Bottom Furrows and Estimated Currents in the Mississippi Delta Region

Summary A simple model of a neutrally buoyant, steady-state benthic boundary layer is used to examine the relationships between the dimensions of secondary roll vortices in the boundary layer and the spacing of a system of bottom furrows located offshore of the Mississippi Delta. The mean free-stream flow velocity predicted by the model, which creates the desired ratio between those two parameters, is an estimate of the free-stream current speed necessary for furrow erosion. In the absence of direct current measurements, such estimates are of value not only for offshore design but also for planning subsequent current measurement programs. Introduction In a recent paper, a system of bottom furrows located in a reentrant at the continental shelf edge in the Mississippi Delta region was described. These furrows and other recognized sedimentary bedforms indicate that bottom processes associated with near-bottom currents are active in the area. Direct observations of bottom currents, however, are scarce. Intensive petroleum exploration activities are being conducted in the potentially productive offshore Mississippi Delta area. Bottom activity and sediment movement are critical factors in the design and placement of oil and gas production and transportation facilities. The bottom furrows provide evidence of the general nature of the operative bottom processes. Using an existing boundary layer model and furrow geometry, it is possible to extract some quantitative information about the currents believed to be responsible for the furrows. These estimates can provide an important parametric input to design studies. In this paper, bottom furrows in the Mississippi Delta region are summarized briefly and their presence is attributed to a secondary circulation consisting of a system of longitudinal roll vortices. A benthic boundary layer model that accounts for the effects of stable stratification on boundary layer structure is used to extract quantitative information on near-bottom currents. Background The Mississippi Delta furrows became known initially as a result of a systematic mapping survey of the offshore Mississippi River delta area by high-frequency echo sounder, subbottom profiler, and side-scan sonar from 1977 through 1979. Closely spaced survey lines [900 ft (275 m)] allowed the furrow distribution to be mapped by sonographic mosaicking techniques. Subsequent work has documented the presence of similar highly localized features 50 miles (80 km) to the west in the Mississippi Canyon, A bathymetric influence on furrow location is suggested by the absence of these features in the intervening topographically uniform bottom area. Erosional furrows have been observed in a number of different marine environments. SPEJ P. 177^