Ultrasonic evaluation of the degradation state of marine archaeological wood

The current strategy for the management of underwater archaeological sites is in situ preservation where possible. Such an approach requires quantitative, non-destructive methods of site assessment and monitoring. A new ultrasonic transmission technique was developed for use on archaeological wooden artefacts together with a new method of signal processing. A broadband pulse with a dominant frequency of 1 MHz permitted measurement of phase velocity, attenuation coefficient and quality factor over the range of 400 - 1200 kHz. While velocity was highest in the longitudinal direction and lowest in the tangential direction, the longitudinal direction possessed the lowest attenuation coefficient and lowest quality factor. Once the ultrasonic properties of undegraded oak and pine had been characterised, measurements of phase velocity were taken on samples subjected to increasing macrofaunal and microfaunal degradation. Degradation was quantified by the decrease in conventional density and corresponding changes in velocity and its frequency dependency were investigated. The velocities of both oak and pine generally showed a decease with increasing degradation, also becoming less anisotropic. While velocity in undegraded samples of both species is generally frequency independent (within confidence limits) this changes as the samples become degraded, varying with species and propagation direction. Calculated reflection coefficients indicate that more degraded material should be easily imaged than undegraded wood by a remote acoustic source (e.g. Chirp profiler) with the exceptions of mildly degraded oak in clay and heavily degraded timbers of both species exposed in the water column. Calculated values were found to be in good agreement with those measured over several wreck sites. Progression of this method will allow more accurate detection and mapping of buried sites and monitoring of the degradation state in situ. In addition, the laboratory system could be used on excavated artefacts to aid the choice of conservation method, which is dependent on the condition of an object.