Single Fold 3D Seismic: Optimum Tool for Deepwater Geohazards

Abstract

Abstract The extraction of two-millisecond sampled seismic traces from 3D exploration data produces high resolution 3D seismic cubes with frequencies of up to 200 Hz for the detailed mapping of geohazards in deep water. Seafloor morphology, bathymetry, subsurface faults, hydrate formations, amplitude anomalies, and sequence stratigraphy can be extracted and mapped in three dimensions to depths of 6,000 feet beneath the sea floor on computer workstations. A single fold 3D data set is used to illustrate the application of such subsets for mapping drilling hazards and the shallow sequence stratigraphy on the continental slope offshore Louisiana. Introduction The use of 3D seismic exploration data cubes for the assessment of deep water geohazards has become common practice over the past several years (Refs. 1-6). There are a number of reasons for this paradigm shift from high resolution 20 to 3D seismic which include: increased spatial coverage (horizontal resolution); manipulation on computer workstations to extract data as a function of both time (depth) and amplitude; and increasing availability of 3D seismic data. The marine geohazards industry has always benefited through the "hand-me-down" technology of its bigger brother, theoffshore seismic exploration companies, and this trickle-down effect is now providing a vast improvement in the assessment of drilling hazards for deep water operators. This paper discusses the benefits of a new methodology and an enhancement of standard 3D exploration data obtained by extracting near-traces to form a single-fold 3D high resolution seismic cube for the optimum assessment of deep watergeohazards. 20 versus 30 Standard practice for deep water geohazards surveys in the Gulf of Mexico, as prescribed by the U.S. Department of the Interior's Minerals Management Service in Notices to Lessees issued since 1972, has been to acquire seismic data using echo-sounders, subbottom profilers operating in the 3.5 kHz range, and single-fold analog or digital multifold "high resolution" seismic data over a 300 meter by 9bo meter (984 × 2953 feet) grid. This sparse coverage of 2D data, does notpermit migration processing and workstation manipulation for the extraction of seismic attributes. These are serious drawbacks when compared to what can be accomplished using standard 3D exploration data (Ref. 2) because seafloor and subsurface hazards may be relatively small and consequently overlooked when using wide-spaced 2D survey grids. For example, the precise lateral coverage of a seafloor fault escarpment and associated authigenic reef, which might harbor chemosynthetic communities, can only be interpolated from 20 data or from a costly deep-tow side scan sonar survey; and since such features must be avoided when drilling or laying anchors, their precise extent needs to be identified and mapped in order to maximize the seafloor area available for drilling and setting of production facilities. Furthermore, the wide acoustic dispersion angles inherent with surface-towed 2D seismic sources produce seismic returns from up-Slope zones within areas of steep bathymetry and diffraction signals from the numerous point sources frequently associated with rugged deep water topography.