Long Coring in Deep Water for Seabed Research, Geohazard Studies and Geotechnical Investigations
Authors: Silva Armand J. , Bryant William R. , Young Alan G. , Schultheiss Peter , Sykora Gabriella , Bean Daniel , Honganen Cary 
Published: October 2008

The overall objective of the research project is to understand and predict seabed processes on the continental slope in the orthwest Gulf of Mexico, related to the regional geology andengineering properties for the design of seafloor installations. This paper is a report of work in progress with a focus on a one-month research cruise to the main study corridor in deep water (800-2700 m W.D.), during which over 1,000 meters of sediment samples were obtained with a large-diameter long piston coring system and a large-diameter gravity corer. Profiles obtained from logging of the cores exhibit the wide range of variability associated with laminated soft sediments; debris flows, turbidity currents, and overconsolidated sediments. There is very good agreement between long core shear strength results and conventional boring data at two floating platform sites, Auger and Jolliet.

Introduction

A new research program on seabed processes in deep water Gulf of Mexico is showing the value of undertaking integrated studies to determine geotechnical and geological properties for geohazard analysis, siting of facilities, and foundation design. A major milestone of the project was the August/September, 1998 research cruise to obtain large-diameter long piston cores of the sediment and perform in situ shear strength and pore pressure measurements. The 5-year joint program by the University of Rhode Island (URI) and Texas A&M University (TAMU) is funded principally by the National Science Foundation with supplemental sponsorship by a consortium of five oil companies (Chevron, Texaco, Mobil, Amoco, and Conoco) and participation of two consulting firms (MARSCO and GEOTEK).

The northwestern continental margin of the Gulf of Mexico (Fig. 1) has a complicated history of construction and evolution resulting in a very complex bathymetry and morphology, especially on the upper and lower slopes. The primary study area (Fig. 2) is a 180 km × 9 km corridor south of Louisiana in water depths ranging from 800 m to 2700 m, which encompasses a wide range of seabed conditions including: stable plateau regions, slopes with various types of instabilities, basins, faulting, uplift areas, sediment flows, diapirism, areas with fluid expulsion and gassy zones. A more detailed map of Beaumont Basin (Fig. 3) shows a relief of 800 m with the basin floor at 2600 m and slope inclinations of over 25° in some areas. Two examples of subbottom and sidescan records within the primary study corridor are shown in Figs. 4 and 5.

The main objective of the project is to understand and predict a wide range of seabed processes related to geological studies and engineering design for seafloor installations such as platforms, pipelines, cables, and moorings. Three other areas in slightly shallower waters where borings have been taken for production platform design were added to the project so that the results of cores and borings could be compared.

The program is an integrated, interdisciplinary effort that includes substantial field work, extensive laboratory analyses and modeling of seabed processes. The majority of the field work, including two deep-tow cruises and the long coring cruise, has been completed and the third deep-tow cruise will occur in early 1999. The laboratory program focuses on determining geotechnical and geological information and parameters necessary for computer modeling of seabed processes, especially slope processes, and for geological interpretations.