Using Multiple Elimination to Image Geologic Structures beneath the

1. Erika Geresi Using Multiple Elimination to Image Geologic Structures beneath the Gas Hydrate Stability Zone in the Northern Gulf of Mexico By Erika J. Geresi, Deborah R. Hutchinson, Patrick E. Hart, Thomas M. McGee

2. Overview • Geographic Distribution • Drilling Hazards • Cruise Instrumentation • Processing Flow • Results • Conclusions

3. Outline of the Area • Gas hydrates are well known for their capacity to change the physical properties of near-surface sediments and have been linked to massive slope failures on continental margins. • As drilling in the Gulf of Mexico has progressed from shallow-water shelf depths (<200m) to deep-water slope depths (>1000m), wells now penetrate the gas hydrate stability zone. In order to understand the consequences of drilling through the hydrate zone, it is imperative to understand the geological setting in which these hydrates exist and to be able to estimate the distribution and concentration of gas hydrate deposits especially in the subsurface.

4. Cruise Overview • In May 2003, the USGS conducted a 14-day cruise aboard R/V Gyre to collect high-resolution multichannel seismic reflection data and develop the geologic framework around two deep-water sites (near lease blocks Atwater Valley 14 and Keathley Canyon 195). Both may be drilled by industry in spring, 2004, to study gas hydrates in the Gulf of Mexico. • The multichannel data from Atwater Valley, in 1300-m water, show many shallow unconformities, deeper dipping reflections, numerous diffractions, and substantial amplitude variations.

5. Geographic Distribution

6. Drilling Hazards

7. Cruise Instrumentation • Primary navigation: Differential Global Positioning System, YoNav software; • Multichannel Seismic: 24-channel high-resolution seismic reflection profiling, 13/13 cu. in³ G.I.Gun; • Bathymetry: Knudsen bathymetry system; • Data Processing: Promax, Datatrend and Lookout 22

8. Processing Flow • Reformating, • Editing, amplitude balancing, • Detrending, • Deep-water delay correction, • FK Filtering, • Spiking Deconvolution, • NMO Correction, • Stacking, • Spherical Divergence Correction, • Bandpass Filtering, • Multiple Suppression.

12. Conclusions • The re-processing and the multiple elimination improved the strength and continuity of the shallow and the deeper reflections. • The multiple suppression helped to get rid of the multiple and gave a better interpretable image. • Results from this reprocessing of selected profiles around one of the vents/sea-floor mounds shows faulting and pathways that more clearly demonstrate the relations between the shallow and deeper structures.

13. Thanks to • Deborah Hutchinson, USGS, 384 Woods Hole Road, Woods Hole, MA 02543, • Patrick Hart, USGS, MS 999, 345 Middlefield Rd, Menlo Park, CA 94025, • Tom McGee, MMRI/CMRET, University of Mississippi, 220 Old Chemistry Building, University, MS 38677, • Bob Woolsey, MMRI/CMRET, University of Mississippi, 220 Old Chemistry Building, University, MS 38677.