Abstract

Submarine hydrocarbon seeps are widespread in the northwestern Gulf of Mexico and give rise to unusual seafloor features that are commonly poorly defined because water depth limits the resolution afforded by sea-surface seismic profiles. Detailed seep images are possible with side-scan sonars because such devices use beam-formed high-frequency sound to give high spatial resolution. Furthermore, side-scan sonars are useful for mapping seeps because their sound waves respond to changes in near-seafloor sediment properties caused 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 where seeps are known to occur. In sonar mosaics from upper-slope study sites, seep features are the main anomalous seafloor response and display high 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 control seepage. Cores from seep-affected areas imply that 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 lower 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, commonly showing crenulations and grooves, which were probably created by sediment failure and erosion by downslope sediment transport. Seeps are commonly difficult to discern because of the abundance of other anomalous features, but seem to be located mainly on basin walls and summits of salt highs.

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