APPENDIX 2

DMS-2000 Recording System

This Appendix presents notes on DMS-2000 seismic recording system on the R/V Maurice Ewing and the SEG-D data format in which we recorded MCS data on the Ewing.

DMS-2000 Recording System

The DMS-2000 records demultiplexed seismic data in a standard SEG-D format (SEG Subcommittee on Field Tape Standards, 1994), using a 20-bit (2.5 bytes per sample) recording format. Data logged from peripheral systems (bird controller, gun controller, navigation, etc.) are not written to the SEG-D header blocks, but are rather recorded in a special auxiliary data trace, referred to in the Digicon documentation as SEGD/Trace0 (Rtrace zeroS). The SEG-D General Header (the header that accompanies each shot record), contains the shot number and shot time to the nearest second.

For EW94-15, the DMS-2000 was configured to record 180 data channels from 46 four-channel digitizing modules (or "cans"). Only 40 of these cans were actually in use in the streamer, resulting in 160 active channels, numbered 1-160. Channel 1 is the far channel, 160 the near channel. Two channels on-board ship (173 and 174) were used to record the sonobuoy receivers. The remaining channels (161-172, 175-180) are physically on tape, but contain no data. Trace0 consists of a 4051-byte data trace preceding the demultiplexed seismic data, with a channel designation -1. It is flagged by the system as an auxiliary trace.

The DMS-2000 also writes a secondary data tape, referred to as the NAVLOG tape. This is recorded in format identical to the SEG-D data tape, but contains only three channels - Trace0 and two data channels. For EW94-15 these were channels 10 and 150, a far-trace and a relative near-trace channel. (Channels to be forked to the NAVLOG tape are selected in the CEO SOL/EOL Auto-Start-End menu of the DMS-2000 system.)

SEG-D Format

The basic layout of the SEG-D tape is:



General Header
IBG
Trace0 record
IBG
Data Channel 1
IBG
Data Channel 2
IBG
:
:
Data Channel 184
IBG
EOF

General Header
IBG
Trace0 record
IBG
Data Channel 1
:
:
Data Channel 184
IBG
EOF



General Header
IBG
Trace0 record
IBG
Data Channel 1
:
:
Data Channel 184
EOF

EOT


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SIZE IN BYTES

96 (three 32-byte blocks)

4051 (variable-length sections)

RECL*2.5 + 20-byte trace header

RECL*2.5 + 20-byte trace header
:
:
:
RECL*2.5 + 20-byte trace header



96 (three 32-byte blocks)

4051 (variable-length sections)

RECL*2.5 + 20-byte trace header
:
:
RECL*2.5 + 20-byte trace header





96 (three 32-byte blocks)

4051 (variable-length sections)

RECL*2.5 + 20-byte trace header
:
:
RECL*2.5 + 20-byte trace header



General Header

The General Header consists of three 32-byte blocks, and contains the following relevant fields:

Byte Number
Data
1-2
3-4
5-6
7-8
9-10
11
12-13
14
15
16
FFID
SEG-D format code (8015 - 20-bit binary demux)
not used
shotpoint
?? (791)
year (94)
day of year
shot hour (GMT)
shot minute (GMT)
shot second (GMT)

These values are recorded in Binary-Coded Decimal (BCD) notation, and are therefore recognizable in a hex dump. The day/hour/minute/second fields above are standard SEG-Y, but the shotpoint location is not defined by the standard. Software that reads SEG-D, such as ProMax or DISCO, will therefore probably read the shot times, but not the shotpoints. The location of the shotpoint field must be explicitly defined in the input module. For example, in ProMax, it is necessary to select "Remap SEGD main header values" in the SEG-D input module, and indicate something like:

where 6.5 indicates the starting location in the header record for the 4-digit BCD value for shotpoint, which is mapped onto the header value SOURCE. The odd value (6.5) for starting location is a peculiarity of ProMax. ProMax SEGD input also converts the shot time to absolute seconds stored in a header word TIM_SHOT. You may choose to explicitly identify the time by hour, minute, and second, with an Input/override entry like:

Trace0

Trace0 contains a wealth of information about the system (e.g., seismic bird information, gun shooting parameters), much of which is also available from other shipboard logging systems. Trace0 data is not accessible from conventional SEGD input software; for access to it, one is advised to use John Diebold's segd_dump program, which reads and reformats to disk various selected portions of Trace0 (Internet address: johnd@lamont.ldgo.columbia.edu). Of course, dumping the Trace0 records from the NAVLOG tapes is much more efficient than reading the data tapes.

A note regarding the Trace0 positional information.

Navigational data is passed from the navigational computer aboard the Ewing ("Moray") to the DMS-2000 for inclusion in Trace0, where it is stored in Section 11, the "Magnavox Nav Data Block". (Digicon uses Magnavox navigational systems on their vessels). This 211-byte navblock contains latitude and longitude fixes for each shot, which can be read with segd_dump. These positions, however, are "real-time" solutions that have not been adjusted or corrected by the post-processing that is done daily on-board the vessel. The 20-byte individual Demux Trace Headers which precede each data trace have little information of use to the processor, nor does the SEG-D standard suggest that they should.

Tape Errors

The recurring, persistent 3480 tape drive errors reported on previous Ewing cruises were resolved prior to EW94-15. However, a few SQRT errors did occur, resulting in two shot records being concatenated into one file, with the second record overwriting some portion of the previous one. These files have a variable number of traces, in excess of the normal 180 and were noted in observer logs when possible. The processing flow must be able to read these abnormal files, and pad the channels to create normal ensembles.

CEO/CSRU and TAGS Logs

The DMS-2000 system also maintains logs and error files of the recording system (CSRU) and the air gun control system (TAGS). The CSRU files are named 'line_id'.rul and 'line_id'.rue for the log and error files, respectively. The corresponding TAGS files are 'line_id'.tal and 'line_id'.tae. All of the information in the TAGS error file is repeated in the TAGS log file, and the CSRU error and log files are complementary. These files are written to DOS floppy disks with the CEO File Handler.

Precise Shot Timing And Location

Processed GPS shot locations and shot-times (precise to 1 ms from the GPS receiver) are available in a separate file, but are not contained in the SEG-D records.