NEW GEOLOGIC DATA FROM THE COMPLEX ESTUARINE ENVIRONMENTS OF LOUISIANA: GROUND TRUTH "CALIBRATED" ACOUSTIC SURVEYS IN SHALLOW WATER

In the Mississippi River delta complex, shallow coastal bays have largely been voids in our geologic knowledge of this large and dynamic depositional system. Because of difficulties in collecting high resolution acoustic data, the construction of the facies architectures of these areas has lagged behind other parts of the coast. Recent development of a high resolution acoustic system (side-scan sonar, chirp sonar, and benthic profiler) has provided the capability to construct a detailed surface and shallow subsurface geologic framework in which critical direct sampling strategies can be knowledgeably implemented. Because of rapidly changing land-water relationships in coastal Louisiana due to both natural and man-induced processes, State and Federal coastal restoration efforts are focusing on diversions of river water and sediment as a promising solution to the state's landloss problem. One major diversion into the St. Bernard Parish marshlands east of the Mississippi River (Caenarvon Diversion) is currently active and another one west of the river (Davis Pond Diversion) is under construction for the Barataria Basin. More diversions are planned for the future. Diverted water and sediment will change the salinity regimes and sediment dynamics within these estuarine settings, so it is imperative to have reliable baseline data in order to evaluate diversion impacts.

Application of digital side-scan sonar (100 and 500 kHz) and a broad-spectrum subbottom profiler (4-24 kHz) for rapidly acquiring both surficial and shallow subsurface data (average water depths 0.7 to ~ 3.0 m) has now been accomplished. These data sets "calibrated" with coring, surface sampling, and other "ground truthing" techniques such as core penetrometer profiles have enormous potential for providing an understanding of: (a) erosion and deposition, (b) geotechnical properties of bottom sediments, (c) locations types and status of oyster reefs, (d) fisheries habitats, (e) shoreline retreat and progradation and (f) shallow subsurface configurations that influence surface conditions. This approach of linking high resolution acoustic data with various direct sampling techniques has been verified in pilot studies. We have initiated a program for establishing a baseline data set for evaluating future change in all of Louisiana's coastal water bottoms.