Advanced marine seismic methods: Ocean-bottom and vertical cable analyses
Carlos Rodriguez Suarez
An overview of ocean-bottom and vertical-cable seismic acquisition and processing techniques, including applications and limitations, is presented.
Shear-wave velocities for ocean-bottom marine sediments were calculated using literature data and offshore-Brazil geotechnical data. Transmission and reflection coefficients for P- and S-wave mode conversion were obtained for sea bottom and Tertiary sediment interfaces. I conclude that most S-wave data recorded on ocean-bottom cables in Tertiary sections are related to upcoming Pto S- conversions at deeper interfaces.
A 2-D seismic line (Valhall Field, Norway) and a 3-D survey (Teal South, Gulf of Mexico), both acquired using four-component (4-C) receivers placed in ocean bottom cables, are processed.
In the 2-D line, P-P reflection data recorded by hydrophone and vertical geophone components did not provide interpretable images of the reservoir region. The P-SV reflections recorded by radial geophone component gives reasonable converted-wave (P-S) images.
In the 3-D survey, the best quality structural data were present on the P-P reflections recorded by the hydrophone, followed by P-P data recorded in the vertical geophone and then P-SV data recorded by radial geophone components. No significant differences were found among three methods used for cable deployment (trenched, sandbagged, and laid) in the P-P data recorded by the hydrophone; the taped system seems to give better results for P-SV data recorded on the radial component. Little compressional wave energy was found on the radial geophone component, but the vertical geophone component is contaminated with SV energy that correlated with the radial motion.
From the analysis of vertical cable geometry, it was found that using a single vertical cable attached to the sea floor, good fold and lateral coverage can be obtained with the use of a reasonable number of receivers per cable and densely spaced surface shot points. However, poor offset and azimuth distribution per bin occurs. When several cables are used fold, offset and azimuth distribution can be improved with optimised cable positioning. Expressions relating coverage with seismic acquisition parameters and water depth were empirically derived; these expressions may give preliminary parameters for vertical cable survey design.