Abstract
Abstract. Knowledge of underwater topography is essential to the understanding of the organisation and distribution of archaeological sites along and in water bodies. Special attention has to be paid to intertidal and inshore zones where, due to sea-level rise, coastlines have changed and many former coastal sites are now submerged in shallow water. Mapping the detailed inshore topography is therefore important to reconstruct former coastlines, identify sunken archaeological structures and locate potential former harbour sites. However, until recently archaeology has lacked suitable methods to provide the required topographical data of shallow underwater bodies. Our research shows that airborne topo-bathymetric laser scanner systems are able to measure surfaces above and below the water table over large areas in high detail using very short and narrow green laser pulses, even revealing sunken archaeological structures in shallow water. Using an airborne laser scanner operating at a wavelength in the green visible spectrum (532 nm) two case study areas in different environmental settings (Kolone, Croatia, with clear sea water; Lake Keutschach, Austria, with turbid water) were scanned. In both cases, a digital model of the underwater topography with a planimetric resolution of a few decimeters was measured. While in the clear waters of Kolone penetration depth was up to 11 meters, turbid Lake Keutschach allowed only to document the upper 1.6 meters of its underwater topography. Our results demonstrate the potential of this technique to map submerged archaeological structures over large areas in high detail providing the possibility for systematic, large scale archaeological investigation of this environment.
Highlights
In archaeological topographic research there is a gap between land surfaces including tidal zones and underwater environments
Our research shows that airborne topo-bathymetric laser scanner systems are able to measure surfaces above and below the water table over large areas in high detail using very short and narrow green laser pulses, even revealing sunken archaeological structures in shallow water
This is unfortunate since knowledge of this shallow offshore underwater topography is essential to the understanding of the organisation and distribution of archaeological sites along and in water bodies
Summary
In archaeological topographic research there is a gap between land surfaces including tidal zones and underwater environments This is a result of two factors, where (1) different archaeological methods are being used for these differing physical environments, and (2) extremely shallow underwater bodies between 0 and 5 m depth can hardly be documented with existing methods. Airborne laser scanning (ALS) systems operating in the NIR region of the electromagnetic spectrum (usually @ 1064 and 1550 nm wavelength) do not penetrate the water column Up to recently, those systems operating with green laser (@532 nm – so called hydrographic or bathymetric ALS systems – Guenther et al, 2000) are designed for maximum water penetration and the moderate pulse repetition rate results in a rather coarse ground sampling distance of several meters It is the so called Secchi depth, which is a measure for turbidity of water and denotes the maximum depth at which a black and white or all white disk with 30 cm diameter disappears from view by the unaided human eye (Preisendorfer, 1986)
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