Hibernia Swath Bathymetry Field Survey

Abstract

ABSTRACT As a result of reservoir and drilling considerations, the decision was made to investigate alternate gravity base structure (GBS) sites for Hibernia. Precise bathymetric data were needed to establish whether a suitably level and smooth site in the proper water depth could be found for the GBS. A swath bathymetry survey was organized and mobilized on a vessel of opportunity over a very short time frame. Besides swath, the program included a tidal measurement program and simultaneous acquisition of sidescan sonar and subbottom profiler data. Survey results, including an evacuation of the swath measurement accuracy, are discussed. A/though not an initial intent of the survey, the swath data in conjunction with sidescan sonar and subbottom profiling provided insight into seabed processes and geology of the Hibernia area improvements to the Swath system as a result of the survey are also discussed. BACKGROUND - SWATH DEVELOPMENT The most common method to measure water depth is to use an echo sounder. Echo sounders are typically mounted on the hull of a vessel and consist of a single piezoelectric transducer that focuses a narrow acoustic pulse (beam between 5 and 20 degrees) onto the seafloor The narrower the beam, the more "point specific" the measurement of water depth and the less likelihood of introducing error from out of plane reflections. Measurement error can be caused by vessel motion, tides, and changes in signal velocity in the water column. These errors can be reduced by introducing motion compensators, monitoring tidal functions, and by frequently measuring sound velocity in the water column. When a bathymetric survey is performed under optimal conditions an accuracy of 0.5 percent of water depth is typically quoted. As a rule, however, conventional echo sounder bathymetric surveys are insensitive in detecting irregular topography Water depths between tracklines are not measured so bathymetric contour maps are a result of interpolation between the tracklines. Therefore, any seabed roughness between tracklines can only be qualitatively inferred from other data such as sidescan sonar images. To improve bathyrnetricdetail with conventional echo sounder surveys, tighter trackline spacing is typically specified. However, even with a tight grid spacing the area between tracklines is not covered and subtle seabed features between tracklines can go undetected. In the past fifteen years, bathymetric surveying has evolved from single beam systems into swath methods where numerous measurements are taken during a single pass across a site. One of the first swath or "sweep" systems consisted of multiple vertical looking transducers (echo sounder)mounted on a boom from a vessel Swath then evolved into systems with a transducer array consolidated onto a single mount. By arranging the beams in a fan pattern, bottom coverage and operating water depth were increased. Numerous swath systems are on the market today including split beam sidescan sonar and multibeam. For more background on these systems, the reader is advised to see references cited at the end of the paper (1, 2, 3, 4, 5, 6).