MARINE ARCHAEOLOGICAL SURVEY BY HIGH-RESOLUTION SUB-BOTTOM PROFILERS BY OLE GRØN¹, ANNE NØRGÅRD JØRGENSEN², AND GERD HOFFMANN³ 1 Langelands Museum. 1994-1996: The National Museum of Denmark. The Research Centre for Maritime Archaeology. 2 Kulturarvs Styrelsen, Copenhagen. 1994-1996: The National Museum of Denmark. The Research Centre for Maritime Archaeology. 3 GEOMAR (Research Centre for Marine Geosciences), Kiel. 1. Introduction Today’s focus on marine resources and environmental factors has supported a rapid development of technology for monitoring the coastal zones. Together with an increasing awareness of the historical and archaeological sites located under water as well as a growing understanding of the enormous research potential of the well-preserved submerged settlements and landscapes as an indispensable supplement to the investigations on land (Grøn & Skaarup 2004, 2006; Skaarup & Grøn 2004), it has been a logical step to attempt application of the technology to practical marine archaeological survey in shallow water (0.5-10m). Where side-scanners have become widely used in marine archaeological survey because of their ability to quickly create a high-resolution acoustic picture of the surface and texture of the sea-bottom, high-resolution sub- bottom profilers have not gained the same popularity in practical survey. It takes longer to cover an area with profiles with a reasonable resolution/ distance between them. Furthermore the sub-bottom profiles are significantly more difficult to interpret than the side-scan data. However, this should be weighed against the fact that a large part of the cultural heritage under water, and that is often the better preserved one, can be located only with high-resolution sub-bottom profilers because it is covered by re-deposited sediments. Furthermore the sub-bottom profilers can provide valuable detailed insights into the bottom’s small-scale geomorphology, which in relation to settlement investigations is added value (van Andel & Lianos 1984, Grøn 1990). 115 Further factors stimulating the archaeological interest in the submerged cultural heritage resources are the industrial peat exploitation and agricultural drainage of the land-based peat-bogs which through the 20th century led to a significant degradation of organic artefacts in these basins. Large numbers of Stone Age sites with well-preserved dwelling remains such as the famous ones excavated at Ulkestrup Lyng and Muldbjerg in Denmark (Andersen 1978; Andersen et al. 1982; Troels-Smith 1960) have lost their important organic component and appear today as mere concentrations of lithic artefacts. We may here have witnessed one of the largest cultural heritage disasters ever. This sad development underlines the special importance of the submerged areas, where intact and well preserved peat basins still can be found, as a last research resort with regard to detailed studies of the Stone Age and of the important process of Neolithisation (Grøn 1990, 1995; Grøn & Skaarup 2004). The presence of entire submerged cultural landscapes from the Stone Age with impressive preservation provide in some areas a unique context for these sites (fig. 2). Where these landscapes have not been damaged by erosion or covered by significant layers of soft sediment, that make them practically inaccessible, such a cultural resource should be managed with extreme care and thoughtfulness. Settlement surfaces from later periods than the Stone Age must, due to the more stable sea-level in this phase, in general be expected to play a less prominent role even though landslides in some cases such as Port Royal are known with disastrous speed to have transferred parts of settlements to submerged positions with good preservation (Marx 2003). However, constructions of poles and stones from these periods, such as harbours, piers, ferry landings, bridges, barrages and other defences against naval attacks are features that have been largely overseen because they are often not preserved above the bottom level and because their surprising preservation can easily be mistaken for the Fig. 1: Locations where important marine-archaeological results were obtained remains of modern pound-nets. with the Chirp system in the period 1994-1996. 116 The main aim of this paper is to demonstrate the practical potential of acoustic high-resolution sub- bottom profilers for marine archaeological large-scale survey in shallow water (0.5- 10.0 m). The focus will be on the so-called ‘chirp’ systems, because they according to our tests produce better results with regard to archaeological sites than single and dual frequency penetration- systems. For general survey the chirp systems also at the moment seem to represent a cost-effective and manageable methodology compared to the experimental 3D-acoustic systems that exist today. Few of the features distinguished with chirp in the Fig. 2: Side-scan recording cases presented in this paper would have been distinguishable with non- showing tree stumps and penetration systems such as side-scanners and multi-beam scanners. trunks on the bottom Furthermore it will provide examples of the use of chirp systems to along the banks of a small demonstrate the significant improvement of the interpretation of submerged watercourse flooded archaeological features they facilitate. The initial development of the approximately 5000 BC cal. method was carried through as a 3-year project period from 1994-1996 by The area shown is only about the authors. The work has since been continued by Grøn. Apart from the one kilometre from the Late authors’ institutions, the Archäologisches Landesmuseum der Christian- Mesolithic dwelling and Albrechts-Universität, Schleswig, the Institut für Ur- und Frühgeschichte nearby boat-grave excavated der Christian-Albrechts-Universität, Kiel, and the Institute of Geology, at the same depth (Skaarup Aarhus, have participated. The results achieved demonstrate that acoustic & Grøn 2004). technology has reached a level where a more general application of it to marine archaeological survey will be profitable. 2. Background and Method Sub-bottom profilers are echo-sounders that penetrate the bottom sediments with acoustic signals and from their reflections generate images (today a digital one) that reflect the features of the sea bottom’s sediments to a certain depth. Low frequencies give a good penetration but poor vertical resolution, 117 and so are not able to distinguish finer details. Higher frequencies yield bad penetration but good vertical resolution: that is a more detailed picture of the features below the bottom surface. Where the older systems employ only one or two frequencies, the chirp systems sweep a frequency interval. In this case a window of 2-23 kHz was used that combines a band of different frequencies with different penetration and resolution and has proved to be well suited for distinguishing archaeological features in shallow waters. Traditional systems with one hydrophone could not be operated successfully at depths less than a couple of metres. The application of more hydrophones to the systems has made it possible to obtain reasonable results at depths as small as 0.5 metres. While small depths are one problem, large depths are another. The longer the acoustic signal has to travel, the more it will be spread and accordingly lose its precision. We found that 0.5-10.0 m depth is an ideal operation area. At larger depths the ‘fish’ (transducer unit) with the hydrophones can be ‘deep-towed’ to obtain a reasonable recording distance to the bottom. An important improvement is the development of real-time processing of the data that facilitate a morphologically correct presentation of features embedded in the bottom. With earlier systems, anomalies could be distinguished only as diffraction hyperbolas that were mostly very difficult to interpret. The efficiency of the method is demonstrated by our findings in Denmark in the project period: 7 barrages (underwater defences against ship attacks), 2 ferry landings and 1 possible breakwater that may have protected an old harbour. One of the barrages found is of a type hitherto unknown in this area. So far, the seven structures from Denmark that have received radiocarbon and dendrochronological datings belong to the period 80-1635 AD (1). Furthermore, 25 acoustic anomalies were observed and Fig. 3: Left: Recording with low degree of processing. Object in water recorded as verified in the harbour area of diffraction hyperbola. Illustration modified from Stümpel & Borth Hoffmann the Viking Age trading centre (1983). Right: Immediately processed chirp-recording (CAP 6600 from Datasonics). Stake, 8 cm in diameter, standing up in water and visible in the soft bottom at Hedeby/Haithabu, Schleswig, sediments as seen in the 2-7 kHz interval (upper profile) and 8-23 kHz interval Northern Germany (Grøn et al. (lower profile). Recording: Ole Grøn and Lars Erik Larsen, 1996. 1998). 118 For the survey a Chirp II system from Datasonics/Benthos (CAP-6600) was used. This system sweeps the interval from 2-23 kHz on two channels and shows good penetration even in sandy sediments with some organic content and in shallow water. It is important that the transducer unit (the ‘fish’) which can be operated in depths as small as 0.3 m is of such a size and weight that it can be transported by a single person. For the dynamic positioning, an Ashtech DGPS-navigation system with its own basis station was used. Every second a position with an accuracy of ± 0.3 m was stored with the seismic data in the seg-y format. The precise dynamic positioning allowed us to deviate from sailing straight parallel lines when this was preferable. Data recorded by sailing straight parallel lines tend to be difficult to interpret. Potentially interesting anomalies registered during such a recording would demand follow-up recording with several cross-profiles to establish their character as much as possible by seismics alone. On the basis of our experiences we trained in instant recognition of anomalies and immediate seismic check of their character instead of accepting a later recognition through processing in the laboratory as the basis for ensuing action.