Sidebar: compiling the inventory

Under the Convention, States Parties are obliged to establish a ‘competent authority’ and to provide for the establishment, maintenance and updating of an inventory of underwater cultural heritage (Art. 22). In practice, this inventory is the archive or the index to the archive in which cumulative information on existing heritage sites is retained. It is a key element in the protection and management of underwater cultural heritage. Preliminary work builds on the inventory, on the one hand, and is one of its major sources on the other. For this reason, the sidebar on inventories has been integrated in the chapter on preliminary work.

In the process of compiling an inventory, the competent authority will be confronted with very different kinds of information. Part of this will be acquired accidentally. In addition, it will typically be enhanced by corroboration and gradual addition whereas other elements of information will be acquired by focused desktop research and active field inventory.


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Reasons for inventorying

Inventories are important for a number of reasons:

  • to enable effective protection of the underwater cultural heritage;
  • to identify and record the underwater cultural heritage;
  • to get an overview of all the heritage sites;
  • to compare sites in order to correctly direct funds and attention to significant heritage;
  • to provide a single point of access to information on the underwater cultural heritage;
  • to provide a major resource for heritage researchers, consultant archaeologists, local government authorities, government agencies, developers and students;
  • to raise support for the endangered heritage;
  • and ultimately, to celebrate the wealth of underwater cultural heritage and to safeguard the underwater cultural heritage.
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Accidentally acquired information


In order to inventory existing heritage, a competent authority can start by actively acquiring data that is collected for other purposes, such as navigation safety, mapping of other resources, clearing of navigation channels or fishing. It can then evaluate this data for its heritage relevance. Various government and private agencies can provide such information on finds in the underwater environment. National authorities, ministries and departments undertaking activities on the seabed or riverbed, as for instance coastguards, the navy, dredging services, research services, fisheries monitoring, etc. should be required to confidentially communicate information on underwater cultural heritage that is found, or on activities concerning or affecting such heritage, to the competent national authorities. Information and cooperation can also be requested from hydrographic and oceanographic services.

Furthermore, fishermen and mariners will also collect relevant data. Private individuals, people in the recreational diving industry, tour operators and others can provide the competent authority with information. Many sites are also likely to be first reported from hearsay. The underwater world is still a world of limited access. Making use of informants is mutually beneficial as it helps the authority and gives the informants a role. It also helps the latter to understand the policies and values of heritage. It is especially in reporting incidental observations that interested recreationists and vocational archaeologists can be of enormous value for a better protection of heritage.

Although it is important to distinguish between established facts and uncorroborated information, it is also important to keep track of even hazy and vague reports by entering them in the inventory with the necessary qualifications and question marks.

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Desktop study and background information


Typically, an inventory also includes the investigation of historical, geological and environmental data that is available in a range of repositories, in just the same way as discussed for preliminary research. A comparison with conditions, processes and heritage found on land can for instance provide insight concerning the possible existence of submerged landscapes and prehistoric sites under water. Library research can provide information on catastrophes. Shipping registers and naval inventories can provide information on shipwrecks.

Before undertaking any practical survey a desk-based inventory and assessment of data would also address questions, such as: are there any records on submerged or sunken heritage? What does the geological record tell us about subsidence and submergence? What does the historical record tell us about beaches, natural harbours and their use? What can a careful analysis of the coastal landscape contribute to a better understanding of the underwater area? Can time series of depth records be constructed that allow for the modelling of erosion and accretion? Have corings or geotechnical soundings been made prior to the construction of breakwaters or offshore installations? Is there other relevant research? A combined scrutiny of such data would first of all help prioritize which bodies of water deserve special attention on the basis of prior knowledge on underwater cultural heritage and its potential for preservation. Predictive modelling in a simple or more advanced Geographical Information System (GIS) can be an enormous help in this process. It provides an inexpensive tool to manage large amounts of very disparate data in combination with expert knowledge.

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Active search


While a ‘passive’ inventory by assessing accidentally acquired information and historic information is relatively inexpensive, it can help more targeted work enormously. It also provides a basis to assess the reliability of different informants and information sources. It is not, however, the only option the competent authority has at its disposal. It can also actively commission or undertake specific surveys, or it may make use of impact assessments for projects to investigate an area.

Specific surveys in the field can be undertaken, and will usually include an on-water phase deploying geophysical techniques and an underwater phase for ground truthing by diving, sampling or remote access through the use of autonomous or remotely operated underwater vehicles. Normally, such inventory would be limited to a project area. This allows for good state-of- the-art surveys, without costs getting out of hand. The project areas should be strategically chosen in order, for instance, to manage the heritage of a specific reserve, or to target areas that are under particular stress. These could be estuaries, harbour approaches or areas of anticipated development.

Active inventory and impact assessment are very often tools that complement each other and that follow the same logic. They differ with regards to the occasion and the costs. Impact assessment is an integral part of a proposed project and is therefore generally regarded as an integrated cost-factor, while an inventory project needs to secure its own funds. It is therefore advisable to aim for synergies and to build up an inventory on already available existing information.

It is preferable, if not indispensable, for all sites in a project area to be assessed individually. If this is the case, a decision on each individual site can be taken. Some will be considered significant enough to warrant full-scale excavation. Others will demand a limited number of observations and others again may be sacrificed in favour of the most important ones or more important purpose. The relative weight that is given to their importance in the context of the development project and relevant policies will inform the selection process.

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Active search - phases and techniques of survey


A full-scale survey should be based on prior desk-based assessment and will then generally involve extensive field-data acquisition in combined on-water and underwater phases. The geological and geophysical techniques involved are seismics, coring and resistance sounding for the general stratigraphic make-up, and such acoustic techniques as sidescan sonar and swath bathymetry or multibeam echo-sounding to map the bottom surface. 

Seismic techniques

Seismic techniques are generally not fine-grained enough to enable the direct location of archaeological sites. The integration of cumulative seismic data in a regional analysis, however, produces fascinating and informative images of submerged palaeo-landscapes. Large amounts of seismic data have been produced by industry during exploration for mineral resources. This applies to all water regions of the world. Although produced for other purposes, their analysis on the basis of archaeological research questions is highly informative, both in inventory and impact assessment. Hence, also, the importance of desk-based work, using data collected for other purposes. 

Acoustic techniques

Like seismics, acoustic techniques, such as sidescan and multibeam sonar, are used for many purposes beyond heritage. Nevertheless, sidescans and multibeams are more regularly put to direct archaeological use. So are magnetometers and sub-bottom profilers. All such techniques acquire their data digitally and can be integrated with GPS position control so that the resulting images give amazing detail.  

Sidescan sonars and GPS instruments

Sidescan sonar and GPS instruments come in different price ranges and are available for mounting on large and small research vessels or even rubber boats alike. In all instances, however, the equipment is only as good as its operator. In preliminary work, there is much to be said for combining different purposes in one survey, undertaken by technically competent operators in combination with analysis by knowledgeable archaeologists. The developer will want to know what kind of obstructions feature on the surface of the seabed and how they warrant the presence of archaeological sites. Side-scanning sonar can obviously be used for both, possibly followed by targeted survey to produce more detailed images by multibeam or video. Magnetometers show the presence of metal and can be deployed to locate metal in underwater cultural heritage as well as to locate lost or dumped ammunition or erratic mines. Sub-bottom profilers are used in the same way as seismics, but for shallower sediments. They are also used for intensive survey of features that only partly show on the surface of the seabed. Scour-marks may, for instance, reflect buried features.

Terms of reference

In addition to being only as good as the operator, surveys are only as good as their terms of reference. It is therefore essential to make the most of the screening and scoping phases of an impact assessment for an industrial project. The resolution that is needed for one purpose may not be good enough for another. It is easier to locate a pipeline for instance, than to interpret a vague feature of potential archaeological importance. Coordinating and agreeing upon terms of reference may avoid requiring a survey to be done twice, and will thus save substantially in costs. Heritage sites that are fully covered by sediment are still hard to locate prior to disturbance of the silt material. It is therefore useful to agree on supervision of a development project during critical phases of dredging or ground moving in sensitive zones, and to agree on set protocols on how to deal with those finds that can be expected to turn up during realization.

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Active research - survey techniques


A survey is usually conducted today using a combination of four techniques:

  • Side-Scan Sonar
  • Magnetometer
  • Swath Bathymetry (Multibeam Echo Sounder)
  • Diver (in shallow water) or video/ROV (in deep water) ground truthing

It is important to train the operating staff  well. The time for the project depends on the area covered.

Data collecting should be integrated with GPS-positioning.

A side-scan sonar survey should include overlapping lanes and cross-angles in sufficient redundancy to warrant discovery.

A magnetometer is of no use for submerged sites and of limited use for wooden wrecks predating the use of iron guns.

Multibeam sonar and visual inspection can be deployed to get a better image of an identified irregularity.

A multibeam area survey, also called Swath Bathymetry can be highly revealing, especially at high resolution. It is still, however, an expensive technique, requiring an expert operator. Besides being attractive for archaeology, it is preferred by more and more water and harbour authorities. This means it is useful to try and combine objectives and seek cooperation.

A survey will only detect objects on the surface of the seabed, so always include depth of sedimentation and prediction of potential in a report.

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Active research - strategic search


The potential for the presence of underwater cultural heritage is a factor in prioritizing where to engage in active survey. Another is the anticipation of political and spatial planning that may result in threats to the long-term preservation of underwater cultural heritage.

Known battlefields, indications on the location of sunken cities or the historic documentation of events in relation to ports or landing places can help focus the survey.

Moreover, it can often be anticipated in which areas future windfarms, offshore installations or artificial islands will be planned.

Prioritizing such areas in an inventory will help in future decisions and will facilitate the prominent inclusion of underwater cultural heritage in the terms of reference for impact studies. Planning major development projects in the maritime environment implies a preliminary study of the assessment of their impact. This should be preliminary to the decision to realize the project. It should also be done before deciding on the project’s final location and spatial scope. One of the objectives is to minimize harm to underwater cultural heritage. Sometimes the most significant heritage sites can thus be preserved and excluded from the development project area, at other times they can be meaningfully integrated. In both cases, destructive excavation can remain limited in favour of in situ preservation. For other sites, this will not be possible. Making the best of them is a major challenge. However, it is also a major opportunity for research through excavation. If researchers strategically address those sites that will be demolished anyway, they can warrant that destruction combines with creation. Preparing a research agenda in advance may be very helpful in this context.

It is still difficult to locate sites that are deeply buried. This is the case on land, but even more so at sea, where planned developments may imply extensive dredging. If deep layers of sand, clay and peat are to be dredged, the preliminary assessment should address the probability of sites being present, whether they have actually been located or not. These can be sites of different categories, for instance sites related to deeply buried land surfaces in an area where such land is submerged, or wreck sites relating to periods of major sedimentation. On the basis of such prediction, a plan can then be drawn. A strict scenario or protocol of mitigation can be included in the planning of the development project. Protocols can be different for each category of potential find. They can, for instance, include crude removal of large remains and more careful treatment of other types of sites. Agreeing on such protocols has a dual benefit. On the one hand, it will urge researchers and heritage managers to think clearly and positively about opportunities and priorities. On the other hand, it will make the planning of contingencies controllable, and that is an asset in complicated project management.

Note that many of the techniques used in archaeological inventory, including desk-based research, on-water survey and underwater truthing, equally apply to elements that are not explicitly identified as heritage. If these constitute dumped or otherwise lost polluted material, containers with toxic substances or ammunition, then it is very important for management to be aware. In planning active inventorying, it is essential to identify synergy through combination of objectives right from the start in the inventory project design.

  •  An example of a site potentially rich in underwater cultural heritage and concerned by intensive development is the reclamation and offshore islands for housing along the coasts of Bahreïn, partly just offshore Qal’at al-Bahrain. The very extensive site was inscribed on the World Heritage list in 2005 as the Ancient Harbour and Capital of Dilmun. The offshore areas are outside the protection zone. Some have been surveyed and inventoried, but most have not, to the great concern of those presently integrating heritage values in planning.
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