SEGY Creation

Create shottables for each OBS. This is where it is handy to have a spreadsheet with all the details from the deployment / recovery. The easiest way to create shottables is with »pyShottab. Its a graphical program, which can handle file sizes of several gigabytes or more. Its also meant to be easy to modify, as it is written in python, and the source is open.

You will need the navigation files (any format), the shotlogs (preferably in the standard GPStar format), OBS deployment position, relative position of GPS antenna to airguns, times for: clock reset, deployment time, recovery time, drift calibration. Also need total clock drift, gun delay, and offset (initial) between ships (GPS) clock, and OBS clock. Very important to do this step properly.

With the shottables from the previious step, and the raw datafiles from the OBS, you can create SEGY files. This can be done using the windows program »dobs2segy2000. The resulting SEGY files are obviously not 'relocated'. The "Vista" SEGY format is little endian (intel/amd), IEEE (ascii) encoded header, and (short) integer formatted data.

OBS Relocation

Jack Wu Yue has made a matlab program for re-locating OBSs. It requires the shottable used to create the SEGY, a file for the 'direct wave pick', actual water depth of OBS, and information about water column velocity from CTD data. Detailed instructions are included inside the zip archive.

Ensure that the direct wave pick is performed on debiased SEGY data. A bias in the SEGY can throw off the picks.

This pick file can be created in SeisWide or Kingdom suite, and easily converted to the appropriate format. This will yield two positions of that the OBS could be relocated to, and the rms error of the result. It is up to the user to decide which position is more reasonable.

The picked arrival must be in terms of 'shotNumber traveltime(ms)', and be sorted from low to high shotnumbers. If its not sorted, it will cause this error: ??? Index exceeds matrix dimensions.

All files must be in a DOS text format (end of line) if using the Windows version of Matlab, and Unix end of line if using the Unix version.

A file containing "depth (m), velocity (m/s)" for the water column is required by this program. The data dumped from the CTD instrument provides information on temperature, salinity, and pressure. Both depth and acoustic velocity must be derived from these. This is easy to accomplish with win32 version of the Seasoft processing and conversion software available from seabird.com »http://www.seabird.com/software/FTPsiteinfo.htm#To%20download%20SBE%20Data%20Processing%20for%20Windows, or in a spreadsheet. Run "derive" from the GUI and specify that depth (salt water, m) and velocity (m/s) are to be included in the output file. This uses an ascii .cnv file as input and writes the output in the same format. It requires the CTD configuration is a .con file (if this is not available, one can be created in the configuration menu).

Once the relocated OBS position is known, new shottables and SEGY files can be created. Alternatively, the existing SEGY files can be modified to a relocated position. These relocated SEGY files are now ready to be processed.

Here is the original version of Jacks »Obsloc3 relocation program, and an modified version which runs on Matlab 6.5.0 in Unix and Widows »Obsloc3.1 (there were some issues with casting of variables to long integers, so these were changed to floats). Both are zipped archives.