Digital Side-scan Sonar Survey of Louisiana Slope Areas Containing Active Oil Seeps and Halokinetic Sediment Modification

Abstract

Abstract We mapped 2400 km2 of Louisiana t:ontinental slope seafloor using the TA M U2 digital side-sean sonar an instrument capable of simultaneous collection of seafloor imagery and bathymetry at frequencies of 12 kHz and 70 kHz. This system has been used effectively for purposes as diverse as cable route surveys and mid-ocean ridge studies. Our goals in this study were twofold: (]) to demonstrate the capabilities of TAMU2 for rapid. high-resolution seafloor imaging on the Gulf of Mexico slope. and (2) to image seafloor features in areas with hydrocarbon seeps and chemosynthetic organism sites. The TAMU22 data show a wealth of features related to halo-kinesis and hydrocarbon seepage. We identified mud mounds authigenic carbonate mounds probable brine lakes hardbottoms faults and sediment flows. Active faults arc shown by their surface expressions and comprise an Interconnected network extending from one diapir crest to another. ringing diapirs and hounding intra-salt basins Along one portion of a fault network where several chemosynthetic sites are known there are numerous examples or sediment flows emanating from the faults and extending downslope. Mud mounds such as the classic "Bush Hill" chemosynthetic site are common. We found 11 in one 26 × 41 km area which have "wipeout" acoustic signatures and infer from geometry that >50 may be present. In a deeper area which includes Pygmy Basin. seepage-related features are seen but halo-kinetic sediment-movement features are striking. Diapir flanks are typically fault-hounded and show numerous gullies. Probable sediment flows are imaged extending from diapir flanks onto basin floors. Some may result from hydrocarbon venting but many appear related to sediments destabilized by salt movement. The rich detail in TAM U2 images enables interpretations of seafloor conditions and processes which are applicable to site engineering evaluations and their regional contexts. Introduction Natural hydrocarhon seeps are common on the continental slope of the northwestern Gulf of Mexico. Photographs from space show tens if not hundreds of slicks on the sea surface. implying seepage from the seafloor 1. These seeps arc of interest because they arc an indicator of where hydrocarbons pool in the subsurface and because seafloor seeps provide habitats for ecosystems based on chemosynthetic organisms that metabolize the organic material2–4. Most Gulf of Mexico seeps and chemosynthetic communities have been located by chance many by the oil industry in the process of high-resolution geophysical hazard surveys of lease blocks. Because of this the number of well-studied seeps and known chemosynthetic organism sites is relatively small. One might expect that there are many more that have gone undiscovered. especially given the large number of sea surface slicks visible from space1. It was this assumption that motivated our study. If there are undiscovered seeps. how does one go about finding them? The logical first step is acoustic profiling techniques from surface ships. Seeps are typically located on faults in areas where the sediment column displays acoustic modification. Acoustic signatures include acoustic turbidity or wipe-out (signal loss) bright spots or layers mud mounds formed by fluid expulsion. and authigenic carbonate mounds (e.g .. refs 5-8).