Page 101 - Manual for Activities directed at the Underwater Cultural Heritage

inventory of underwater cultural heritage. This is due to the fact that impact on heritage is considered to be part of the collateral costs that are integral to the project. Benefits and collateral costs make up the balance sheet of political decision-making in the process of authorization. Consequently, this kind of survey is usually paid for by the enterprise.
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
100 acquired accidentally. In addition, it will typically
© National Museum of Underwater Archaeology. ARQUA.Towfish of a side scan sonar.The side scan sonar is a technical device that is used to locate, map and investigate sites of archaeological interest. It can also be used in repeat surveys of archaeologically sensitive site to examine site development over a certain time period.
The side-scan system originally developed in the 1950's
from experiments using echo sounders tilted at an angle from vertical. Initial experiments were conducted to detect shoals of fish, but results immediately showed the potential of this method for studying seabed geology and the detection of wrecks.
The side scan sonar uses narrow beams of acoustic energy (sound) transmitted from either side of the towfish and across the bottom. Sound is scattered back from the bottom and
from objects to the towfish.The intensity of the backscattered signal (reverberation) is a direct function of the bottom roughness and the angle of incidence.
The rougher the bottom, the stronger the reverberation. However, roughness is a relative term and is dependent upon the frequency (and more importantly the inherent wavelength) of the acoustic pulse.
The sonar image is constructed one line of data at a time. In general, hard objects reflect more energy causing a dark (black) signal on the image, whilst soft objects do not reflect as much energy and are displayed in lighter tones of grey.The absence of sound that as shadows behind objects show up as white areas on a sonar image causes.
Preliminary work