Marine Engineering Geophysics: Meeting The Deepwater Geohazards Challenge

Abstract

Engineering geophysics is playing a critical role in meeting the considerable geohazards challenge in deepwater. Exploration drilling for hydrocarbons is taking place in nearly 10,000 ft of water, and production is now on-line in as much as 7,200 ft. Total investment for some deepwater developments is more than $1 billion. Geohazards Geohazards and soil conditions at deepwater sites can be complex and can cause expensive problems for exploration and development activities. Local deepwater geohazards include: 1) irregular, sometimes rocky, topography with sharp relief ranging up to several tens of feet; 2) steep and potentially unstable slopes; 3) both modern and ancient landslides; 4) over-pressured gas or water sands (with potential for shallow-water-flow); 5) mud volcanoes or other fluid vents and associated flows; 6) active faults with seafloor scarps ranging up to more than 500 feet high; 7) gas hydrates that may be subject to reduced shear strength and thaw settlement when heated; and 8) materials ranging from weak, underconsolidated soils to rock. Deepwater geophysical tools Geophysical tools used to help characterize deepwater sites and assess geohazards include: 1) deep-deployed autonomous underwater vehicles (AUV’s) outfitted with side-scan sonar, subbottom profiler, and swath-mapping multi-beam bathymetry systems; 2) surface-deployed 2-D and 3-D highresolution seismic systems; 3) lower-resolution 3-D exploration seismic data; and 4) very-highresolution borehole logging tools. Robust interpretation and visualization of deepwater engineering seismic data are now being facilitated by PC-based workstations. This paper summarizes deepwater conditions and the principal hazards encountered to date, and describes some of the geophysical tools and methodologies used to investigate them. A case history is given to help illustrate the AUV micro 3-D seismic survey methodology recently used to do detailed mapping of shallow faults at a proposed deepwater production-spar site.