--> ABSTRACT: Considerations and Pitfalls of High-Resolution Seismic Acquisition, Processing, and Data Analysis: Gulf of Mexico, Offshore Louisiana, by Susan E. Nissen, Janet M. Combes, H. Joseph Mckinley, Anthony G. Nekut, William B. Pramik; #91020 (1995).

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Considerations and Pitfalls of High-Resolution Seismic Acquisition, Processing, and Data Analysis: Gulf of Mexico, Offshore Louisiana

Susan E. Nissen, Janet M. Combes, H. Joseph Mckinley, Anthony G. Nekut, William B. Pramik

As part of an industry consortium project studying Pleistocene sedimentation on the outer continental shelf and upper slope of the Gulf of Mexico, 684 km of shallow, high-resolution, 2-D seismic data were collected offshore Louisiana. The acquisition parameters for the seismic survey were designed to image stratigraphic features at scales comparable to those seen in core and outcrop (approximately 1/2 meter to 10's of meters). Digital data were acquired to 600 msec below the seafloor at a sample interval of 0.25 msec, using an anti-alias filter of 1440 Hz. The receiver was a 100 m-long, 16-channel streamer, with a group interval of 6.25 m, and the source was an 8 cu. in. sleeve gun array (4 × 2 cu. in.), with a shot interval of 3.125 m.

Acquisition design parameters specified a source and receiver depth of 1 m to attempt to optimize temporal resolution. Subsequent analyses of arrival times for direct waves, the primary and multiple sea floor reflections and the associated ghost arrivals indicated that the source and receiver depths were different than specified by the design parameters. These analyses suggest that the streamer was tilt (near group) to 3 m (far group) and that the source depth varied from less than 1 m to 4 m. This led to several complications in the processing of the data, particularly with respect to determination of stacking velocities. The incorrect acquisition geometry required anomalously slow velocities (between 1250 and 1350 m/s) to stack the sea floor and sub-bottom reflections for most CDP g thers. Conversely, ghosted arrivals stacked at abnormally high velocities (up to 2000 m/s). Initially, the higher stacking velocities were used in processing, causing the primary reflections to be attenuated and the ghost arrivals to be enhanced. The net result of this is that the events on the stacked sections are phase reversed and slightly delayed from what would normally be expected. However, the quality of the processed data was still excellent.

Frequencies of at least 400 Hz are needed to image (with a vertical resolution of 1.5-2.2 m) the detailed internal geometry of the shelf-edge deltas targeted by this study. Analyses of amplitude spectra and filter panels of the stacked seismic data show that frequencies up to 1000-1200 Hz were actually recovered from the subsurface to approximately 150 msec (112.5 m) below the seafloor, permitting vertical resolution of beds as thin as 0.4 m. Within the deltaic clinoforms, which dip at angles up to 8°, frequencies above 850 Hz may be spatially aliased at the 3.125 m CDP spacing of this survey. In order to avoid spatial aliasing of the 1200 Hz events at a dip angle of 8°, a maximum CDP spacing of 2.2 m is required.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995