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USGS Open-File Report 03-474, G1-03-GM, Gas Hydrates Cruise Report, R/V Gyre, 1-14 May 2003, Northern Gulf of Mexico

Data Processing

Seismic Processing Lab

The seismic processing lab, contained two Unix-based seismic processing computers, two seismic processing software packages (ProMAX 2D and FOCUS), and a DVD writer. Processing was split between the two systems. The bulk of the geometry merging was done on ProMAX, and the bulk of data processing with FOCUS. After SEG-D input, all data were stored on the dedicated Snap Server until archived to DVD. A 12 inch OYO Geospace thermal plotter was used to plot preliminary and final sections at a scale of 5 inches/second. The ship-board processing sequence follows (steps in bold indicate where data were archived):

Using Promax:

  • SEG-D demultiplexed format input
    (Multiple lines per tape; shot coordinates read from SEG-D headers)
  • SEG-Y output of raw shot records
    (One line per file; 0.5 ms sample interval; archived on DVD)
  • Geometry input
    (CDP and Receiver UTM coordinates written into trace headers)
  • CDP sort
  • SEG-Y output of CDP sorted records
    (1.0 ms sample interval; archived on DVD)

Using FOCUS

  • Edit noise spikes and noisy channels
  • Whole-trace amplitude balance
  • Deep-water delay correction
  • FK filter
  • Spiking deconvolution
  • NMO correction
  • Stack
  • Spherical divergence correction
  • 60-360 Hz bandpass filter
  • SEG-Y output of stacked profiles
    (1.0 ms sample interval; archived on DVD)

Several conventions were followed in the processing: original ffid's were preserved, but all shots were renumbered starting at 1 at the beginning of every line. All geometry was calculated in absolute coordinates, i.e., UTM positions calculated from the latitude/longitude positions supplied by YoNav. Keathley Canyon was in UTM zone 15; The Atwater Valley region was in UTM zone 16. The initial geometry definition involved extracting the position information from the headers in the SEG-D field records, calculating a corrected UTM x and y position for the actual shot location (using a combination of MATLAB and ArcGIS), then using ProMAX to do full geometry to accurately locate all CDP and receiver locations. The SEG-Y data with geometry were read back into FOCUS for processing through stack. Although data were recorded to 5 s, stacks were only done to 4.5 s. Stacking was not sensitive to velocities because of the short streamer length (240 m) and large water depths (1300 m). A generic velocity was used that consisted of 1500 m/s rms to the sea floor, then increasing linearly to 2000 m/s at 3 s twtt and 2500 m/s at 4.5 s twtt. For the multichannel data with the 13/13 GI gun, data generally were stacked to 6 fold at 5-m cdp spacing. Final navigation was extracted from the cdp x and y locations (SEG-Y trace header locations 181-184 and 185-188) and converted back to latitude and longitude values. Ray Sliter and Erika Geresi ran the seismic processing lab. Photographs of the seismic processing lab are shown in Appendix 6 (Photos 26-27).

GIS lab

A separate GIS lab provided GIS support for the cruise. A desk-top computer running ARCGIS and ARCView maintained all the master files and master calculations for cruise data. This computer was used to define all track lines, perform mid-cruise planning adjustments, and calculate corrected shot positions. Initial (i.e., planned) track lines were exported to YoNav in both the main lab and the bridge for underway navigation. Metadata for all new shape files and tabulated information were also created and archived here. Seth Ackerman and Jen Dougherty oversaw the GIS lab. Photographs of the GIS Lab are shown in Appendix 6 (Photos 28-29).


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Title Page / Abstract / Introduction / Acknowledgments / Geologic Settings / Cruise Objectives / Cruise Strategy / Instrumentation / Data Processing / Data Handling / Marine Mammal Mitigation / Operational Summary / Meeting Objectives Success / Summary / References / Appendices
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