ADVANCES IN DEEP AND ULTRA-DEEP WATER SITE INVESTIGATION AND GEOHAZARD ASSESSMENT DURING THE PAST 50 YEARS

The specialty field of deep and ultra-deep water engineering geology is less than 50 years old and its evolution has paralleled advances in offshore petroleum exploration and production technology. Fifty years ago, in 1963, the world’s first semi-submersible platform was limited to 100 m water depth. The deep water exploration threshold of 1000 ft (300 m) was crossed in 1979 and ultra-deep water threshold of 5000 ft (1500 m) in 1997. As of early 2013, exploratory wells had been drilled in more than 10,000 ft (3000 m) of water. Maximum production depths lag slightly behind exploration depths, but have already exceeded 9500 ft (2800 m). Complete darkness, extreme pressures, and the ability to work only remotely in areas that can be both geomorphogically and structurally complicated further increase the challenges of deep water engineering geology. The same conditions make most kinds of mitigation work impossible; hence, engineering solutions must either accept existing conditions (which can include seafloor sediments with undrained shear strengths of only a few kPa, or less than 1 psi) or avoid problems by careful site selection. Among the technological advances that have made modern deep water engineering geology possible are 3-D high resolution and 2-D ultra-high resolution seismic surveys; autonomous underwater vehicles (AUVs) capable of collection high resolution bathymetric, side-scan sonar, and sub-bottom profiler data as well as photographic images; and seafloor drilling rigs capable of operating at extreme depths. Piston cores, jumbo piston cores, and box cores can also be recovered in water as deep as 4000 m using gravity-driven and mechanically triggered devices lowered from vessels. Geologic interpretation is facilitated by bathymetric modeling to help delineate seafloor features and observational results can be supplemented using the results of physics-based numerical models to simulate processes such as seismic slope instability and sediment gravity flows. Future developments will likely include increased use of probabilistic techniques to account for natural variability and uncertainty, geologically driven optimization of site or route selection using composite geo-cost indices, and refinement of inversion methods to infer sediment geotechnical properties from near-seafloor seismic data.