Relationship between wood density and ultrasound propagation velocity: a non-destructive evaluation of waterlogged archaeological wood state of preservation based on its underwater acoustic properties

With current progress in marine geophysics equipment, survey and processing techniques, we can be now closer to support needs emerging after decades of maritime archaeology and conservation practice worldwide. Acoustics have been suggested as an appropriate non-intrusive method for investigating marine archaeological sites and as a proxy for degradation of submerged archaeological timbers whilst in situ. The present study has significantly contributed to the use of ultrasound by conservators and archaeologists in the field of Maritime Archaeology. A new scientific way has been developed to evaluate the degree of degradation of waterlogged archaeological wood, using a reliable relationship between its physical properties during degradation and its corresponding acoustical properties. For this purpose a new reliable experimental ultrasound set-up and measurement methodology were first developed, followed by initial experiments on “fresh” waterlogged wood, degraded with a new artificial degradation process simulating closely the degradation patterns of wood recovered from marine environments. These experiments led to the establishment of reliable calibration curves between wood’s density and ultrasound velocity. To evaluate the efficiency of the calibration curves, waterlogged archaeological samples from the National Museum of Denmark were successfully tested with ultrasound. The technique would be invaluable for conservators working in the laboratory for assessing the state of preservation of small wooden mobile artefacts and waterlogged timbers. Further, the data produced were used to improve reflection coefficients for waterlogged archaeological wood and add on our understanding and potential of its remote acoustic characterization whilst in situ. On-going research will support the correlation between the laboratorial results and the real world