Repeat Surveys to Evaluate Seasonal Variability in Seafloor and Shallow Sub-surface Acoustic Properties, Shallow Water Gulf of Mexico

摘要

To investigate the seasonal variability in seafloor and shallow sub-bottom acoustic properties in the shallow water Gulf of Mexico. We chose three sites for this purpose. The sites, and their primary objectives, are: MURI Fluid-Mud site, Louisiana. Acquisition of a baseline geophysical data set in summer 2007 along the two primary transects identified as the focus of ONR- sponsored research as part of the Fluid-Mud MURI. Groundtruthing of the acquired acoustic/geophysical data with extensive coring, including additional sites where our colleagues from NSP/SIO deployed their waves/pressure sensors. The direct sampling measurements (grain size, viscosity) will add to their wave model. The area was re-surveyed in winter 2009, with additional survey lines acquired over new MURI instrumentation sites. West Delta area, Louisiana. An area tightly related to the offshore industry, we chose the West Delta / South Pass area offshore the Mississippi River bird's foot delta for our study. Specifically, a survey conducted in the area of the West Delta 109 pipeline, which broke during the 2005 hurricane season, imaged numerous mud flows, with some areas characterized by 6 to 10 m of net accumulation over a 20 year period. The SS Virginia shipwreck occurs in the field area, and had been mapped previously on three occasions, showing that the mud flow in WD109 moved more than 400 m in the 2005 event. Previous mapping in the area did not extend to a hypothesized source of the mud flows at the West Pass channel outlet and/or the designated Corps of Engineers dredge dump location. Our goal at this site was to determine seasonal variability due to Mississippi River sediment delivery and to examine the connection between the previously mapped mud flows, possible sources, and the connection to anthropogenic influences. We documented temporal changes between the 2005/2006 industry survey, the 2007 ONR survey, and our last survey of February 2009, which showed that the mud flows in WD109 mov.