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When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES SCHOOL OF OCEAN AND EARTH SCIENCE 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 Angeliki Zisi Thesis for the degree of Doctor of Philosophy October 2015 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES SCHOOL OF OCEAN AND EARTH SCIENCE Thesis for the degree of Doctor of Philosophy 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 Angeliki Zisi 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. i ii Table of Contents Chapter 1....................................................................................................................... 1 1.1 Rationale ......................................................................................................................... 1 1.2 Value of archaeological material ....................................................................................2 1.3 Work significance ...........................................................................................................3 1.3.1 Preservation in situ .....................................................................................................3 1.3.1.1 Defining state of preservation of a shipwreck ................................................... 4 1.4 Evaluating density of waterlogged wood whilst in situ: Current options ..................... 7 1.4.1 Destructive and near non-destructive methods....................................................... 7 1.4.2 Non-destructive ..................................................................................................... 9 1.4.2.1 Ultrasound propagation in wood ...................................................................... 10 1.5 Research aims ............................................................................................................... 15 1.6 Chapter outline ............................................................................................................. 17 Chapter 2 .................................................................................................................... 19 2.1 Introduction .................................................................................................................. 19 2.2 Shipbuilding .................................................................................................................. 21 2.2.1 Wood for shipbuilding .......................................................................................... 21 2.2.2 Shipbuilding traditions and timber conversion techniques................................ 33 2.3 Wreck formation: Prospects of wood hull preservation ...............................................38 2.4 Character of marine waterlogged archaeological wood ............................................... 51 2.4.1 Disintegration due to physical, biological and chemical processes .................... 51 2.4.2 Physical and mechanical properties of marine waterlogged archaeological wood 53 2.5 Acoustic properties of waterlogged archaeological wood ............................................54 2.5.1 Influence of structure and density ....................................................................... 57 2.5.2 Influence of insonification angle in relation to tree growth axes ...................... 62 2.5.3 Influence of moisture content ............................................................................. 64 2.6 Chapter conclusions .................................................................................................... 66 Chapter 3 ................................................................................................................... 69 3.1 Introduction ................................................................................................................. 69 iii 3.2 Wood test-piece preparation ........................................................................................ 70 3.2.1 Selection of the appropriate wood sample .......................................................... 70 3.2.2 Production of test-pieces...................................................................................... 72 3.2.3 Geometry of the radiation field – Phenomena affecting wave propagation in wood 74 3.2.3.1 Wavelengths passing through wood ................................................................ 75 3.2.3.2 Near / far field ................................................................................................ 76 3.2.3.3 Beam divergence – Edge effects ........................................................................ 77 3.2.4 Test-piece waterlogging procedure...................................................................... 78 3.3 Equipment ....................................................................................................................80 3.4 Method ......................................................................................................................... 83 3.4.1 Propagation of uncertainty ..................................................................................86 3.5 Results .......................................................................................................................... 87 3.5.1 Reliability of the measuring system ..................................................................... 87 3.6 Chapter conclusions .....................................................................................................90 Chapter 4 .................................................................................................................... 93 4.1 Introduction ................................................................................................................. 93 4.1.1 Test-piece preparation .............................................................................................96 4.1.2 Artificial degradation........................................................................................... 103 4.1.2.1 Drilling procedure ........................................................................................... 104 4.1.2.2 Calculating wood density after drilling .......................................................... 106 4.1.2.3 Chemical treatment with alkali .......................................................................107 4.1.3 Ultrasound testing .............................................................................................. 108 4.2 Results .......................................................................................................................