Preserving underwater cultural heritage: combining satellite images with a novel flash lidar platform

Abstract

Les Argilliez, part of the UNESCO World Heritage Site “Prehistoric Pile Dwellings around the Alps,” is in Lake Neuchâtel (Switzerland) and dates to the Classical Cortaillod (3841-3817 BC) and Late Cortaillod (around 3500 BC) cultures. Among other artifacts, it consists of 4,834 wooden piles found over a 7000 m2 area ranging from 2 m to 3 m depth below the water surface. Two dangers threaten the preservation of the site: erosion and the proliferation of invasive mussel species: Dreissena rostriformis bugensis (quagga mussel) and Dreissena polymorpha (zebra mussel), which pose the specific threat risk of degrading the wooden piles. This study presents two methods developed for the monitoring of the erosion and mussel populations at Les Argilliez: a new flash lidar based 3D imaging platform and satellite image data analysis. Together, the two data collection schemes allow for efficient identification, quantification and tracking of environmental risks threatening underwater archeological sites, such as this one. The flash lidar is optimized for underwater applications and designed to collect 3D point cloud data from a medium-sized unmanned surface vehicle (USV). The lidar system is batterypowered and features a 128×128 pixel focal plane array for high resolution point cloud capture. When mounted to the USV, the lidar enables regular and efficient lake surveys, allowing consistent comparison of the same locations over time. The point cloud data enables measuring the height and orientation of the wooden piles and the lakebed profile. The first demonstrative lidar measurement results are presented in this study. In addition, spectral data from satellite images taken by Sentinel-2 was compared with simulated reflectance spectra for sand, macrophytes and mussels. The processed satellite data successfully identifies spectral anomalies correlated to the proliferation of quagga mussels in the area over a period of four years, and is confirmed by underwater surveys done by divers.