Evaluating Soils and Hazards in the Mississippi Canyon

ABSTRACT During the period 1979–1984 a comprehensive program was conducted to study the surface and near surface seafloor conditions in the Gulf of Mexico's Mississippi Canyon. Included were a high resolution geophysical survey, a deep soil boring, studies of ongoing consolidation settlement and downslope creep and a long-term program of acoustic position measurements to detect seafloor movements. This paper presents an overview of the entire program, which indicated the feasibility of placing petroleum development structures and systems within a certain region of the canyon seafloor. INTRODUCTION The Mississippi Canyon is one of the most prominent bathymetric features in the northern Gulf of Mexico. Of particular interest is the four block area shown in Figure 1. This area is approximately 49 miles south-southeast of Grand Isle, Louisiana, in 1500 ft of water. The studies reported here were undertaken in 1979 in order to establish whether the Mississippi Canyon and the four block area in particular were safe for the sitting of petroleum production structures and facilities. At the time the studies were undertaken, several factors contributed to concern over potential well maintenance and foundation problems. Results of a soil boring in the area had shown that at least the upper half of the 2OOO-ft deep soil fill in the canyon was highly under consolidated. Moreover, the existing geophysical data and interpretations (references [1] through [7]) of the canyon's history and present condition included the possibility of ongoing seafloor motions. It had been suggested that the canyon was formed through erosion by turbidity currents over the period from 70,000 to 30,000 years ago and that the canyon fill sediments were turbidity current and mass transport deposits. Features interpreted from geophysical records included erosional channels, diapirs, slump scars, slides, and folded and faulted sediments. The studies being discussed here were a high resolution geophysical survey, a geotechnical investigation, studies of ongoing consolidation settlement and downslope creep and a long term program of acoustic position measurements to detect seafloor movements. The findings of these studies were applied to the evaluation of geotechnical and geologic hazards for petroleum production systems in the Mississippi Canyon and to the development of foundation design criteria and methods for both shallow and deep foundations. The results showed that the Mississippi Canyon can be developed safely. The highly under consolidated soils are undergoing consolidation settlement and possibly some downslope creep, but the magnitudes of expected soil movements can be accommodated in foundation designs. GEOPHYSICAL INVESTIGATION In early 1980, a high resolution geophysical data base was acquired over a 58 square mile area including the four block area shown on Figure 1. The data gathered included 12-channel digitally acquired and processed mini sleeve exploder seismic profiles, 3.5 kHz subbottom profiler records, precision depth profiles and digitally acquired overlapping scale-corrected side scan sonar records.