Maritime Archaeological Registration 2: sediment echo-sounding:

Use of sediment echo-sounding for location of archaeological sites under water


It falls naturally within the work of the National Museum's Centre for Maritime Archaeology to further the development of methods for locating archaeological sites on the sea-bed. Visibility in Danish waters is generally poor. Often one can only see a couple of metres, and sometimes nothing at all. It therefore goes without saying that surveying by diving and swimming through large areas is time-consuming and disproportionately expensive.When it is a matter of needing to see what lies hidden in the sea-bed, divers swimming around are of even less use. Where the seabed is soft a diver can of course reach down and feel around, but anyone who has tried this will know that it is a tiring and very lengthy process.

In contrast to light, sound has very good qualities of penetration through water, regardless of whether the water is clear or cloudy. Sound is therefore much more usable when we want to 'see' far underwater into the water-saturated sediment. During the last 20-30 years work has been carried out on developing echo-sounders which are not only capable of showing the surface of the sea-bed, but can also 'see' down into the sediment below. These are described as 'penetration echo-sounders' or 'sediment echo-sounders'. The developments in transportable computer-technology within recent years have meant that it has become possible to process the returning sound-signals so that many sources of noise-interference can be removed. The results, in the form of printed echograms, have therefore become more reliable and much more detailed than hitherto.

The use of this advanced technology is subject to a number of fundamental 'rules of the game', which need to be carefully applied to obtain optimal benefits. The lower the frequency one uses, the greater the penetration-quality of the sound. But at the same time its ability to show small structures decreases. This is known as coarse resolution. The higher the frequency one uses, the smaller the objects one is able to observe, but then the ability of the sound to penetrate the sediment is drastically reduced. As a rule, a frequency of around 30 kHz (30,000 oscillations per second) can reach a couple of metres down into the sediment and also make it possible to observe structures of a size of about 5cm (this is known as a resolution of around 5 cm). This is an acceptable compromise for archaeological purposes.

Equipment of the 'boomer' type, which sends out a 'burst' of frequencies all at the same time (e.g. 0.7-10 kHz), or the new CHIRP II, which sends out a 'swirp' of frequencies from 8 to 22 kHz, gives fine resolution combined with good penetration-capacity. The disadvantage with this is the processing of the returning sound-signals, which is much more complex than in the case of conventional sediment echo-sounders.

Our knowledge of how different types of relics show up on sediment echo-sounders is very meagre. In the course of 1994 and 1995, therefore, trials were made over a number of underwater sites with historic remains which had already been registered and surveyed. The objective was to compare different makes of equipment showing the same structure, to find out which was best suited for archaeological use.

The main trial area selected was the mouth of Haderslev Fjord, with two blockages from the Iron Age and Middle Ages: Margrethes Bro and Æ Lei. Their siting - typically - was at a narrow part of the fjord. Æ Lei is situated well below the surface of some very soft sediment, as is found in many sheltered fjords, while Margrethes Bro, because of its site close to the mouth of the fjord, lay in significantly sandier deposits which would constitute a greater problem for sound-penetration.

As expected, the blockage further inland, Æ Lei, gave the better result. Margretes Bro could also be discerned, and the results have to be said to exceed our expectations; the prospects look good for the detection of so far unknown blockages in coming years with the help of sediment echo-sounders.

The very wet winter of 1994/95, with flooding of large low-lying field-areas, brought with it the idea of trying out sediment echo-sounders in areas which were normally 'inland sites'. We found out that it is not possible to shoot sound through flooded grass. Its high air-content simply blocks the sound. There was a surprising result, however, when we tried to run the sediment echo-sounder in a fishpond which is dug out of Ravning Enge near Vejle, just above the course of a major Viking-Age bridge. The posts of the bridge could be seen about 1 m down into the sediment. They were standing where we expected them to be, but just a few metres to one side there was an identical stretch of bridge. The posts we could see on the echo-sounder could also be felt when we probed the sediment. Mogens Schou Jørgensen's planned excavation in the fishpond has thus suddenly taken on a completely different character, and we are now waiting in suspense for the Agency for Forestry and Nature's decision as to what should happen to the fishpond in future.

It is important to establish whether it is possible, with the use of a sediment echo-sounder, to find layers from Stone-Age settlements which are now under water. In recent years, just off the small island of Blak, in Roskilde Fjord, the Færgegården Museum has been excavating a settlement from the Kongemose culture in water a couple of metres deep. At the south boundary of the excavation it has been shown that the culture-layer is situated about 1.5 metres under the surface of the sediment. A seismic profile produced by a Chirp I in fact shows a layer starting at the edge of the now filled-in excavation-area, about 1.5 m under the surface of the sediment, and continuing downwards. An attempt will be made in March 1995, by drilling, to establish whether what we can see is the known culture-layer, or whether it is something else. Whatever the result, it is important to note whether it will prove possible to identify such thin layers.

In conclusion it has to be said that the provisional results of trials with sound-penetration seem promising. Either we have just been extremely lucky so far, or else the method is capable in future providing a considerable increase in the quantity of known underwater remains. The testing out of the different types of equipment described in this article was made possible as a result of help and support from the following firms: EIVA in Hasselager, Denmark; NAUTIKARIS in Saandport, Holland; JON B. JOLLY Inc., Seattle, USA; KNUDSEN ENGINEERING Ltd., Ontario, Canada; and FURUNO DANMARK, Denmark.

Ole Grøn


Jorn.Sjostrom@natmus.min.dk
Copyright © 1995 The National Museum of Denmark
Last Updated august 6, 1995