U.S. Geological Survey
Open-File Report 2005-1411
High-resolution multichannel seismic-reflection data acquired in the northern Gulf of Mexico, 2003
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2003 USGS Research Cruise Summary
In early 2003, the JIP site selection committee chose two targets for its Gulf of Mexico hydrate drilling program; primarily based on interpretation of exploration industry 3D seismic data sets. These sites are in lease blocks Keathley Canyon 195 and Atwater Valley 14. Researchers from the Coastal and Marine Program of the USGS conducted a 14-day cruise aboard the Research Vessel Gyre to these two sites to help guide the planned drilling program. (Figure 1) The DOE provided funding for this cruise to acquire seismic reflection images of the proposed drill sites at higher resolution than available from existing industry data. A detailed description of operations, equipment, data acquisition equipment, navigation and shipboard data processing for this cruise (USGS cruise ID G1-03-GM) can be found in the on-line report (Hutchinson and Hart, 2003) at: http://pubs.usgs.gov/of/2003/of03-474/index.htm
The two locations surveyed during the cruise are each on the northern Gulf mid-slope at approximately 1300 m water depth, but in very different geologic environments. Keathley Canyon 195 is near the junction of four mini-basins in a region of significant salt tectonics. Gas hydrate is speculated to exist near the base of the hydrate stability zone here, because a Bottom Simulating Reflection (BSR) is observed on seismic profiles. Atwater Valley 14 is on the floor of the Mississippi Canyon in an area with several prominent seafloor mounds that appear to be related to the venting of brines and natural gas that may have created deposits of gas hydrate. 1033 km of high-resolution 2D multichannel seismic reflection data were acquired during the cruise; 779 km at Keathley Canyon and 254 km at Atwater Valley. Trackline maps are shown in Figure 2 and Figure 3. Data quality was excellent with very good imaging to subbottom depths of greater than 1500 m.
The seismic data acquisition system consisted of a dual chamber GI (generator-injector) air gun, a 24-channel 240-m analog hydrophone streamer, and a PC-based digital recording system. For most of the program the GI gun used had a 13 cubic inch generator chamber and a 13 cubic inch injector chamber and was fired at 20-m shot spacing. Four lines in the middle of the program (lines KC-60, GB-61, GB-62 and GB-63) used 24 cubic inch chambers fired at 30-m shot spacing. Testing at the beginning of the program and one line included with this report, line KC-T1, used a 15 cubic inch water gun source, fired at 10-m shot spacing.
Shot records, 5 seconds in length, were digitized at 0.5 ms sample interval and recorded on 8mm tapes in SEG-D format. A differential GPS system that utilized wide area augmentation system corrections (WAAS) collected UTM coordinates that were entered into the seismic data trace headers during shipboard processing. The Keathley Canyon portion of the program was in UTM zone 15R and the Atwater Valley portion was in UTM Zone 16R. Preliminary stacks were made of all of the multi-channel seismic lines using a shipboard USGS data processing system to help in making mid-cruise adjustments to the planned program and for distribution to the JIP working group immediately after the cruise.
Multi-channel Seismic Data Processing
Subsequent to the cruise, all of the lines were reprocessed at the USGS data processing facility in Denver, Colorado. A complete set of final stacks and migrations was created and is available for download from the SEG-Y and Image Data section of this report. The data processing sequence for all lines except the water gun test line KCt2, is listed in the table below. The processing for the water gun line is fully discussed in Lee et al, 2002 at: http://pubs.usgs.gov/bul/b2181/
TABLE 1: Processing Sequence for Multichannel GI Gun Seismic Data
1. SEG-D input and resample to 1.0 millisecond
2. Geometry assignment and sort to CDP at 5 m spacing
3. Wavelet deconvolution (Derived wavelet using 3 shot gathers and 16-18 ms window)
4. Predictive deconvolution (1st-zero crossing, operator length = 60 ms and a single design window)
5. Trace edit
6. Single window balance
7. Band-pass filtering
8. Common-Offset DMO
9. Velocity analysis and NMO
10. Stack (12-fold where 10-m shot spacing, 6-fold where 20-m shot spacing, 4-fold where 30-m shot spacing)
11. True amplitude gain application on stacked data.
12. Post-stack 2nd-zero crossing deconvolution (Operator length = 30 msec and single design window)
13. Migration velocity analysis based on the stacking velocity: Iteratively Adjusted
15. Post-stack 40-400 Hz band-pass filtering
16. Water-bottom mute
SEG-Y and Image Data
Seismic reflection profiles and associated data are provided with this report. The data files are written in SEG-Y exchange format, (Barry and others, 1975).
Each profile is available as a digital data file (SEG-Y format) of the stacked data and the post-stack migrated data, and as a JPEG image file of the migrated data.
Stacked multichannel air gun profiles: the stacked SEG-Y data are written in 32-bit IBM floating point format at a 1.0 millisecond sample interval; record length is 4.0 seconds including up to 1.0 seconds of deep-water recording delay. The EBCDIC header for each file contains additional descriptive information as shown in table 2.
TABLE 2: SEG-Y EBCDIC Header for 2003 Multi-channel seismic data
C 1 U. S. GEOLOGICAL SURVEY COASTAL AND MARINE GEOLOGY TEAM
C 2 SURVEY_ID: G1-03-GM YEAR: 2003
C 3 AREA: NORTHERN GULF OF MEXICO VESSEL: R/V GYRE
C 5 ACQUISITION:15 CU IN WATER GUN ; 10 METER SHOT INTERVAL
C 6 24 CHANNEL 240-METER SOLID CORE HYDROPHONE STREAMER
C 7 0.5 MS RECORDING SAMPLE INTERVAL, 3.0 SEC RECORD LENGTH
C 8 RECORDED IN SEG-D FORMAT
C10 PROCESSING: RESAMPLE TO 1.0 MS, GEOMETRY ASSIGNMET, CDP SORT
C11 WAVELET AND PREDICTIVE DECONVOLUTION, TRACE EDIT AND BALANCE
C12 BAND-PASS FILTER, DMO, VELOCITY ANALYSIS, NMO AND 12-FOLD STACK
C13 TRUE AMPLITUDE GAIN RECOVERY, POST-STACK DECON, 40-400HZ FILTER
C16 OUTPUT: 32-BIT IBM FLOATING POINT SEG-Y
C17 DATA LENGTH 3.0 SEC
C18 RECORD LENGTH 4.0 SEC ON MOST LINES TO INCLUDE DEEP-WATER DELAY
C20 CDP NUMBER STORED IN BYTES 21-24
C21 CDP LOCATIONS IN UTM COORDINATES STORED IN TRACE HEADERS
C22 IN THREE SEPARATE LOCATIONS
C23 CDP-X IN BYTES 73-76, 81-84, AND 181-184
C24 CDP-Y IN BYTES 77-80, 85-88, AND 185-188
C34 FOR FURTHER INFORMATION CONCERNING THIS DATASET, CONTACT:
C35 PATRICK HART (650) 329-5160 HART@USGS.GOV
C36 U. S. GEOLOGICAL SURVEY, M/S 999
C37 345 MIDDLEFIELD RD., MENLO PARK, CALIFORNIA 94025
CAUTION: SEG-Y files may be very large, and therefore long to download.
ASCII files of CDP locations, in UTM coordinates for the multichannel profiles are listed in the following files.
The datum is WGS84 and the UTM zone is 15R for the Keathley Canyon lines (KC01 - KC60), the test line (KCt2), and the Garden Banks lines (GB61 - GB63). The UTM zone is 16R for the Atwater Valley line (AV64-AV99) and the Mississippi Canyon lines (M100 and M101).
No software is provided for preview or display of the SEG-Y, image or navigational data. SEG-Y data can be displayed, printed or further processed using a variety of seismic processing programs that run on UNIX, Windows and Macintosh systems. A public-domain seismic processing package, SU, is available via the Internet at: http://www.cwp.mines.edu/cwpcodes/index.html.
JPEG images of the seismic lines can be opened in any of numerous graphics packages.
Navigation files can be plotted using commercial GIS software such as ARC/INFO. The Generic Mapping Tools (GMT), a public-domain mapping package for visualizing geographic vector data on Unix systems, was used to create the trackline figures, and is available via the Internet at: http://gmt.soest.hawaii.edu/
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