SIDE-SCAN SONAR IMAGING OF HYDROCARBON SEEPS ON THE LOUISIANA CONTINENTAL SLOPE

Submarine hydrocarbon seeps are widespread in the northwestern Gulf of Mexico and give rise to unusual seafloor features that are often poorly defined because water depth limits the resolution afforded by sea surface seismic profiles. Detailed seep images images are possible with side-scan sonars because such devices use high-frequency sound waves formed into beams to provide high resolution. Furthermore, side-scan sonars are useful for defining seeps because the high-frequency sound waves respond to variations in near-seafloor sediment properties, which are modified by seep activity. To examine seep sonar signatures, we used a broad-swath, sea surface side-scan sonar to survey three areas of the Louisiana continental slope, totaling 2400 km2, where seeps are known to occur. In sonar mosaics from the two upper slope study sites, seep features are the primary anomalous seafloor response, resulting in enhanced backscatter of sonar waves. The mosaics show fault traces, fault scarps, authigenic carbonate mounds, irregular patches of seep-affected seafloor, mud mounds and mud volcanoes, craters, and sediment flows. Most of these features are located along faults that apparently control seepage. Cores from the seep-affected areas indicate that the enhanced sonar backscatter results from a combination of the perturbation of seafloor sediment physical properties and the introduction of seep-produced foreign material, including gas, oil, carbonate nodules and fragments, and chemosynthetic organism debris. In the deeper slope survey area, tectonic activity and resultant mass sediment movements make identification of seeps more difficult. In this area, many sonar backscatter anomalies appear to result from sediment flows and mass movements on the edges and bottoms of intrasalt basins. Basin walls appear as highly dissected, often showing crenulations and grooves, probably caused sediment failure and erosion by down-slope sediment transport. Seep features are often difficult to recognize because of the abundance of other anomalous signals, but seem to be located mainly on basin walls and summits of salt highs.