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The acoustic imaging, reconstruction and characterization of buried archaeological material
No full textMaritime archaeologists are increasingly required to investigate important sites in a nondestructivemanner. Therefore, this thesis aimed to adapt and apply sub-bottom acoustictechniques to shallowly buried shipwrecks and submerged palaeo-landsurfaces.The imaging of archaeological sites requires a geophysical source with high resolving power. Amodel was developed to determine the vertical resolution of the Chirp sub-bottom profiler, thesource used throughout this thesis. The horizontal resolution, on the other hand, was determined bymodelling the directivity pattern of an array of Chirp transducers. The results were then used tochoose a suitable Chirp sweep to image shallowly buried objects. Three sites were selected to testthe ability of the system to image, reconstruct and characterize archaeological material. The firstsite comprised of a shipwreck buried within muddy riverine sediments: the Grace Dieu (1418).The survey utilized both a 2D- and 3D-Chirp system. In both cases, the source was pushed over thesite by divers. Close survey line spacing, accurate navigation and decimeter-scale resolution dataenabled the construction of a pseudo- and full 3D-image of the site. In the second site, theYarmouth Roads Wreck (16th century), an acoustic blanking zone, a consequence of the highlyattenuating character of degraded wood, was detected and used to create maps of the buriedremains. The final site concentrated on the detection of peat layers in the English Channel, whichare crux to palaeo-landscape reconstructions. Results suggest that peat buried in fine to mediumgrained sediments has an easily identifiable acoustic signature. The seismic data acquired for theaforementioned case studies was subsequently used for quantitative studies of the buried material.Firstly, reflection coefficients were calculated and used to determine the degradation state of thehull remains. Secondly, the Quality factor was calculated for the Yarmouth Roads Wreck site toobjectively demonstrate the highly attenuating character of degraded wood buried in sediments incomparison to the surrounding sediments.Overall, this PhD has demonstrated the potential of the Chirp sub-bottom system forarchaeological purposes, by creating (pseudo-) 3D reconstructions and making objectcharacterization possible
3D reconstruction of a shallow archaeological site from high-resolution acoustic imagery - a case study
Get PDFThis paper compares a 2D and 3D-Chirp geophysical investigation of a buried shipwreck: the Grace Dieu (1418). The site, covered by 2-5m of water, is buried within muddy inter-tidal sediments. At exceptionally low tides, a few of the marginal timbers are exposed. The marine survey utilised a 2D-Chirp systems with a DGPS navigational system, and a 3-Chirp system with RTK-positioning capability. In both cases, the source was towed over the site by divers. Close survey line spacing, accurate navigation and decimetre scale resolution data enabled the construction of a pseudo- and full 3D-image of the site, which were calibrated against an RTK-GPS-terrestrial survey. The exercise demonstrated the improved spatial resolution provided by the 3D-Chirp system and the ability of a well-constrained 2D survey to produce a pseudo-3D reconstruction of the gross ship structure
Mapping of the buried Yarmouth Roads wreck, Isle of Wight, UK, using a Chirp Sub-Bottom Profiler
No full textRemains of the 16th-century Yarmouth Roads wreck, buried in sediments, were imaged using high-resolution geophysical techniques. The remains appear as strong reflectors underlain by an acoustic blanking zone, which was used to create maps of the wreck material. Close survey line-spacing allowed the construction of contour maps and hull sections, which revealed that the bow and stern sections are tilted to port at different angles, implying that they are detached. The seismic data have enhanced our understanding of the site beyond what was known from conventional archaeological investigations and confirms that this wreck was once a large carrack.<br/
High resolution acoustic imagery from a shallowly buried shipwreck: a case study (abstract of poster presented at the EGU General Assembly 2005, Vienna, 24-29 April 2005)
Get PDFShallow water, inter-tidal to immediate sub-tidal (< 5 m), archaeological shipwreck sites present unique challenges to geophysical investigation. Retrieval of reliable data is very difficult through the combination of issues of water column bubble turbulence and the simple geometry of the acoustic. This poster will present an acquisition method that tackles both of these problems through the combination of a purpose built, high frequency (150 kHz) narrow beam (3o) transceiver, Real-Time Kinematic (centimetric accuracy) navigation and a non-motorised deployment method.
The test site for this system was the Grace Dieu (1420), the flagship of Henry V, which was scuttled at its berth in the Hamble River (UK). The site is typically covered by a maximum of 2-5 m of water only being exposed at exceptionally low tides during the spring equinox. The site is partially buried within muddy inter-tidal sediments to a maximum depth of 1.5 m.
The marine survey utilised the 150 kHz transceiver mounted on a catamaran along with an Ashtec marine RTK-GPS navigational system and attitude sensor. The catamaran was towed over the site using divers. Close survey line spacing (< 1 m), centimetric accurate navigation and a theoretically < 10 cm vertical and horizontal resolution enable the accurate presentation of a pseudo-3D acoustic image of this buried wreck. A full and pseudo-3D interpretation of the site has been calibrated against known archaeological site investigation data and an RTK-GPS terrestrial survey.
This poster will demonstrate not only the effectiveness of this system for shallow water archaeological survey but also shallow water detection of any buried object
3D reconstruction of a shallow archaeological site from high-resolution acoustic imagery: The Grace Dieu
Get PDFAcoustic imaging and characterisation of buried objects (and in particular archaeological materials) in shallow-water (<5 m) is often unsuccessful owing to problems related to vessel-induced bubble turbulence and the restricted acoustic geometry of the system. A 2D surveying method that tackles these problems has been tested on the known wreck of the Grace Dieu (1418), Henry V’s flagship, currently buried within the inter-tidal sediments of the Hamble River. The wooden hull is recognisable in the seismic sections as a high amplitude anomaly underlain by an acoustic blanking zone. Close survey line spacing (ca. 1 m) allowed the construction of time slices, identifying the ovate plan of the hull. High, predominantly negative, reflection coefficients suggest this anomaly corresponds to degraded oak timbers buried within the sediment. Combining the data enabled the construction of a (pseudo)-3D image, revealing the dimensions and shape of the hull remains for the first time
Sedimentology and depositional history of Holocene sandy contourites on the lower slope of the Faroe-Shetland Channel, northwest of the UK
No full textThermohaline ocean currents with near-bed velocities of a few tens of cm s-1 have the ability to transport and sort sand, which is often deposited in thin sheets referred to as ‘contourite’ sand sheets. We examine a small sandy contourite sheet located in 800-1000 m water depth on the west Shetland slope in the Faroe-Shetland Channel. We describe the sedimentology of the contourite, and the relationship between sediment transport processes and contourite distribution through the last deglaciation and Holocene, based on a set of 17 short cores. Four sedimentological units ranging from very poorly sorted mud at the base (unit 1) to moderately sorted, slightly muddy, fine sand at the top (unit 4) are recognised. Ice-rafted detritus (IRD) occurs in the lower three units, but is absent in unit 4. The upper two units, 3 and 4, make up the contourite sand sheet. Analysis of the sand fraction of units 3 and 4 shows that the mean sand grain size is remarkably constant at around 3 throughout, except in some unit 3 samples with unusually high IRD content, where the mean can be coarser. IRD content correlates strongly with mean grain size, sorting and skewness throughout unit 3 even when the IRD content is small, showing that IRD input is one of the key processes controlling the sedimentology of unit 3. We conclude that the muddy sand contourites form by incomplete winnowing of the underlying glaciomarine sediments. The mud component is transported with the sand from the same source, either as small particles attached to sand grains or as sand-sized mud aggregates. Thickness maps of units 3 and 4 show that they have different depocentres, with unit 4 displaced northwest (downslope) relative to unit 3. Sedimentological trends in unit 3 are inconsistent and show no consistent transport direction. In contrast, clear across slope trends are seen in unit 4, with weak alongslope trends from the centre to both ends of the elongate contourite body. This suggests that the upper part of the contourite, unit 4, is derived from the downslope reworking of unit 3 sediment. There is no evidence for significant alongslope transport in either unit 3 or unit 4. The patchy occurrence of sandy contourites along the west Shetland slope is interpreted to result from localised export of sand from further upslope. The usefulness of the contourite sand in palaeoceanographic terms is limited. No meaningful dates can be derived from the sediments because of reworking and bioturbation. However, the presence of IRD within unit 3 shows that the Holocene circulation pattern, with strong bottom water flow through the Faroe-Shetland Channel, was established while there was still significant floating ice in the area.<br/
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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