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Marine Engineering Geology and the Challenge of Offshore Geohazards

Kerry J. Campbell
Fugro GeoServices, Inc, Houston, TX

Offshore geohazards and other foundation-zone and tophole geologic conditions are increasingly presenting significant challenges to offshore energy exploration and development. This paper describes some offshore geohazards and the tools used to characterize them, and discusses some of the most pressing challenges now facing marine engineering geoscience and geohazard specialists. Geohazards now being routinely dealt with, especially at deepwater sites, include, among others: landsliding and other seafloor instability; faults and faulting; gas and gas hydrates; shallow overpressures; fluid venting; and seafloor current scouring. In addition, erratic soil properties in buried landslide deposits, sands, rock, and gas hydrates, present a variety of geotechnical problems for tophole drilling and foundation design, installation, and performance. Although conventional 3-D exploration seismic data is routinely used for some geohazards applications, specialized geophysical and geotechnical tools are also being used to identify, map, and characterize conditions to the high level of detail needed for facilities siting and foundation engineering purposes. Among these tools are: AUV survey systems providing 3 to 10 kHz, 2-D seismic profiles and micro 3-D seismic data volumes; high-resolution (150 to 300 Hz) 3-D seismic systems; very-high-resolution (~2 cm) borehole logging systems; and several types of geotechnical soil sampling and insitu soil testing tools. Seismic workstations and other technologies being used for prospect evaluation and reservoir characterization are now being used to analyze, integrate, and visualize the large amounts of disparate engineering geophysical and geotechnical data available for typical deepwater development projects. Site characterization is followed by quantitative geohazards risk assessment to provide direct input to economic analysis of prospects. Ongoing challenges include increasing the reliability of tophole pore-pressure prediction and refining geophysical means to predict materials at the high level of detail required for foundation engineering.