A uniquely detailed geologic survey was completed in 1977-79 for 100 lease blocks (900 square miles) in the mudslide area off the Mississippi Delta in water depths of 30-800 feet. Line spacings were 840-1200 feet. Subbottom data were acquired from 3.5-12 kHz sources for near-surface sediments and 50-100 Hz sparkers for 500-800-foot generation. Overlapping side-scan sonar data at scales of 1:1500 and 1:2000 provided almost complete sea-floor coverage. Mosaics were constructed from digital, scale-rectified sonographs. Data were initially analyzed at 1:12,000, then reduced to a set of 28 maps at a scale of 1:48,000. Historical bathymetry, surface morphology, isopachs of disturbed sediments, and subsurface geologic structure were mapped. Resurveys during 1979-80 used the original data or comparative studies. The maps allowed an assessment of the location, magnitude, and frequency of sea-floor processes for siting and design of structures and pipelines. The methods of survey and data synthesis are applicable to other areas which impose engineering constraints.
The offshore Mississippi Del ta region is subject to a variety of sea-floor instability processes that pose particular engineering problems for the oil and gas industry. The types of instability have been described in several papers, 1-6 and their impact on offshore structure has been documented. Specifically, some pile-supported production platforms have been moved and damaged, pipelines broken and jackup rigs displaced byshallow rotational slides, mudslides/flows, and subsidence/ collapse processes.
Successful engineering design in such areas requires interdisciplinary studies with input from construction engineers, soil mechanics and geotechnical specialists, oceanographers, and marine geologists. A number of complementary contributions are necessary, including(a) definition of regional and site geologic history, (b) identification and location of bottom instabilityfeatures, and (c) assessment of the magnitude and frequency of process activity. This essentially geologic component is a necessary precursor to, and provides a context for, geotechnical and engineering evaluation and design which aims at quantification of potential stresses on structures.
In view of the intensive industrial use of the offshore delta-front slopes, a survey was begun in 1977 to provide basic geologic data for the area and to map, in considerable detail, the distribution of sediment instabilities. The survey of the entire delta was completed during 1979 and the data reduced to map form during 1980. The maps and report were made available toindustry by the Bureau of Land Management as Open File Report 80-01.
The near-surface geology and instability distribution maps were prepared from high-resolution geophysical and side-scan sonar surveys supplemented by precision bathymetry, bottom samples, and data from soilfoundation borings.
