Archaeological Survey

When we think of underwater archaeology, we often think first of technology - the remarkable modern tools which allow us to probe the ocean floor, calculate coordinates and discover artefacts that, without technology, might remain among the secrets of the seas.

But most underwater archaeology begins just as any archaeological excavation for which the precise location of the site or artefact is not known: in a library, or amid other historical archives. This research must sometimes proceed for years before the surface of the water is broken!

In most cases, however, the best that archival research can do is to narrow down an area, a hypothetical "zone of exploration", which can vary from a few square nautical miles to a vast area of ocean, depending on the historical context and on how much specific information is known.

The next step is the geophysical survey, which takes place within the zone of exploration. The survey is conducted using different sensors to measure the physical characteristics of the site. The purpose of this is to reveal anomalies, which may correspond to archaeological remains.

A boat equipped with measuring instruments and computer systems, systematically scours the entire search area along parallel lines spaced 30-80m apart depending on the average depth of the site and the nature of the remains to be discovered.

Nuclear magnetic resonance magnetometers (NMRs) and a side scan sonar, operated by electrical winches, are towed behind the boat: barely sub-surface in shallow water or submerged by hydrodynamic depressors in deeper water. An acoustic positioning system continuously transmits the position of the towed sensors. The catamaran is also equipped with high-resolution echo sounders.

In on-land archaeology, these measurements would be calibrated and compared against a constant measure of longitude, latitude, and elevation - since like all sciences archaeology requires measurable and duplicable results. At sea, we have an intensified use satellite-based Differential Global Positioning System, or DGPS, to provide this constant positioning.

All the recorded parameters are subjected to an initial analysis during the course of the survey. Daily processing of the data leads to the generation of bathymetric maps (showing a relief of the ocean bed) and magnetometric maps (showing magnetic fields) of the area under survey, completed by a sonar image of the sea-bed (using acoustic photography).

The analysis of the data generated allows the geophysicists to determine the most promising positions for further exploration.

When an anomaly has been measured, experienced divers or remotely operated vehicles (ROVs) make an initial inspection in order to identify its origin. Even in clear water the remains are usually rarely visible. The process of uncovering detected material, which is often buried or covered with thick deposits, generally requires soundings to be taken.