THE UNDERWATER SITE OF QAITBAY - Methods and techniques

The Aquameter

Myriam Seco Alvarez and Laure Déodat

The site off Qaitbay is a muddle of ancient blocks spread over an area of 1.3 hectares. To understand such a site it is necessary to create a complete inventory of the sunken blocks and a map plotting their position. While this work can be done using traditional topographical techniques, it can certainly be aided by the use of a tool adapted for an underwater environment. Thus, since 2001, the divers at the Qaitbay site have been using the D100 Aquameter.

This is an acoustic tool for aquatic topography that allows for the precise localisation of objects in three dimensions within a zone with a diameter of less than 200 metres. Fully autonomous and waterproof, it can be used by a single diver. Measurement set up and retrieve are performed using standard PC based software.

Photos André Pelle - ©CEAlex - all rights reserved

The aquameter is composed of two principal elements:
- The base station, which constitutes the reference Cartesian coordinate system {0,0,0}), at the top of a 2 metres (6 feet) mast.
- A portable pointer, which includes a small keyboard and LCD display panel, allowing the diver to measure positions and save them in the back-up memory.

To chart an architectural block, the diver must place the pointer upon the points to be plotted (for example, the four corners). By turning the pointer towards the base station, it then calculates the distance between the two elements and memorises it. Before starting the exercise, the divers have already fed the identification number of each block to be plotted into the pointer via the computer programme developed with the aquameter. Once the block is recorded, the divers can move on to the next block by a mere tap on the pointer’s keypad. At the end of the dive, all the plotted points are transferred onto a computer thanks to the same programme mentioned above, and then exported into AutoCad. The points then appear perfectly positioned on the screen. Thus, the map draws itself in a semi-automatic fashion. One need only join the dots in order to draw the different blocks.

While this tool may appear very useful, its application is somewhat problematic. In effect, the limits of its efficiency are restricted by certain conditions: the absence of noise, since it is an acoustic tool, and the absence of swell, to avoid interference between the base and the pointer. Moreover, the aquameter gives relative coordinates with the base being point zero. Thus direct survey topography (see elsewhere in this site) is still essential to establish certain reference points.

At the moment, the tool is still in an experimental phase, but the prospects presented by the use of the aquameter do appear promising.