Seismic data processing

Seismic data processing on board CCGS HUDSON  (June/July 2001)
Below is a description of the seismic data handling during CCGS HUDSON cruise 2001-28. This includes a description of
•the data download
•the preparation of navigation files
•the creation of SEG-Y files
•the data archival

Data download
After retrieval of the OBS the clock drift was determined by the OBS technicians. The hard disk with the data was then removed from the instrument and was brought to the drawing room on the bridge deck. During transport, the hard disk was protected by a plastic case used for videos.
In the drawing room, the hard disk was plugged into a OBS unit, which was powered by the "DC Power Supply 1610" (12 V). The serial interface of the OBS was connected to the serial port of a DELL Latitude laptop computer. The data port of the OBS was connected to a DAQ card-DIO 24, which was plugged into one of the PCMCIA slots of the computer. The other PCMCIA slot was used for a SCSI card, to which a CD writer (Yamaha CDR400tx) and 4mm DAT tape recorder (Hewlett Packard Sure Store DAT 8) were connected.
Before the download can start, some configuration is necessary with regard to the input/output port settings of the DAQ Card. The system will automatically assign these port numbers depending on the combination of PCMCIA cards that are installed in the notebook. These port settings are necessary as input for the program pc7up in order to establish proper communication. The necessary parameter can be found by clicking on
Start ® Settings ® Control Panel ® System ® Device Manager ® Data Acquisition Devices ® DAQCard-DIO-24 ® Properties à Resources
Then there will be a window showing the Input/Output Range with a setting of e.g. 1400 – 141F. The first number, in this case 1400, is the number wanted for the input parameter of pc7up. This number will not change as long as one does not change any PCMCIA card.
For the download a DOS window was opened on the laptop computer and mirror.exe was started to establish the communication between the computer and the OBS. In mirror, the settings  for the Dalhousie OBS are chosen (type the number from the menu and press return). The TTZ> prompt appears after pressing RETURN several times. Type dr+ to turn the hard disk on and type the command run. Error messages (Error reading EEPROM) can be ignored. (Some other commands that can be used here are dir to get a listing of the files on the hard disk and dr- to turn the hard disk off.) Now mirror can be quitted by pressing the F2 key. The data download with the program pc7up.exe has to be done in the same DOS window that was used for mirror.
pc7up downloads only one datafile. To download several files in one operation, the program doall.exe can be used in a batch file. Appropriate batch files were created (obs<line#><station#>.bat). The syntax is
doall batchfile first-datafile last-datafile	(e.g. doall obs217 0 237). This will download all datafiles from datafile.000 to datafile.237
The batch file is the name of a batch that actually calls the program pc7up. The batchfile (e.g. obs217.bat) contains the following two lines
pc7up –b 9600 –i 0x1400 datafile.%1
move datafile.%1 c:\data\dat217\datafile.%1
In the first line, only the –i option has to be adapted to the input/output range of the DAQ Card (see above). In the example above, the number was 1400 and that translates into –i 0x1400. A range of 1100 would require the option –i 0x1100. The second line moves the downloaded datafile into a subdirectory of the working directory, in the example to the directory dat13. Please note that you have to create this directory before the download process (mkdir name).

Navigation files
One essential input file for the creation of SGY files from the raw data is the table with the times and positions of the shots. These tables were calculated from shipboard navigation data provided by Paul Girouard. Two navigation files were provided for each day, one with the interpolated ship position for every minute and one with all raw navigation strings. These files are named
jd<XYZ>.raw   -raw data
jd<XYZ>.pos   -data for full minute
with XYZ being the Julian day number.
Run program get-head.f to extract the heading from the raw data file for each minute. Add the value for 0.00 UTC manually by taking the closest heading value from the raw file. The position file (jd*.nav) and the heading file are merged into one navigation file by means of the program merge-head-pos.f. All relevant navigation files for one shot line where then manually merged into one single file which then was manually edited to delete minute marks with no airgun shots.
The final shot table was created by  the program gun-corr.f . This program applies a gun delay and a correction for the offset between GPS antenna and airguns. The gun delay was 150 ms on line 2 and 60 ms on lines 1 and 3. The GPS antenna was located 58.1 m from the aft of the ship and 3.3m off the midships line towards starboard. The airguns were towed 100 ft (30.5 m) behind the ship. The depth of the airguns in the water was 20 ft. Final shot tables are named line[line#].nav.

Creation of SEG-Y files
1. Creating preliminary shot tables with the program maketab.for using the input file s[line#][station#].inp and the transformed navigation files line[line#].nav . Output files  s[line#][station#].sht
The .inp files contain the following 11 lines
line2.nav			ß Navigation file
0				ß Offset between gun and GPS (set to ZERO, because already corrected)
s201.sht			ß input shot table
44.186295,-62.780191,-61.0	ß OBS position (latitude,longitude) and central meridian
171,15,0,0			ß clock start (day,hour,minute,second)
177,23,41,0			ß clock check (day,hour,minute,second)
0.0				ß clock drift in ms (use ZERO, will be corrected in next step)
0				ß gun delay (set to ZERO, is already corrected for in shot table)
0				ß offset between GPS/OBS clock (set to ZERO, will be corrected in next step)
171,15,0,0			ß start time to extract shots (day,hour,minute,second)
177,23,41,0			ß end time to extract shots  (day,hour,minute,second)

2. The program maketab.for has difficulties with determining the correct sign for the ranges. The output shot tables should be checked manually to determine shot numbers with negative offsets. The program offset.for then corrects the signs of the ranges. The program prompts for input and output files and for the minimum and maximum shot point number with negative offsets. After this step, the input shot tables were deleted and the output shot tables were renamed to the name of the input shot table.
The shot tables have than to be corrected for the clock drift and any static time corrections. This is done by the program drift.for . The input files are the shot tables and files with the name mcs[line#][station#]tc.inp and s[line#][station#]tc.inp  (tc stands for time correction). These .inp files contain six lines of the following format
s201.sht			ß input shot table
s201tc.sht			ß output shot table
-5424				ß static clock correction (msec)
196,19,12,0			ß clock start (day,hour,minute,second)
204,8,41,0			ß clock check (day,hour,minute,second)
-16				ß clock drift (msec)
Note: In addition to the static shift shown in the drop sheets, an additional shift of +2 msec has to be applied.
The output shot tables show the actual OBS clock time of individual shots. The names are  s[line#][station#]tc.sht 
3. Now the SEGY files can be compiled using the program Dobs2Sgy2000.exe (please note bugs below). Click on the  program icon, click okay and browse to the directory with the raw datafiles. Click on the first datafile, move to the last datafile, hold the SHIFT key and click on the last datafile. Press OK. In the next field browse to the appropriate time corrected shot table s[line#][station#]tc.sht, and specify the output filename in the lowermost browse window. Record length is 60 seconds and choose 0 for all channels. The channel number will be added automatically to the SEGY file name. 
Filenames: s[line#][station#]ch[channel#].sgy
With	ch1	the hydrophone component
	ch2	the vertical geophone
	ch3/4	the horizontal geophone components
The SEGY files are NOT standard IBM format but with ASCII encoded header and short integer formatted data.

Dobs2Sgy2000 bugs
•The program does not apply corrections for clock drift, gun delay and clock offset – even if specified in the header of the shot table. è Individual shot times in the shot table have to be corrected accordingly.
•When choosing just channel 2 as opposed to all channels (0) for the output, the resulting file will be channel 1 and not channel 2. It has not been tested, if similar errors occur when other single channels are processed. 

Data archival
After downloading the data from the Dallhousie instruments on a Dell Latitude notebook, the raw data files were immediately saved on a CD ROM using the Yamaha CD writer and Adaptec Easy CD creator software. The data was kept on the hard disk of the laptop computer until an additional copy was saved on a 4mm DAT-tape, which was written with Adaptec EZ-SCSI 5.0 Software (data on tape is not compressed).
Data from the GSC instruments were downloaded in the Forward Lab on a PC. The data files were stored on the server Hud1/Science05from which they were copied on CD (Dave Heffler's PC for Line 2 OBS 1 (Dal instrument!) and OBS 6; all other instruments on a ship's PC with HP writer software). Data from the CD's were then copied to the Dell notebook from which they were saved to DAT tape (see above).

Comments
The measurement of the clock drift of the GSC instruments during the first deployment (Line 2) caused some difficulties. It was not recorded, at what time the clock calibration was carried out. For this reason, a time was assumed that postpones the recovery of the instrument by some 15 to 30 minutes. The clock calibration for GSC instrument 3 on position 7 (Line 2)revealed a value of -75.1440 ticks x 8 ms/tick= 601.152 ms. It is unclear at this moment of this is caused by a linear drift or if a clock jump occurred during the deployment or recovery. For this reason, the SEGY files for station 7 are not corrected for any clock drift. However, preliminary analysis of the direct wave for close offset shots  indicate that the clock jump probably occurred during the recovery of the instrument.
The same instrument had a large clock drift (-28.640 s) on the second line (Line 1 OBS 13). The clock jump probably occurred during the recovery since the direct wave is at its expected position.
OBS 4 on Line 1 did not record data (channels 3 and 4 have zero values, channel one has zero values with some occasional spikes, and channel 1 is oversaturated with no discernible signal in it).
Some hydrophones have recorded a 1 second pulse (Line 2 OBS 1 and 13, Line 1OBS 12).