As part of an ongoing investigation of the wrecks and artifacts remaining off the American sector D-Day beaches of Normandy, a team from the U.S. Naval Historical Center's Underwater Archaeology Branch, Reson Inc. and the University of New Hampshire's Center for Coastal and Ocean Mapping/Joint Hydrographic Center, carried out a series of surveys aimed at exploring the applicability of using a hydrographic- quality, high-resolution multibeam sonar for marine archaeological studies. The system used for these surveys (the Reson 8125) operates at 455 kHz with dynamically focused near field beam-forming that allows 240, 0.5° (at nadir) beams to be formed over a 120° sector. The short pulse length and very narrow beam widths of this sonar provide extremely high-resolution and very reduced sidelobes that allow for the robust tracking of complex targets. We surveyed 21 areas in water depths ranging from about 3 - 30 m. Some sites contained individual targets such as landing craft, barges, a destroyer, a troop carrier, etc., while others contained multiple smaller targets such as tanks and trucks. Of particular interest were the well-preserved caissons and blockships of the artificial Mulberry Harbor deployed off Omaha Beach and destroyed by a Force 6 - 7 storm that started on 19 June 1944.
Unlike traditional marine archaeological search tools (sidescan sonar and magnetometers), the multibeam sonar can provide detailed, undistorted, and quantitative information on the 3-D geometry of the target being surveyed from a platform that is safely above target. Unlike traditional visual or photographic inspection, the multibeam sonar insonifies a relatively large area (tens to hundreds of meters) allowing the full context of targets to be established quickly. When combined with state-of-the-art 3-D visualization techniques that allow the viewing of both rendered surfaces and individual points, the data returned provides an unprecedented level of detail including the ability to recognize individual components of the wrecks (ramps, gun turrets, hatches, etc.), the state of preservation of the wrecks, and the impact of the wrecks on the surrounding seafloor. Given these capabilities we suspect that the multibeam sonar will play an increasingly important role in future marine archaeological studies.