OCS Study MMS 95 - Data Center

2.4 Remote-Sensing Detection of Natural Oil Seepage2.4.1 Use of Geophysical Methods for Indirect Detection of ChemosvntheticCommunitiesOur study suggests that geophysical techniques can be used to narrow theareas of detailed study needed to locate and classify chemosynthetic communities.Ultimately, no geophysical method will allow the reliable identification of suchcommunities without in situ observation .However, by using known geophysicalsignatures and affinities, likely locations can be identified for more detailed study .Seeps are located along faults and faults are usually identified in regionalseismic exploration profiling surveys .Thus, the first step in locatingchemosynthetic communities is to map the faults along which seeps could occur .The likeliest faults to harbor seeps are those that are active and thus extend to thesurface and those that show a significant displacement .Having found the faults, the next step would be to look for seep relatedtopography and sediment disturbance . Topographic features would be mudmounds, carbonate mounds, and pock-marks . The mud mounds are typically morethan 10 m in height and more than 100 m in diameter and would be evident onhigh-resolution bathymetry maps and side-scan sonar images . Carbonate moundscan be somewhat smaller, one to a few meters high and 10 m wide, or less . Inaddition, pock-marks are variable in size, from small, shallow craters a few metersacross to large craters many meters deep and hundreds of meters wide .Sediment disturbances can usually be found using acoustic profiling andimaging techniques . On side-scan sonar records, the disturbance can appear as anincrease in backscatter, particularly in conjunction with small mounds . In 3 .5 kHzand other echo sounder profiles, the disturbance may show up as an attenuation ofsignal reflections ("wipe out"), as an enhancement ("hardbottom"), or as2-8