PASISAR: A new tool for near-bottom seismic profiling in deep water

In order to obtain a better seismic image of the sea-bottom in deep water and particularly to decrease the size of the Fresnel reflecting zone, IFREMER decided in 1993 to equip its deep towed side-scan sonar SAR with a single channel seismic streamer performing as a conventional seismic streamer towed near the sea surface. In such a system, the seismic source can be either an air-gun, a water-gun or a sparker depending on the water depth and on the expected resolution and penetration. Several cruises were made in 1993 in order to validate the concept and to collect first near bottom seismic data in water depth ranging from 200 m to 5200 m. Compared to conventional surface seismic data, both spatial and vertical resolutions are considerably increased. Off-line data processing allows correction of some artefacts due to the specific geometry of the system. Using the SAR, it is now possible to collect simultaneously on the same profile up to 6000 m water depth: high resolution side-scan sonar data along a 1500 m wide corridor (resolution less than 1 m), microbathymetry along the profile, near-bottom 3.5 kHz subbottom echo-soundings data, near-bottom seismic data and conventional surface seismic data. It is becoming a highly performant tool for detailed studies of the geometry of sedimentary bodies, for site surveys and mapping in deep waters