator by both starting and ending with it. For example: -D:xname:yname:zname:scale:offset:invalid:title:remark: Use quotes to group texts with more than one word. Note that for geographic grids (-fg) xname and yname are set automatically.
- Let nsamp override number of samples in each trace.
- Fix number of traces to read in. Default tries to read 10000 traces. -M0 will read number in binary header, -Mntraces will attempt to read only n traces.
- No data. Set nodes with no input sample to this value [Default is NaN].
- Can be used to change two different settings depending on mode:
-Qxx-scale applies scalar x-scale to coordinates in trace header to match the coordinates specified in -R.
-Qys_int specifies sample interval as s_int if incorrect in the SEGY file.
- Set variable spacing; header is c for cdp, o for offset, or bnumber for 4-byte float starting at byte number. If -S not set, assumes even spacing of samples at the x_inc, y_inc supplied with -I.
- -V[level] (more ...)
- Select verbosity level [c].
- -r (more ...)
- Set pixel node registration [gridline].
- -^ or just -
- Print a short message about the syntax of the command, then exits (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 exits.
- -? or no arguments
- Print a complete usage (help) message, including the explanation of all options, then exits.
To create a grid file from an even spaced SEGY file test.segy, try
gmt segy2grd test.segy -I0.1/0.1 -Gtest.nc -R198/208/18/25 -V
Note that this will read in 18-25s (or km) on each trace, but the first trace will be assumed to be at X=198
To create a grid file from the SEGY file test.segy, locating traces according to the CDP number, where there are 10 CDPs per km and the sample interval is 0.1, try
gmt segy2grd test.segy -Gtest.nc -R0/100/0/10 -I0.5/0.2 -V -Qx0.1 -Qy0.1
Because the grid interval is larger than the SEGY file sampling, the individual samples will be averaged in bins