15 research outputs found
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
No full textThis 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)
No full textShallow 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
No full textAcoustic 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/
De Kerf bij Schoorl: inventarisatie van geomorfologie en kalkgehalten
No full textDe Schoorlse duinen worden gekenmerkt door een afwisseling van grote loopduincomplexen met uitgestrekte loopduinvlaktes. Het grootste deel van het landschap, met uitzondering van de zeereep, is gestabiliseerd, er zijn vrijwel geen actieve verstuivingen. De ingreep, het graven van de kerf, en afplaggen van de achterliggende Parnassiavallei, heeft hier verandering in gebracht. De Pamassiavallei is tijdens storm reeds een aantal malen overspoeld. Geulen die ontstaan bij volstromen van de vallei zijn kortstondige verschijnselen. Door de werking van de zee is een strandwal-achtige vorm ontstaan met een hoogte vergelijkbaar met het omliggende strand. Hierdoor zal de overstromingsfrekwentie af nemen. Aan de kerf zelf zijn door afslag kleine klifjes ontstaan. De vormen zijn deels ten gevolge van betreding door recreanten uitgewist, deels door aanstuiving. In de vallei is (zout)water achtergebleven. Het water vormt een barrière voor doorstuiving. Door de vloed bevindt zich een vloedmerk op een hoogte van circa 2.5 m NAP. Het overstroomde oppervlak is voor een deel bedekt met fijn, kalkrijk slib. Mogelijk kan dit slib verdere verstuiving belemmeren. Op verschillende plaatsen is de vegetatie overstoven. Het stuifzand is uit de vallei zelf afkomstig en is overwegend kalkloos. De vallei zelf wordt door recreanten betreden. Verwacht kan worden dat de betreding de vestiging van pioniervegetaties zal tegen gaan. Het zand in de Parnassiavallei is zeer kalkarm tot kalkloos. Het zand van de aangrenzende zeereep is matig kalkhoudend. Er is sprake van een scherpe kalkgradiënt. Na het graven van de kerf is dit niet wezenlijk veranderd. In de vallei bevindt zich nog steeds overwegend kalkloos zand aan het oppervlak. Hier overheen is wel een zeer dun laagje kalkhoudend materiaal afgezet tijdens overspoeling door de zee. Het stuivende zand binnen de kerf is niet kalkhoudend. Er is nog geen sprake van echte doorstuiving van strandzand. Waarschijnlijk is dit een gevolg van de barrièrewerking van het in de vallei aanwezige water. Het strand ten N en ten Z van de kerf is gesuppleerd met gebiedsvreemd materiaal. Het suppletiezand is zeer kalkrijk. De kalkgradiënt in de gesuppleerde gebieden is groot. De achterliggende valleien zijn voornamelijk kalkloos, met uitzondering van kleine complexen van kopjesduinen. Op de zeereep zelf is suppletiezand aangetroffen, herkenbaar aan de afwijkende kleur. Het is wellicht aan te bevelen het gebied af te sluiten voor publiek om de natuurlijke ontwikkeling niet te verstoren.EVA/DYNAMIE
Acoustic emmission gedrag van verschillende grondsoorten
No full textDit rapport vat de resultaten samen van de meetrapporten "Acoustic Emission" (A.E.) gedrag van verschillende grondsoorten door ing. F.M. Schenkeveld, LGM en "Acoustic emission laboratory testing of sand, clay and peat soils" door Prof. R.M. Koerner, Drexel University, U.S.A. Het onderzoek werd verricht in opdracht van het Centrum voor Onderzoek Waterkeringen (COW), in samenwerking met het Laboratorium voor Grondmechanica (LGM). Projectleider voor het COW was ir. A. Penning. Het doel van het onderzoek is de evaluatie van de "acoustic emission"-methode met betrekking tot stabiliteitsproblemen bij Nederlandse dijken. Tijdens het vooronderzoek is kontakt gelegd met Prof. R.M. Koerner (R.M.K.), Drexel University, U.S.A. Om een goede basis te geven aan het onderzoek werd met Prof. Koerner een gezamenlijk project uitgevoerd. "Acoustic Emissions" zijn spanningsgolven die gegenereerd worden in een materiaal onder spanning tijdens deformatie. Soms zijn deze spanningsgolven hoorbaar (kraken van hout), maar vaak niet vanwege de lage amplitude en hoge frequentie. Een piezoelectrische sensor wordt gebruikt om de "acoustic emissions" te detecteren. De opnemer produceert een electrisch signaal evenredig met de amplitude van de waargenomen trilling. Het signaal wordt dan versterkt, gefilterd en geteld of op een andere wijze gekwantificeerd. Achtergrondruis en andere ongewenste verstoringen worden gefilterd of apart gekwantificeerd en afgetrokken van de resultaten. De tellingen of andere bewerkte resultaten van de emissies worden dan gecorreleerd met het materiaal gedrag om empirisch de veiligheid van de onderzochte situatie te bepalen. Indien er geen acoustic emissions zijn is het materiaal in het algemeen in rust en stabiel. Evenwel als er emissies worden waargenomen is het materiaal in beweging, hetgeen uiteindelijk kan leiden tot bezwijken. De komponenten van het huidige "acoustic emission" systeem bestaan uit: een stalen staaf (om het signaal vanuit de grond naar een geschikt waarnemingspunt te brenger.) , een sensor (geofoon, versnellingsopnemer,hydrofoon of een andere opnemer die een mechanische trilling in een electrisch signaal omzet), een voorversterker (om het signaal te versterken indien een lange kabel wordt gebruikt), filters (om ongewenste gedeelten van het signaal te elimineren), een versterker (om het signaal verder te versterken voor signaalbewerking) en een signaalbewerkingssysteem. Het signaalbewerkingssysteem is over het algemeen een teller die telt als een bepaalde drempelwaarde wordt overschreden.TAW/EN
Cooperative interactions within the family enhance the capacity for evolutionary change in body size
No full textArticleThis is the author accepted manuscript. The final version is available from Nature Publishing Group via the DOI in this record.Classical models of evolution seldom predict the rate at which populations evolve in the wild. One explanation is that the social environment affects how traits change in response to natural selection. Here we determine how social interactions between parents and offspring, and among larvae, influence the response to experimental selection on adult size. Our experiments focus on burying beetles (Nicrophorus vespilloides), whose larvae develop within a carrion nest. Some broods exclusively self-feed on the carrion, while others are also fed by their parents. We found that populations responded to selection for larger adults, but only when parents cared for their offspring. We also found that populations responded to selection for smaller adults, but only by removing parents and causing larval interactions to exert more influence on eventual adult size. Comparative analyses revealed a similar pattern: evolutionary increases in species size within the genus Nicrophorus are associated with the obligate provision of care. Combining our results with previous studies, we suggest that cooperative social environments enhance the response to selection, whereas excessive conflict can prevent a response to further directional selection.This project was funded by a European Research Council grant (310785_Baldwinian_Beetles), and a Royal Society Wolfson Research Merit Award, both to R.M.K. We are very grateful to S.-J. Sun and D. Howard for providing unpublished information about other burying beetle species, and to C. Creighton for discussion. We thank S. Herce Castañón for help with MATLAB; M. Barclay and R. Booth from the Natural History Museum, London for their help with the beetle collections; and K. MacLeod and P. Lawrence for commenting on earlier drafts. A. Backhouse, S. Aspinall and C. Swannack maintained the beetles while A. Attisano, E. Briolat, A. Duarte and O. de Gasperin helped in the laboratory
Towards p-adaptive spectral/hp element methods for modelling industrial flows
No full textThis is the author accepted manuscript. The final version is available from Springer Verlag via the DOI in this record.There is an increasing requirement from both academia and industry for high-fidelity flow simulations that are able to accurately capture complicated and transient flow dynamics in complex geometries. Coupled with the growing availability of high-performance, highly parallel computing resources, there is therefore a demand for scalable numerical methods and corresponding software frameworks which can deliver the next-generation of complex and detailed fluid simulations to scientists and engineers in an efficient way. In this article we discuss recent and upcoming advances in the use of the spectral/hp element method for addressing these modelling challenges. To use these methods efficiently for such applications, is critical that computational resolution is placed in the regions of the flow where it is needed most, which is often not known a priori. We propose the use of spatially and temporally varying polynomial order, coupled with appropriate error estimators, as key requirements in permitting these methods to achieve computationally efficient high-fidelity solutions to complex flow problems in the fluid dynamics community.D.M. acknowledges support from the EU Horizon 2020 project ExaFLOW (grant 671571) and the PRISM project under EPSRC grant EP/L000407/1. D.S. is grateful for the support received from CNPq (grant 231787/2013–8) and FAPESP (grant 2012/23493-0). D.E. acknowledges support from the EU ITN project ANADE (grant PITN-GA-289428). S.J.S. acknowledges Royal Academy of Engineering support under their research chair scheme. R.M.K. acknowledges support from the US Army Research Office under W911NF1510222 (overseen by Dr. M. Coyle). Computing resources supported by the UK Turbulence Consortium (EPSRC grant EP/L000261/1) and the Imperial College HPC service
