Interpretation of side-scan sonar images from hydrocarbon seep areas of the Louisiana continental slope

Abstract

Side-scan sonar images from the Louisiana continental slope were examined to study hydrocarbon seepage and related surficial geologic seafloor features. Three study areas are located in the Green Canyon area and the Garden Bank area. Hydrocarbon seeps are crude oil or gas that occurs on the earth's surface, having migrated from underlying buried source strata along faults. Hydrocarbon seeps can alter acoustic properties of the sediment in which they occur and these alterations can be detected by high-resolution geophysical devices, including side-scan sonar and subbottom echo-sounder. Interpretation of the data obtained by these geophysical devices can provide the knowledge of seep existence and distribution, as well as other geologic features associated with hydrocarbon seeps. The data base includes 11/12 kHz long range TAMU² side-scan sonar images, 3.5 kHz reflection profiles, piston and gravity core data, which were obtained by the survey vessel, R/V Gyre, 2-12 kHz chirp sonar data, which were obtained by the U.S. Navy's research submarine NR-1, and other miscellaneous data, including 3 multi-channel seafloor seismic amplitude maps from the oil industry. Side-scan sonar images show six anomaly seafloor features: mud mounds, crater/ depression, faults, sediment flow, mass wasting and seepage-affected seafloor spots. Mud mound, crater/depression, and seep-affected seafloor spots are thought to be related to hydrocarbon seeps and are found to be distributed together with faults. Mud mounds are often located in the center of the anomalous seafloor spots. Faults are abundant in all these seep-affected areas. Two types of faults exist: regional faults and local faults. They provide migration pathways for hydrocarbon seeps to the seafloor. Hydrocarbon seeps exist in all three study areas. Mud mounds/craters or seep-affected areas are often covered with carbonate-cemented seafloor spots. These lithified seafloor spots caused hardbottom reflection and high backscatter.