Acoustic investigations of natural seeps at GC600

Abstract

We examine the acoustic signature of natural seeps in the Green Canyon block 600 of the Gulf of Mexico. The survey site was 2200'1600 m2 at a depth of 1200 m. Near-bottom multibeam backscatter, side-scan sonar mosaic, and chirp sub-bottom profiles were collected using an AUV cruising at 40 m above the seafloor. The water-column profiles collected by the Kongsberg EM2040 at 200 kHz central frequency were utilized to detect and localize seeps. Geological information from the chirp sonar and side scan images will also augment these data to further our understanding of the geologic structure of natural seeps. Identified seeps from the backscatter intensity were visited with a Comanche ROV for a visual inspection. An oily plume was selected for further investigations and vertical upwelling will be monitored with an Acoustic Scintillation Flow Meter (ASFM), for the first time. Concurrent measurements include nearby Acoustic Doppler profilers, a vertical array of conductivity/temperature instruments and two video cameras (VTLC) for identifying bubble/droplet sizes and distribution and will provide invaluable reference data on temporal variability.