Stephen G. Wright

Spatial Profiling/Inference of Subsurface Conditions From Seafloor Observations

Erik Liedtke (MS student), Nicholas Rodrigues (Undergraduate)

The objective of this study is to develop methodologies for combining multiple sources of data to infer site subsurface conditions. The study is motivated by the need to develop alternative methods for site characterization in deep water where conventional exploration schemes are becoming less feasible. Both geophysical methods (like the SASW method) and back-analyses of slope stability can provide useful information regarding subsurface strength and stratigraphy. The goal of this study is to develop methods to combine results of both methods to reduce the uncertainty associated with each method alone. As this study has progressed it has evolved to place increased focus on slope stability and the large amount of uncertainty due to difficulties in measuring accurately the bathymetry and slope geometry in deep water.

OTRC seeks to develop enabling technologies for the safe and efficient utilization of deep water resources. Characterization of the geotechnical engineering properties of the subsurface beneath the seafloor and assessment of slope stability are an essential element of this effort. The research seeks alternative methods of site characterization as well as addresses the evaluation of slope stability where the slope profile is highly variable and there is large uncertainty in geometry.

This project began with an effort develop methods for extracting stratigraphic and geotechnical information from knowledge of the subsurface topography and slope stability. The eventual goal is to be able to combine stratigraphic information that can be inferred from slope stability with the stratigraphics and property information that can be obtained from geophysical methods (SASW) and thereby improve the resolution over what can be obtained by each method alone. We have completed a number of deterministic studies to determine what level of resolution might be obtained in stratigraphy and strength from slope stability and, conversely, how certain stratigraphic features may affect slope stability (e. g. layer thicknesses, shear strength contrast). During the past 12-18 months we have been examining an actual offshore slope at one of the OTRC "theme" sites in Pigmy Basin, Gulf of Mexico. Our analyses revealed that there was considerable variation and uncertainty in topography, which in turn lead to significant variations and uncertainties in slope stability. We have spent a significant amount of effort and made good progress on developing robust methodologies for assessing the spatial variation in slope stability at sites where there is large variation in the slope inclinations. We have had to modify and extend some of the methods conventionally used to evaluate stability on land in order to enable them to be used for sites where there is significant variation and non-uniformity in slope geometry. Future work will incorporate more realistic slope geometries and will attempt to better quantify the uncertainties that can be expected in the slope geometry and how this uncertainty affects slope stability.

Prior OTRC research projects have developed the basis for application of the SASW method offshore. Other research at The University of Texas has developed methods for the deterministic analyses of slope stability. The results of both of these efforts are being utilized in the present study. In addition, other, on-going research on slope stability is providing analytical methods that are being used in the OTRC project.

This study focuses on both the development of new methods of site characterization offshore and the development of refined analytical methods for evaluating slope stability. The uncertainties associated with back-analyses of stratigraphy are being quantified to enable them to be used better in reliability analyses for site characterization. In addition existing analytical methods are being adapted to better handle stability analyses where the slope surface profile is highly variable. Examination of the bathymetry for the "theme site" in Pigmy basin has shown that slope geometry is likely to be much more variable offshore and require different approaches for evaluation of stability than are currently used onshore and in shallow water.

October 1994 to September 1998