Survey is essential to any site protection program in order to establish inventories of the existing underwater cultural heritage and to ensure that measures are inscribed in a global protection scheme.
Applicable survey methods vary according to the physical conditions on the location of the submerged archaeological site, such as accessibility, depth, currents, visibility, temperature and financial means available.
Survey methods, allowing the establishment of an inventory on underwater cultural heritage present in a region, include:
1. Visual survey: based on human observation through diving. Its efficiency is proportional to the size of the area, number of divers, visibility and depth. Visual survey can be improved by the use of metal detectors working on pulse induction. It is limited to water depths, where air or gas mixtures, Trimix and others, allow diving respecting decompression limitations.
2. Remote Sensing means using electronic devices controlled from a boat on the water surface to collect large amounts of information in a short period and at greater depth than accessible to divers. It allows to operate without visibility and detect elements buried under sediment.The use of remote sensing equipment requires experience and training to carry out the survey and correctly interpret the data obtained. Only then can wreck features be identified as being different from natural formations. Overlapping corridors should be established to ensure maximum coverage. Historical search is essential to minimise the area of search.
- Echo-sounders: among the economic and simpler forms of electronic search equipment. A streaming remote sensing “fish” is towed to a ship. It transmits a pulse of energy and the return image or “echo” is registered, interpreted and displayed on a computer screen. Echo sounders produce high resolution images of seabed types and depths. The computer-generated graphics produced can be accompanied by other information, such as GPS coordinates, in order to register the exact location of any possible anomalies.
- Magnetometer: measures the strength of the earth’s magnetic field and can detect variations caused by the presence of ferrous material on or below the seabed. A signal is generated, which is directly proportional to the strength of the earth’s field. Magnetometers can detect buried magnetic material.
- Side-scan sonar: works similar to an echo-sounder, however, electromagnetic energy is pulsed towards the seabed on a cone shape. Two separate fan-shaped beams are directed on either side of a tow-fish and selected signals are interpreted and displayed graphically. The graphic is usually reflected on a wide paper roll marked by a stylus which indicates the topography of the seabed as a form of a negative image consisting of acoustic highlights and shadows. The more dense an object is, the stronger the return signal and the darker the image.
- Sub-bottom profiler: in particular used to detect buried archaeological materials. It works on the same principle as the echo-sounder, but with a lower operating frequency allowing the penetration of sand and sediment. The “fish” is towed to the ship and kept close to the seabed.