Acoustic documentation

At sea, the terms acoustic imaging, acoustic documentation and acoustic data refer to images of the seabed or underwater sites obtained using a side-scan sonar or a depth sounder, equipment that rely on the way that the seafloor reflects sound waves.


The term admiralty refers to both the corps of admirals and the high command of the navy of a given state.

Amphibious tank / Sherman DD

Amphibious tanks (called Duplex Drive, or DD) were designed by British General Percy Hobart to manoeuvre in water as well as on land. The chassis of the conventional Sherman tank was modified to make it watertight, and to improve its buoyancy it was fitted with a canvas flotation screen held in place with steel hoops, as well as two swivelling propellers for propulsion and steering. It could reach a speed of four knots on water. Its armaments were similar to land models: two 7.62 mm Browning machine guns and a 75 mm gun. Its crew of five included a commander, driver, gunner, loader, and co-driver/bow gunner. Once on land, the screen, or skirt, could be lowered in just a few minutes making the gun operational.

Archaeological survey

The process of searching for and locating archaeological remains, using reconnaissance methods adapted to the natural environment and the elements sought. Thus, the way a survey is conducted may be pedestrian, aerial, underwater, geophysical, magnetic, etc. Appropriate methods permit archaeological remains to be located without being damaged or destroyed.


This underwater ultrasonic detection device is an ancestor of sonar. Designed by British, French and American scientists in the 1920s, it was used during the Second World War to detect enemy submarines up to 2000 m away.


While the 18th-century term aviso referred to a ship responsible for carrying orders or notifications, the avisos of the 20th century were light, fast vessels mobilised for escorting convoys, coastal protection or anti-submarine combat.


An ocean-going vessel, large and heavily armed, whose vulnerable parts are protected by thick armour.


Concrete or steel floats weighing 19 tonnes that supported the floating roads of the artificial ports constructed for the Normandy landings.


A merchant or military vessel deliberately sunk in shallow water to create an obstacle to the onslaught of the sea and thus protect, for example, a landing area within a Gooseberry.

Bomb disposal diver / Clearance diver

A navy diver trained to search for, identify, render safe and dispose of explosive devices at sea and in port areas.


Floating breakwaters anchored off artificial harbours to mitigate the swell inside the port. These metal caissons were 60 m long and cruciform in cross-section, arranged over a total length of 1,800 metres approximately 5.5 km from the coast.

Buoy tender

Vessel equipped for the installation and maintenance of beacons and buoys.


Negative ions of chlorine. Present in seawater, they are the cause of metal corrosion.


The states and territories of the former British Empire that share a certain solidarity, today more cultural than jurisdictional.


While the term once designated an intermediate-sized naval sailing ship, in the contemporary era, it refers to a small vessel used as an escort and in anti-submarine warfare.


Destruction of a mine by using other explosive charges.


A cruiser is an intermediate warship, between a battleship and a torpedo boat, carrying out protection, surveillance and reconnaissance missions.


A destroyer is a fast warship, of small to medium tonnage, originally designed to fight against torpedo boats. It is a versatile weapon, useful for both offence and defence. During the Second World War, the destroyer was one of the key weapons used to combat submarines.


The vertical distance between the waterline and the bottom of a ship’s keel.

Escort vessel

A vessel responsible for protecting merchant ships and convoys.


Construction using ferrocement, reinforced concrete or reinforced cement, entails building a metal frame that is then covered with hydraulic cement. The first reinforced cement boat was designed by Joseph Lambot in 1848. Although ferrocement barges, pontoons and service ships were built from the early 20th century onward, this technique developed significantly during the two World Wars due to the shortage of steel. As early as 1917, the first ferrocement oceangoing cargo ships were launched and during the Second World War, reinforced concrete was used to build caissons for artificial harbours, cargo ships used as breakwaters and landing barges. From the late 1960s onward, this technique was also used in the yachting industry, especially in amateur boatyards.


Formerly, in the sailing navy, a frigate was a lighter vessel than a ship of the line, with no more than 60 guns. During the Second World War, the term designated a medium-sized warship, an intermediary between a corvette and a cruiser, providing protection for convoys against submarines and aircraft.

Geophysical survey

The search for archaeological remains relies on geophysics, the knowledge of the physical characteristics of the Earth. This involves studying the Earth’s surface layers, or the seabed, with a range of equipment (sonar, depth sounders, sediment penetrators, magnetometers, etc.), in order to detect variations or anomalies that may correspond to archaeological sites or ancient manmade landscapes.


Gooseberry is the code name used by the Allies for the breakwaters created at Utah, Omaha, Gold, Juno and Sword Beaches to protect the landings of troops and equipment. These breakwaters consisted of lines of deliberately scuttled old ships, blockships, that protected smaller vessels from the ocean swell.


Acronym for Her/His Majesty’s Rescue Tug. Prefix designating the salvage tugs of the British Royal Navy


Acronym for Her/His Majesty’s Ship. Prefix signifying that a ship belongs to the British Royal Navy.

Hospital ship

A ship equipped to transport and care for the sick and wounded in times of war.


Expert in the measurement of sea and river beds, currents and tides.


At sea, speed is expressed in knots, one knot being equivalent to one nautical mile per hour, or 1,852 metres per hour.


The German Navy, between 1935 and 1945, under the Third Reich.

Landing Barge Flak (LBF)

A barge transporting guns to the beaches and also capable of defending beaches from the sea at a distance of 3,000 to 5,000 yards (2750 to 4500 m). LBFs were 60 to 90 feet long (approximately 18 to 27 m). They were equipped with two 40 mm Bofors guns and either two 20 mm Hispano guns or two twin Lewis machine guns.

Find out more : note on ONI 226

Landing Barge Kitchen (LBK)

A barge used as a floating kitchen for small landing units. LBKs were 79 feet long (approximately 24 m), could accommodate up to 25 crew members and were unarmed.

Find out more : note on ONI 226

Landing Barge Oiler (LBO)

A tank barge designed to supply fuel to small landing craft near the beaches. They were equipped with a 40-tonne fuel tank, sometimes divided in two to accommodate two types of fuel. They measured 80 to 90 feet long (approximately 24 to 27 m) and were equipped with a twin Lewis .303 calibre machine gun (i.e., 7.7 mm).

Find out more : note on ONI 226

Landing Barge Vehicle (LBV)

A commercial barge converted to transport vehicles during the early stages of the landings. There were three types of LBV (small, medium and large), measuring between 70 and 82 feet long (approximately 21 to 25 m). Depending on their size, LBVs carried loads of up to 100 to 200 tonnes, including up to three trucks, for the largest model, as well as guns.

Find out more : note on ONI 226

Landing Barge, Emergency Repair (LBE)

A workshop barge for carrying out emergency repairs on small landing craft. LBEs were 75 to 85 feet long (approximately 23 to 26 m) and 18 to 23 feet wide (approximately 5 to 7 m). They were equipped with a 20 mm Oerlikon gun.

Find out more : note on ONI 226

Landing Craft Assault (LCA)

A unit designed to shuttle soldiers from large troop transports to the landing sites. It was also used to transport equipment. British-made, it was the smallest troop carrier (41 feet long or approximately 12 m), the American equivalent was the larger LCVP. Its capacity was 35 soldiers or 800 pounds of equipment (approximately 360 kg). It was armed with a Bren gun, two Lewis .303 calibre (7.7 mm) guns and two mortars.

Find out more : note on ONI 226

Landing Craft Gun (LCG)

A support unit for assault troops tasked with destroying coastal defences. During the Normandy landings, two types of LCG were used: the Mark 3 and Mark 4, derived from the LCT 3 and 4, whose landing ramp was converted into a simple bow. Measuring approximately 190 feet long (58 m) they were armed with two British 25 pounder guns, two 20 mm guns and two machine guns. They could accommodate up to 23 gunners.

Find out more : note on ONI 226

Landing Craft Infantry (LCI)

This large barge was used to land an infantry company, once the beach had been secured. Depending on the type, it could carry between 102 and 182 soldiers, or 75 tonnes of cargo. LCIs were between 105 and 158 feet long (approximately 32 to 48 m) and armed with a 40 mm Vickers gun or several 20 mm Oerlikon guns.

Find out more : note on ONI 226

Landing Craft Personnel (LCP)

A small craft, often wooden, used to transport troops. They were loaded on deck or suspended from the davits of larger units. Depending on their type, LCPs used first by the British and then by the Americans, measured between 20 and 38 feet long (approximately 6 to 12 m) and could transport between 18 to 36 soldiers. The British LCPs were armed with a Lewis .303 calibre (7.7 mm) machine gun and the American ones with two Browning .30 calibre guns.

Find out more : note on ONI 226

Landing Craft Tank (LCT)

A barge for landing tanks. There were 11 categories of LCT (Mark 1 thru 9, LCT Armoured and LCT Rocket), launched initially by the British, then by the Americans. Varying according to category, the LCTs measured between 117 and 226 feet long (between 35 and 69 m) and could transport from three 40-tonne tanks up to 8 heavy tanks, trucks or cargo ranging from 250 to 350 tonnes. LCTs were armed with cannons or machine guns.

Find out more : note on ONI 226

Landing Craft Vehicle Personnel (LCVP)

Heavily used by the Americans for landing troops, LCPs were 36 feet long (approximately 11 m), capable of transporting 36 soldiers (or a platoon), a cargo of 8,100 lb (approximately 3,600 kg) or vehicles. They were equipped with two .30 calibre machine guns.

Find out more : note on ONI 226

Landing Ship Infantry (LSI)

LSIs were large troop transports, built from 1942 onwards or requisitioned, used in the landing operations. Depending on their type, they measured between 360 and 500 feet long (approximately 110 to 152 m). The largest of them could carry up to 1,100 soldiers and 20 LCA assault barges.

Find out more : note on ONI 226

Landing ship tank (LST)

An ocean-going vessel designed to be able to land armour, other types of vehicles (trucks, jeeps) and troops, directly at the landing site, through an opening bow. LST types 1, 3 and 4 were of British construction and LST type 2 was an American design. From 328 to 390 feet in length (approximately 100 to 120 m), they could transport 70 vehicles and nearly 200 soldiers, as well as LCAs and LCTs.

Liberty ship

A military cargo ship built in large series during the Second World War: 2,751 of them were built in the United States between 1941 and 1945, in 70 days on average (and only 7 days, 14 hours and 29 minutes for the Robert E. Peary). It was a simple functional ship, rapidly constructed from prefabricated, welded components. Standard Liberty ships measured 134.5 m long and could transport 10,000 tonnes of cargo.


Within the artificial harbours, Loebnitz platforms were floating steel quays, anchored to pylons, with concrete extensions to facilitate unloading. Several quays were built within Mulberry B; the west quay was designated for ammunition convoys, the central quay for unloading goods and the east quay was reserved for soldiers and vehicles.


A device for measuring the magnetic field. In underwater archaeology, it is used during geophysical surveys. Towed above the seafloor by a ship, it detects metallic masses lying on the seabed or just slightly buried. It is considered a “passive” measurement, in the sense that the device simply “listens” without emitting waves.

Metal salvage

Dismantling and recovery of sheet metal and parts for recycling.


A Latin term, often used by collectors, designating man-made objects used in military activities.

Military engineering

In the military domain, this term designates the branch of the army responsible for building military works including the excavation, fortification, and installation of infrastructure and equipment for communication and transmissions.


A small warship specialised in the neutralisation of underwater mines. During the First World War, early minesweepers were often wooden trawlers whose fishing gear was modified to tow a mechanical sweep. The aim was to cut the anchor cables of contact mines to bring them to the surface and then destroy them by cannon or gunfire. It was not until the Second World War that specialised minesweepers appeared, capable of exploding magnetic or acoustic mines by triggering their detonators. Unlike subsequent minehunters, minesweepers could not detect mines and were thus working blindly.


Mulberry was the code name given to the two artificial harbours designed by the Allies for the Normandy landings to compensate for the lack of deep-water ports. Mulberry A (for American) was set up off Omaha Beach and Mulberry B (for British) off Gold Beach. Both ports were protected externally by bombardons. They consisted of sea walls formed by lines of blockships and Phoenix caissons, and housed docks and floating roads connected to the beaches.

Multibeam echo sounder

A device, usually fixed to the hull of a ship, that emits a beam of “acoustic waves”, and measures the return signal of these sound waves after they are reflected from the seafloor. By calculating the travel time of the sound waves, the water depth can be simultaneously estimated over the entire width of the swathe. Multibeam echo sounders can be used to determine the bathymetry of the oceans and to produce nautical maps. In underwater archaeology, they are used during geophysical surveys to detect large masses, such as metal wrecks protruding from the seabed.

National archaeological map

A cartographic database, of France in its entirety, listing all known archaeological sites, the regulatory protections to which they are subject and the relevant archaeological research operations. This information is updated by the regional archaeological departments (DRAC) for terrestrial sites and by the DRASSM for underwater and foreshore sites.


Neptune is the code name used for the Normandy landings. Operation Neptune was only one part of Operation Overlord.


The Allies used the code name Overlord to refer to the ensemble of operations intended to create a new front in Western Europe. One of these was Operation Neptune, which specifies the Normandy landings.


Phoenix caissons were parallelepipedal structures made of reinforced concrete, that were submerged in the artificial harbours to complete the blockship breakwaters thereby creating an artificial dyke approximately 2 km from the shore. Six types of Phoenix caissons, varying in length from 53 to 61 m, were constructed in Great Britain. Hollow and equipped with simple bulkheads on the inside, they were floated across the Channel before being sunk off the French coast by filling their compartments using water valves. Some of them (types A and B) were fitted with anti-aircraft equipment.

Professional diver

A trained professional diver charged with carrying out underwater research or work.


The act of bringing a vessel back to the surface of the water after it has sunk.

Royal Navy

The British Navy.


The act of deliberately sinking a ship.

Service vessel

A vessel performing a service either for other ships, or supporting a human activity, either directly (pilotage, towing, etc.) or indirectly (dredging, buoy tending, etc.). In modern military terminology, the term also includes tankers, among others.

Side-scan sonar

A sonar device for obtaining an acoustic image of the seafloor. It is equipped with two transmitting antennas that send out acoustic pulses (or sound waves). The receiving antennas then analyse the echo reflected by the seafloor. Differences in the amplitude of the measured signals provide information about the nature of the seabed. In underwater archaeology, it is used mainly in the context of geophysical surveys. Towed over the seabed by a ship, it can detect wrecks, archaeological sites and objects lying on the seafloor or protruding from the sediment.


Acronym for Steam Ship. Prefix designating steam powered vessels.


A vessel used for transporting liquids.

Territorial waters

Territorial waters or seas refer to the zone extending 12 nautical miles offshore (approximately 22 km) of a country’s coastline. According to international law, this area belongs to the respective coastal state and its legislation is the same as the rest of the national territory.

Torpedo boat

A torpedo boat is a small, fast, manoeuvrable warship designed to attack large surface naval vessels with torpedoes.


A service vessel that can rescue a ship in distress by towing or pushing it to facilitate berthing and unberthing. Tugs are characterised by a very powerful propulsion system, a bulky silhouette, with superstructures gathered toward the bow, leaving the stern of the vessel free for the winch or tow-hook of the tug. A distinction is made between harbour or coastal tugs, ocean-going tugs, and salvage tugs, equipped for firefighting and other accidents at sea.

Underwater mine

An explosive device placed on the seabed or floated just below the surface. Laid by surface ships, submarines or aircraft, marine mines explode on contact with a ship’s hull, either by magnetic influence (magnetic mine) or by sound (acoustic mine).


Acronym for United States Ship. Prefix signifying a ship belonging to the US Navy.


Metal gangways, each 24 m long, resting on concrete or steel floats, linked to form floating roads connecting the quays of artificial harbours to the shore. Some of them were reused after the war to replace bridges destroyed by bombing.