Excavation techniques


The underwater excavation of a wreck follows several stages.

A preliminary survey can give us an idea of the date and origin of the wreck. It is from these data that the justification of an excavation could be given and that research objectives can be defined.

Before undertaking an excavation, the first stage is to perform a basic survey in order to determine the ascendancy of the wreck and if possible its orientation in space.
If such crucial data as the ship's environment, research objectives, and the general position of the wreck can be defined, an excavation strategy can be developed as well as the extent of the work, the work procedures and the respective stages in order to achieve the goal.

1 - Installation of a location system

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The goal of this system of location is to be able to have a reference system which remains available all the time, to which would be attached all the measurements carried out on the bottom.

The environmental conditions from this viewpoint are very significant, for the setting up of the system depends on depth, the sediment covering the wreck, the nature of the sea floor around the wreck, the visibility, the movements of the sea and the current. The archaeologist has an array of means that he will have to adapt according to the site because even the most adapted tools cannot always be implemented. One must construct around the site a series of fixed points as resistant as possible which can be used as needed for several years. The installation of these fixed points surrounding the wreck must make it possible to take at least three measurements from any given point on the site. A line of distance must be installed, for example parallel to the longitudinal axis of the wreck, and be maintained taut.

Each sector of excavation used during a given period, must be subjected to a particular system of squaring or triangulation, and attached to the network of general reference points.

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2 - Clearing away of the sediment

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This involves the removal of the sediment enshrouding the wreck. If this sediment is movable such as sand or mud, one can use either a water dredger or an air dredger to remove it. Both techniques create an aspirating water current channelled through a pipe. The water dredger is pump-driven and uses pressurized water whereas the air dredger injects pressurized air generated by a compressor at the base of an immersed pipe. As the air rises, it dilates and creates a strong, aspirating current. Water is rejected at the surface or into a water-filled recipient.

The removal of the sediment must be carried out in successive horizontal layers. It is indeed the most effective way to understand the structures and to preserve the objects that appear. The beginning of the removal is one of the most delicate phases of the excavation since one does not know what he is going to find and also due to the fact that the first structures to appear are generally decayed and thus the most fragile. The destruction of these first discoveries can be a great loss to the operation, contrary to well-preserved structures which are easier to understand.

During this phase the discovered objects are taken according to methods which must be defined in advance according to their nature. The most fragile objects such as organic matter and ferrous concretions can be taken only after examination of their positioning. Before removal, certain objects must be numbered and if needed they are photographed on site and a partial listing is done.

3 - Study of the structures

The structures must then be identified and marked.
They are then measured one by one in order to reconstitute their form. A series of accumulated measurements must make it possible to recreate the function of each item or element as a whole. Particular care is taken to the study of the assemblages such as the shaping of wood and the use of nails or pegs. Certain structural elements will have to be momentarily surfaced in order to be photographed and drawn.

The cutting and disassembly of some structures will be necessary to reach inaccessible parts of the site.

The reconstitution of the forms will be possible by cross-referencing information gathered from several types of measurements:
- cross sections are situated accordingly, with respect to the longitudinal axis of the wreck,
- photogrammetric listings which will allow a three dimensional reconstitution.

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All of the measurements taken will allow for to the preparation of individual maps, overview maps, and then instructional maps for the restoration of the structures (axonometric reconstitutions for example).
Models taking into account the archaeological data must allow for the study of the hull according to the pre-established research axes and lead to the completion of overall reconstitution models.

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4 - Collection of archaeological artefacts

The found objects will be indexed while at the same time keeping in memory their position on the site. If necessary they will undergo a preliminary preventive conservation procedure: for example rinsing and desalting ceramic objects, conditioning the organic matter, conserving the wooden objects in fresh water containing fungicide. The objects will then be dealt with by a laboratory specialized in the conservation of archaeological artefacts obtained from underwater excavations.

© Max Guérout