Pushing the Limits of Seismic Resolution in Deep Water

Obtaining very high resolution seismic images of sediments near the sea floor in deep water is fraught with problems. The conventional approach has been to tow a subbottom profiler near the bottom but, if the water depth is a kilometre or more, vessel speed and manoeuverability are so restricted that it becomes more economical to mount the profiler on an autonomous underwater vehicle. Either way, the survey would be expensive. Moreover, in depths of a kilometre or more, source energy levels are so restricted that the depth of subbottom penetration is limited to a few tens of metres. A research program at the University of Mississippi has been investigating ways of improving seismic resolution while achieving significant penetration and has obtained some remarkable results in the Gulf of Mexico. Deploying a commercial watergun just below the surface of the water and towing a single-channel hydrophone array at a few hundred metres depth, the program has demonstrated that, in a thousand metres of water, sea-floor resolutions of tens of centimetres and penetration of hundreds of metres can be achieved. Vessel speeds were typically 2-4 knots but it is likely that higher speeds are possible. The demonstrated results were achieved by combining rapid digitization with state-of-the-art signal processing. Processing was done on a desktop computer and easily could have been done on board ship. The method is discussed and illustrated by examples of both raw and processed data.