On 6 June 1944 the combined Allied forces began Operation Overlord, the invasion of Hitler's Fortress Europe, along the beaches of Normandy, France. This operation, described by Winston Churchill as "undoubtedly the most complicated and difficult that has ever taken place," was the decisive turning point of the Second World War (Bowden, 2002). The logistics of this effort were staggering. By the end of June more than one million men, 177,000 vehicles and 586,000 tons of supplies landed on the Normandy beachhead.

In support of the invasion, Allied naval forces mounted Operation Neptune involving nearly 5,000 vessels and an ingenious strategy to float concrete caissons across the English Channel, and create, in a matter of days, two fully functional ports (codenamed Mulberry). The creation of these ports was a key component of Operation Overlord, as the initial invasion avoided the strongly defended harbors of Cherbourg and Le Havre, but required the delivery of approximately 5,000 tons of material per day to the Allied troops. The capacity of these artificial ports, constructed in less than two weeks, would equal that of the Port of Dover that had taken seven years to construct (Ferrand, 1997).

During Operation Neptune, several hundred Allied vessels and tons of war material were lost off the Normandy coastline. Over the years, salvage operations removed many of the wrecks deemed hazardous to safe navigation and activities of the local fishing communities. While salvagers completely removed some ships, other vessels remain, some heavily impacted and others relatively intact - all an important but vulnerable testament to the courageous efforts of the Allied troops and to one of the most important operations in U.S. military history.

The Naval Historical Center's (NHC), Underwater Archaeology Branch, recognizing the potential historical significance of US Navy wreck sites off the D-Day beaches and seeking to fulfill its mandate to manage and preserve historic ship and aircraft wrecks, undertook a three-year remote-sensing study off the Normandy coast. The specific objectives of this study were to: 1- locate and confirm the existence of U.S. Navy wrecks associated with Operation Neptune; 2- provide identification and an indication of the state of preservation for each wreck site; 3- compare historical cartographic documents to remote-sensing analyses, and; 4- identify the authorities and agencies that have an interest in the preservation of these possibly significant historical resources and make the appropriate recommendations (Neyland and Schmidt, 2002).

In the first two years of its study, the NHC used the traditional tools of marine archaeology, sidescan sonar, magnetometer, and ROV video imagery, to locate and document potentially significant targets off Omaha Beach, Utah Beach and Point du Hoc. The study revealed nearly 3000 magnetic anomalies and more than 700 acoustic targets, including the submerged caissons that formed the Mulberry Harbor at St. Laurent sur Mer off Omaha Beach. After closer study, the NHC selected 30-40 targets as high-priority sites warranting further investigation (Neyland and Schmidt, 2002).

In the summer of 2002, the NHC, in collaboration with Reson Inc. and the Center for Coastal and Ocean Mapping/Joint Hydrographic Center at the University of New Hampshire (CCOM), returned to the waters off Normandy to conduct detailed surveys of these high-priority sites. This survey used innovative new, very high-resolution multibeam sonar operated to hydrographic standards. When combined with state-of-the-art visualization techniques, the data collected with this sonar provided an unprecedented view of the nature and state of the wrecks, addressing many of the key issues of concern to the marine archaeological community. This paper documents this operation and, in so doing, outlines the tremendous potential of multibeam sonar as a tool for underwater archaeology.