If we wish to understand the surviving shipwreck record and to inform strategies of heritage, man... more If we wish to understand the surviving shipwreck record and to inform strategies of heritage, management and conservation an improved knowledge of the variable impact of marine physical processes is required. I am approaching this problem at a range of scales from full wreck to individual artefact scale, through a combination of bathymetric survey, diver monitoring and physical modelling in a controlled laboratory environment. The first phase of this work has involved the integration of time-lapse studies (months to years) of 8 shipwreck sites in a range of oceanographic conditions (e.g. tidally dominated, wave dominated and combined flow regimes). Despite having had decades to hundreds of years to reach equilibrium these wreck sites still exhibit metres of change in bedlevel change over periods of months. Preliminary results suggest these sites adhere to predictable patterning derived from the initial physical models. To determine the reproducibility of these results a much larger ...
The Outer Thames Estuary Regional Environmental Characterisation (REC) provides an environmental ... more The Outer Thames Estuary Regional Environmental Characterisation (REC) provides an environmental reference statement defining marine and seabed conditions within the study area. Prior to this study, regional environmental assessment of the Outer Thames Estuary was based upon ...
ABSTRACT Evidence from the Severn Estuary demonstrates that this region was exploited by Mesolith... more ABSTRACT Evidence from the Severn Estuary demonstrates that this region was exploited by Mesolithic hunter-gatherer-fishers (Bell, 2007). The potential for future archaeological discoveries (Bell, 2007; Webster, 2007: 273; Bell and Warren, 2013: 39), and the well-preserved palaeoenvironmental evidence in the fine-grained and organic sediments of the Somerset, Avon and Gwent Levels (Hosfield et al., 2007a: 40) makes the area of importance for archaeological study. Small quantities of worked flint have been recovered from the foreshore around Stolford, Porlock and Minehead Bay (Mullin et al., 2009; Canti et al., 1995) implying human activity in the present intertidal zone, which is further enhanced by the suggestion of possible deliberate burning of reed swamps (Jones et al., 2005) similar to that postulated in the Severn Estuary (Brown, 2005 and Timpany, 2005; Bell, 2007).
The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall eve... more The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall event on the 18 May 2005. This event triggered widespread landslips and large debris flows in the Awatarariki and Waitepuru catchments behind Matata. The Light Detection and Ranging technology (LIDAR) data sets flown prior to and following this event have been differenced and used in conjunction with a detailed field study to identify the distribution of debris and major sediment pathways which, from the Awatarariki catchment, transported at least 350,000 ± 50,000 m3 of debris. Debris flows were initially confined to stream valleys and controlled by the density and hydraulic thrust of the currents, before emerging onto the Awatarariki debris fan where a complex system of unconfined sediment pathways developed. Here, large boulders, clasts, logs and entire homes were deposited as the flows decelerated. Downstream from the debris fan, the pre-existing coastal foredune topography played a significant role in deflecting the more dilute currents that in filled lagoonal swale systems in both directions. The differenced LIDAR data have revealed several sectors characterised by significant variation in clast size, thickness and volume of debris as well as areas where post-debris flow cleanup and grading operations have resulted in man-made levees, sediment dumps, scoured channels and substantial graded areas. The application of differenced LIDAR data to a debris flow event demonstrates the techniques potential as a precise and powerful tool for hazard mapping and assessment.
The wreck of the SS Richard Montgomery, sunk off the Isle of Sheppey, UK, in 1944, has been surve... more The wreck of the SS Richard Montgomery, sunk off the Isle of Sheppey, UK, in 1944, has been surveyed with high resolution swath bathymetry, a total fourteen times over a seventeen year period. The structure sits on a sandbank at a maximum depth of 20m and is sub-aerially exposed at all tidal states. Ambient sediment transport is restricted to the flood tide and during storm conditions, resulting in a triple-lobed scour pit elongated in the flood tide direction. Despite maximum bed-level change, within the zone hydrodynamically influenced by the structure, occurring over a range of 2.5m/yr, total bed-level change over the survey period was only -0.6m, with yearly averages of <0.1m. This long-term (inter-annual) balance between bed-level loss and gain is suggestive of a self-contained system.
High resolution acoustic surveying for buried objects in the shallow waters of the inter-tidal to... more High resolution acoustic surveying for buried objects in the shallow waters of the inter-tidal to sub-tidal zone is a major challenge to many sectors of the marine surveying community. This is a consequence of a number of issues such as the relationship between water depth and acoustic acquisition geometry; problems of vessel induced bubble clouds reducing the signal-to-noise (SNR) ratio; and the necessity of high spatial survey accuracy in three-dimensions. These challenges are particularly acute for the marine archaeological community, who are frequently required to non-destructively investigate shallow-water (< 5 m) sites. This paper addresses these challenges and demonstrates the potential of imaging buried objects in extremely shallow environments by describing a seamless marine archaeological and geophysical investigation of a buried shipwreck: Henry V's `great flagship', the Grace Dieu (1418). The site, located in the Hamble River (UK), is typically covered by 2-5 m of water, and is partially buried within muddy inter-tidal sediments. At exceptionally low tides, during the spring equinox, a few of the marginal timbers are exposed. The marine survey utilised three different deployment methods of a Chirp system: two 2D Chirp systems, each emitting different frequencies and accompanied by different navigational systems (DGPS versus RTK), and a 3D Chirp system with RTK positioning capability. In all cases, the source was towed over the site using diver power. Close survey line spacing, accurate navigation and decimetre scale vertical and horizontal resolution acoustic data enabled the construction of a pseudo and full 3D image of this buried wreck site. This has been calibrated against known archaeological site investigation data and an RTK-GPS terrestrial survey. This data has identified the true plan form and dimensions of the remaining segments of the vessel, supporting the assertion that it was the most significant naval design for over two centuries. It has also been possible to identify the presence of a horizon of incoherent timbers associated with the scuttling of the vessel.
The 3D Chirp high--resolution sub--bottom profiler provides unparalleled imaging of coastal and i... more The 3D Chirp high--resolution sub--bottom profiler provides unparalleled imaging of coastal and in--shore seabed and sub--seabed structure by combining the known, highly repeatable source waveform of Chirp profilers with the coherent processing and interpretation afforded by true 3D seismic volumes. Comprising 60 hydrophone groups arranged around four Chirp II transducers, 3D Chirp permits the acquisition of a true 3D volume with a horizontal resolution of 12.5 cm, providing the perfect base for shallow-water engineering, archaeological, and geological studies. Here we present the results of a survey in an a--tidal basin on the south coast of England. With dimensions of 150 by 200 metres, the study area provided a series of unique challenges. These include a large number of anomalous objects (ranging from 10s centimetres to several metres in size) buried in a thin (0.5 to 1.5 metres) veneer of unconsolidated silt/sediment, overlaying a bedrock surface showing a high acoustic contrast, short-- wavelength roughness, and two distinct changes in topography. Surface and isopach maps, along with extracted seismic attributes, are used to confirm that the detector is acquiring a true 3D volume and illuminating aspects of the subsurface. Particular attention is paid to the properties and distribution of unidentified objects, including detailed analysis of the size, shape, and amplitude of their diffraction hyperbola.
We use > 150 km of multi-channel Boomer seismic reflection data acquired using a 60 channel, 80 m... more We use > 150 km of multi-channel Boomer seismic reflection data acquired using a 60 channel, 80 m streamer to interpret the depositional history for Lake Windermere, Lake District, UK, since the LGM (c. 21 ka BP). The large source-receiver offsets provided by this system allow pre-stack velocity analysis to aid the interpretation of glacial/glacio-lacustrine/lacustrine sediment facies. The data are complemented by > 100 km of single channel Chirp and Boomer seismic profiles providing 50 - 100 m line spacing, and the collated borehole data acquired in the lake throughout the twentieth century. Windermere is a glacial ribbon lake, c. 17 km long, up to 1.5 km wide, and has been glacially overdeepended to 105 m below lake level (65 m below sea level). The Younger Dryas glaciation was not as extensive in the Lake District leaving retreat deposits of the British and Irish Ice Sheet, and post-BIIS sediment stratigraphy well preserved. We identify five major seismic units which provide a new Late Quaternary deglacial history of the lake: 1. Lodgement till, with high seismic velocity (c. 3500 m/s), deposited within hollows in the bedrock during BIIS advance or initial retreat stages. 2. Multiple moraine systems deposited during BIIS retreat, which can be distinguished by their morphology and seismic velocity. A sequence is recorded of major still-stands during retreat, punctuated by minor smaller scale re-advances, producing De Geer moraine sequences. 3. Glacial-lacustrine varves of fine outwash sediments, up to 45 m thick, deposited during, and after, ice retreat from the lake. These have an average velocity c. 1570 m/s. 4. Slump deposits and late-glacial and interstadial sediments reworked during the Younger Dryas (YD). 5. Holocene (post-glacial) gyttja drape up to 5 m thick, with a seismic velocity of 1470 m/s. Until now reconstructing the glacial history of the Lake District prior to the Younger Dryas is difficult due to a dearth of terrestrial LGM deglaciation retreat structures in the region, and so it has been previously suggested that rapid climate amelioration at the end of the LGM caused the glaciers to down-waste in situ. A range of retreat moraines identified within this study indicate active-ice removal from the lake throughout the BIIS retreat phase, further their locations and morphology provide insights into the patterns and processes of a more complex deglacial history. The lake also provides a high resolution record of the immediate de-glacial sedimentary environment, the Younger Dryas fluvio-glacial impacts and terrestrial and lacustrine slope failures, and the climatic amelioration associated with the Holocene.
We present high-resolution "Chirp" sub-bottom profiler data from Thingvallavatn, a lake in Icelan... more We present high-resolution "Chirp" sub-bottom profiler data from Thingvallavatn, a lake in Iceland's western rift zone. These data are combined with stratigraphic constraints from sediment cores to show that movement on normal faults since 9 ka are temporally correlated with magmatic events, indicating that movements were controlled by episodic dike intrusion. Sediment depo-centres and the focus of subsidence migrated westwards over 3 -4 kyrs towards the locus of subsequent brittle failure. We interpret this subsidence as related to dike intrusion a few Bull et al. 2 kilometers along strike, originating from the Hengill volcanic system, which occurred prior to major diking, faulting and subsidence within the lake at 1.9 ka.
The Whakatane Graben is a young (<2 Ma) and highly active structure situated on the New Zealand c... more The Whakatane Graben is a young (<2 Ma) and highly active structure situated on the New Zealand continental shelf where the sediment supply has been sufficient to infill accommodation space created by the rapidly displacing faults, thus providing a complete record of past fault motions. The shallow water (up to 200 m depth) of the graben on the continental shelf means it is well placed for acquisition of high-resolution seismic data and core samples. One of the most active faults in the Whakatane Graben is the Rangitaiki Fault, a 20 km long linked, segmented, normal fault. The central 9 km of the Rangitaiki Fault is imaged by over 97 closely spaced chirp sonar, 3.5 kHz and Boomer profiles covering an area approximately 5 km by 9 km, the outer sections of the fault are imaged by 12 more widely spaced high-resolution profiles. These data provide excellent quality high-resolution images of the top 50 - 60 m of sediments which include the post-glacial (< 18 ka) transgressive sequence. Within the post-glacial sequence there are several strong and laterally continuous reflectors displaced by active extensional growth faults. Displacements on four of these horizons, dated 17 +/- 1, 13.3 +/- 1, 11.1 +/- 1 and 9 +/- 1 ka, are used to create displacement profiles and to calculate displacement rates. The segments of the Rangitaiki Fault are hard-linked throughout the last 18 ka, but the post-glacial displacement accumulation is still dominated by the location of the relict segment boundaries. The maximum displacement rate observed on the Rangitaiki Fault is 4.6 +/- 2.3 mm/yr measured over 2 ka, a maximum of 3.4 +/- 0.2 mm/yr measured over 18 ka. Displacement profiles of the last 9 ka of fault movement are regular and similar to profiles showing the last 300 ka of fault movement. In contrast, profiles showing only 2 ka of fault movement are highly irregular and show points of zero displacement on the larger faults. The variability of fault displacement rates over short (< 2 ka ) timescales suggests that fault activity rates measured from geodetic surveys may not be adequately sampling the fault activity, and longer timescale estimates of displacement rate, such as are presented here, are more reliable an indicator of seismic risk and earthquake recurrence intervals.
The acoustic properties of marine sediment containing free gas bubbles are considerably more comp... more The acoustic properties of marine sediment containing free gas bubbles are considerably more complex, and therefore more poorly understood, than the acoustic properties of saturated sediment. A more detailed understanding of gassy sediments would therefore be advantageous to a wide range of marine users, ranging from marine surveyors to those responsible for homelands defence. To provide a simplified system through which the acoustic properties of gassy sediments can be investigated, laboratory experiments have been performed. Samples of both gassy mud and a synthetic bubbly gel mimic were examined. Results for the compressional wave velocity and attenuation coefficient are presented from 30 to 110 kHz. These are compared to the predictions of linear theoretical models for both sediment and water based bubble distributions. In addition, the results of combination frequency techniques are presented. These exploit the non linear interaction between the acoustic field the bubbles, therefore allowing bubble size distributions to be inferred though the analysis of the sum and difference frequency scattered fields. All of the results are compared to bubble size distributions measured independently using a X-Ray CT scanner, which is able to resolve bubbles with radii greater than 20 micrometers.
This paper will present the first true 3D high resolution acoustic imagery of a wreck site buried... more This paper will present the first true 3D high resolution acoustic imagery of a wreck site buried in the marine environment. Using a 3D Chirp system developed at the University of Southampton, a marine seismic survey of the mid-eighteenth century wreck site has been undertaken. The Invincible was a 74 gun warship built by the French in 1744, captured by the British in 1747 and subsequently lost off Portsmouth, UK in February 1758. The wreck was re-discovered by divers in 1979, partially buried on the margins of a mobile sandbank in approximately 8 metres of water. In 2004 the system was surveyed using a 60 channel, rigid framed 3D Chirp (1.5-13 kHz source sweep) system with integral RTK GPS and attitude systems. An area of 160 m x 160 m, centered over the wreck site, was surveyed with a total of 150 Gb data being acquired. The data was processed, using 3D Promax, to produce 25 cm bins with typical 3-6 fold coverage. The stacked traces have been visualized and interpreted using Kingdom Suite software. The final imagery shows at unprecedented resolution the full three-dimensional buried form of the wreck and it's relationship to the surrounding sedimentary sequences, enabling the full evolution of the site to be discussed. Further, the data is compared to previously acquired swath bathymetry and 2D seismic data in order to illustrate the impact of such a device for underwater cultural heritage management.
In situ techniques provide the most reliable method of examining the geoacoustical properties of ... more In situ techniques provide the most reliable method of examining the geoacoustical properties of marine sediments. In the past, individual in situ surveys have only been able to examine compressional waves over a maximum frequency range of 100 Hz to 50 kHz. A new in situ acoustic device, the Sediment Probing Acoustic Detection Equipment, or SPADE, has been developed, which can emit a variety of pulses, e.g. tonal and swept-frequency, over a continuous frequency range of 10 -100 kHz. Data from a recent field trial are analysed to obtain the in situ velocity and attenuation over frequency increments of 5 kHz between 10 -75 kHz. Results imply that scattering is a dominant attenuation mechanism from 10-75 kHz and the media is dispersive for frequencies between 60 and 70 kHz and below 20 kHz. Biot theory cannot accurately model the observed velocity and attenuation.
We present results from decimetre resolution 3D seismic volumes acquired over Late Quaternary mas... more We present results from decimetre resolution 3D seismic volumes acquired over Late Quaternary mass movement deposits in both Lake Windermere, UK, and the Trondheim Harbour area, central Norway. Both deposits were imaged using the 3D Chirp sub-bottom profiler, which combines the known, highly repeatable source waveform of Chirp profilers with the coherent processing and interpretation afforded by true 3D seismic volumes. Reflector morphology from these two volumes are used to identify and map structure on scales of 10s cm to 100s metres. This shows the applicability of the method for the interpretation of failure mechanism, flow morphology and depositional style in these two environments. In Windermere, Younger Dryas deposits have been substantially reworked by the episodic redistribution of sediment from the steep lakesides into the basin. Within the 100 x 400 m 3D seismic volume we identify two small debris flow deposits (1500 m3 and 60,000 m3) and one large (500,000 m3) erosive mass flow deposit. These two depositional mechanisms are distinct. The debris flows have high amplitude, chaotic internal reflections, with a high amplitude reflector representing a lower erosional boundary, discontinuous low amplitude top reflector, and thin out rapidly with distance from the lake margin. The thicker mass flow unit lacks internal structure, and has high amplitude top and base reflectors,. In the Trondheim Harbour we image the down-slope extent of three large slide blocks (which have a net volume > 1 x 106 m3), mobilised by a landslide in 1990, in the 100 x 450 m 3D seismic volume. The morphology of these mass movement deposits is distinct again; demonstrating translational failure along a clear slip plane, leaving well defined slide scars, and forming prominent compressional/extensional structures.
The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall eve... more The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall event on the 18 May 2005. This event triggered widespread landslips and large debris flows in the Awatarariki and Waitepuru catchments behind Matata. The Light Detection and Ranging technology (LIDAR) data sets flown prior to and following this event have been differenced and used in conjunction with a detailed field study to identify the distribution of debris and major sediment pathways which, from the Awatarariki catchment, transported at least 350,000 ± 50,000 m3 of debris. Debris flows were initially confined to stream valleys and controlled by the density and hydraulic thrust of the currents, before emerging onto the Awatarariki debris fan where a complex system of unconfined sediment pathways developed. Here, large boulders, clasts, logs and entire homes were deposited as the flows decelerated. Downstream from the debris fan, the pre-existing coastal foredune topography played a significant role in deflecting the more dilute currents that in filled lagoonal swale systems in both directions. The differenced LIDAR data have revealed several sectors characterised by significant variation in clast size, thickness and volume of debris as well as areas where post-debris flow cleanup and grading operations have resulted in man-made levees, sediment dumps, scoured channels and substantial graded areas. The application of differenced LIDAR data to a debris flow event demonstrates the techniques potential as a precise and powerful tool for hazard mapping and assessment.
If we wish to understand the surviving shipwreck record and to inform strategies of heritage, man... more If we wish to understand the surviving shipwreck record and to inform strategies of heritage, management and conservation an improved knowledge of the variable impact of marine physical processes is required. I am approaching this problem at a range of scales from full wreck to individual artefact scale, through a combination of bathymetric survey, diver monitoring and physical modelling in a controlled laboratory environment. The first phase of this work has involved the integration of time-lapse studies (months to years) of 8 shipwreck sites in a range of oceanographic conditions (e.g. tidally dominated, wave dominated and combined flow regimes). Despite having had decades to hundreds of years to reach equilibrium these wreck sites still exhibit metres of change in bedlevel change over periods of months. Preliminary results suggest these sites adhere to predictable patterning derived from the initial physical models. To determine the reproducibility of these results a much larger ...
The Outer Thames Estuary Regional Environmental Characterisation (REC) provides an environmental ... more The Outer Thames Estuary Regional Environmental Characterisation (REC) provides an environmental reference statement defining marine and seabed conditions within the study area. Prior to this study, regional environmental assessment of the Outer Thames Estuary was based upon ...
ABSTRACT Evidence from the Severn Estuary demonstrates that this region was exploited by Mesolith... more ABSTRACT Evidence from the Severn Estuary demonstrates that this region was exploited by Mesolithic hunter-gatherer-fishers (Bell, 2007). The potential for future archaeological discoveries (Bell, 2007; Webster, 2007: 273; Bell and Warren, 2013: 39), and the well-preserved palaeoenvironmental evidence in the fine-grained and organic sediments of the Somerset, Avon and Gwent Levels (Hosfield et al., 2007a: 40) makes the area of importance for archaeological study. Small quantities of worked flint have been recovered from the foreshore around Stolford, Porlock and Minehead Bay (Mullin et al., 2009; Canti et al., 1995) implying human activity in the present intertidal zone, which is further enhanced by the suggestion of possible deliberate burning of reed swamps (Jones et al., 2005) similar to that postulated in the Severn Estuary (Brown, 2005 and Timpany, 2005; Bell, 2007).
The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall eve... more The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall event on the 18 May 2005. This event triggered widespread landslips and large debris flows in the Awatarariki and Waitepuru catchments behind Matata. The Light Detection and Ranging technology (LIDAR) data sets flown prior to and following this event have been differenced and used in conjunction with a detailed field study to identify the distribution of debris and major sediment pathways which, from the Awatarariki catchment, transported at least 350,000 ± 50,000 m3 of debris. Debris flows were initially confined to stream valleys and controlled by the density and hydraulic thrust of the currents, before emerging onto the Awatarariki debris fan where a complex system of unconfined sediment pathways developed. Here, large boulders, clasts, logs and entire homes were deposited as the flows decelerated. Downstream from the debris fan, the pre-existing coastal foredune topography played a significant role in deflecting the more dilute currents that in filled lagoonal swale systems in both directions. The differenced LIDAR data have revealed several sectors characterised by significant variation in clast size, thickness and volume of debris as well as areas where post-debris flow cleanup and grading operations have resulted in man-made levees, sediment dumps, scoured channels and substantial graded areas. The application of differenced LIDAR data to a debris flow event demonstrates the techniques potential as a precise and powerful tool for hazard mapping and assessment.
The wreck of the SS Richard Montgomery, sunk off the Isle of Sheppey, UK, in 1944, has been surve... more The wreck of the SS Richard Montgomery, sunk off the Isle of Sheppey, UK, in 1944, has been surveyed with high resolution swath bathymetry, a total fourteen times over a seventeen year period. The structure sits on a sandbank at a maximum depth of 20m and is sub-aerially exposed at all tidal states. Ambient sediment transport is restricted to the flood tide and during storm conditions, resulting in a triple-lobed scour pit elongated in the flood tide direction. Despite maximum bed-level change, within the zone hydrodynamically influenced by the structure, occurring over a range of 2.5m/yr, total bed-level change over the survey period was only -0.6m, with yearly averages of <0.1m. This long-term (inter-annual) balance between bed-level loss and gain is suggestive of a self-contained system.
High resolution acoustic surveying for buried objects in the shallow waters of the inter-tidal to... more High resolution acoustic surveying for buried objects in the shallow waters of the inter-tidal to sub-tidal zone is a major challenge to many sectors of the marine surveying community. This is a consequence of a number of issues such as the relationship between water depth and acoustic acquisition geometry; problems of vessel induced bubble clouds reducing the signal-to-noise (SNR) ratio; and the necessity of high spatial survey accuracy in three-dimensions. These challenges are particularly acute for the marine archaeological community, who are frequently required to non-destructively investigate shallow-water (< 5 m) sites. This paper addresses these challenges and demonstrates the potential of imaging buried objects in extremely shallow environments by describing a seamless marine archaeological and geophysical investigation of a buried shipwreck: Henry V's `great flagship', the Grace Dieu (1418). The site, located in the Hamble River (UK), is typically covered by 2-5 m of water, and is partially buried within muddy inter-tidal sediments. At exceptionally low tides, during the spring equinox, a few of the marginal timbers are exposed. The marine survey utilised three different deployment methods of a Chirp system: two 2D Chirp systems, each emitting different frequencies and accompanied by different navigational systems (DGPS versus RTK), and a 3D Chirp system with RTK positioning capability. In all cases, the source was towed over the site using diver power. Close survey line spacing, accurate navigation and decimetre scale vertical and horizontal resolution acoustic data enabled the construction of a pseudo and full 3D image of this buried wreck site. This has been calibrated against known archaeological site investigation data and an RTK-GPS terrestrial survey. This data has identified the true plan form and dimensions of the remaining segments of the vessel, supporting the assertion that it was the most significant naval design for over two centuries. It has also been possible to identify the presence of a horizon of incoherent timbers associated with the scuttling of the vessel.
The 3D Chirp high--resolution sub--bottom profiler provides unparalleled imaging of coastal and i... more The 3D Chirp high--resolution sub--bottom profiler provides unparalleled imaging of coastal and in--shore seabed and sub--seabed structure by combining the known, highly repeatable source waveform of Chirp profilers with the coherent processing and interpretation afforded by true 3D seismic volumes. Comprising 60 hydrophone groups arranged around four Chirp II transducers, 3D Chirp permits the acquisition of a true 3D volume with a horizontal resolution of 12.5 cm, providing the perfect base for shallow-water engineering, archaeological, and geological studies. Here we present the results of a survey in an a--tidal basin on the south coast of England. With dimensions of 150 by 200 metres, the study area provided a series of unique challenges. These include a large number of anomalous objects (ranging from 10s centimetres to several metres in size) buried in a thin (0.5 to 1.5 metres) veneer of unconsolidated silt/sediment, overlaying a bedrock surface showing a high acoustic contrast, short-- wavelength roughness, and two distinct changes in topography. Surface and isopach maps, along with extracted seismic attributes, are used to confirm that the detector is acquiring a true 3D volume and illuminating aspects of the subsurface. Particular attention is paid to the properties and distribution of unidentified objects, including detailed analysis of the size, shape, and amplitude of their diffraction hyperbola.
We use > 150 km of multi-channel Boomer seismic reflection data acquired using a 60 channel, 80 m... more We use > 150 km of multi-channel Boomer seismic reflection data acquired using a 60 channel, 80 m streamer to interpret the depositional history for Lake Windermere, Lake District, UK, since the LGM (c. 21 ka BP). The large source-receiver offsets provided by this system allow pre-stack velocity analysis to aid the interpretation of glacial/glacio-lacustrine/lacustrine sediment facies. The data are complemented by > 100 km of single channel Chirp and Boomer seismic profiles providing 50 - 100 m line spacing, and the collated borehole data acquired in the lake throughout the twentieth century. Windermere is a glacial ribbon lake, c. 17 km long, up to 1.5 km wide, and has been glacially overdeepended to 105 m below lake level (65 m below sea level). The Younger Dryas glaciation was not as extensive in the Lake District leaving retreat deposits of the British and Irish Ice Sheet, and post-BIIS sediment stratigraphy well preserved. We identify five major seismic units which provide a new Late Quaternary deglacial history of the lake: 1. Lodgement till, with high seismic velocity (c. 3500 m/s), deposited within hollows in the bedrock during BIIS advance or initial retreat stages. 2. Multiple moraine systems deposited during BIIS retreat, which can be distinguished by their morphology and seismic velocity. A sequence is recorded of major still-stands during retreat, punctuated by minor smaller scale re-advances, producing De Geer moraine sequences. 3. Glacial-lacustrine varves of fine outwash sediments, up to 45 m thick, deposited during, and after, ice retreat from the lake. These have an average velocity c. 1570 m/s. 4. Slump deposits and late-glacial and interstadial sediments reworked during the Younger Dryas (YD). 5. Holocene (post-glacial) gyttja drape up to 5 m thick, with a seismic velocity of 1470 m/s. Until now reconstructing the glacial history of the Lake District prior to the Younger Dryas is difficult due to a dearth of terrestrial LGM deglaciation retreat structures in the region, and so it has been previously suggested that rapid climate amelioration at the end of the LGM caused the glaciers to down-waste in situ. A range of retreat moraines identified within this study indicate active-ice removal from the lake throughout the BIIS retreat phase, further their locations and morphology provide insights into the patterns and processes of a more complex deglacial history. The lake also provides a high resolution record of the immediate de-glacial sedimentary environment, the Younger Dryas fluvio-glacial impacts and terrestrial and lacustrine slope failures, and the climatic amelioration associated with the Holocene.
We present high-resolution "Chirp" sub-bottom profiler data from Thingvallavatn, a lake in Icelan... more We present high-resolution "Chirp" sub-bottom profiler data from Thingvallavatn, a lake in Iceland's western rift zone. These data are combined with stratigraphic constraints from sediment cores to show that movement on normal faults since 9 ka are temporally correlated with magmatic events, indicating that movements were controlled by episodic dike intrusion. Sediment depo-centres and the focus of subsidence migrated westwards over 3 -4 kyrs towards the locus of subsequent brittle failure. We interpret this subsidence as related to dike intrusion a few Bull et al. 2 kilometers along strike, originating from the Hengill volcanic system, which occurred prior to major diking, faulting and subsidence within the lake at 1.9 ka.
The Whakatane Graben is a young (<2 Ma) and highly active structure situated on the New Zealand c... more The Whakatane Graben is a young (<2 Ma) and highly active structure situated on the New Zealand continental shelf where the sediment supply has been sufficient to infill accommodation space created by the rapidly displacing faults, thus providing a complete record of past fault motions. The shallow water (up to 200 m depth) of the graben on the continental shelf means it is well placed for acquisition of high-resolution seismic data and core samples. One of the most active faults in the Whakatane Graben is the Rangitaiki Fault, a 20 km long linked, segmented, normal fault. The central 9 km of the Rangitaiki Fault is imaged by over 97 closely spaced chirp sonar, 3.5 kHz and Boomer profiles covering an area approximately 5 km by 9 km, the outer sections of the fault are imaged by 12 more widely spaced high-resolution profiles. These data provide excellent quality high-resolution images of the top 50 - 60 m of sediments which include the post-glacial (< 18 ka) transgressive sequence. Within the post-glacial sequence there are several strong and laterally continuous reflectors displaced by active extensional growth faults. Displacements on four of these horizons, dated 17 +/- 1, 13.3 +/- 1, 11.1 +/- 1 and 9 +/- 1 ka, are used to create displacement profiles and to calculate displacement rates. The segments of the Rangitaiki Fault are hard-linked throughout the last 18 ka, but the post-glacial displacement accumulation is still dominated by the location of the relict segment boundaries. The maximum displacement rate observed on the Rangitaiki Fault is 4.6 +/- 2.3 mm/yr measured over 2 ka, a maximum of 3.4 +/- 0.2 mm/yr measured over 18 ka. Displacement profiles of the last 9 ka of fault movement are regular and similar to profiles showing the last 300 ka of fault movement. In contrast, profiles showing only 2 ka of fault movement are highly irregular and show points of zero displacement on the larger faults. The variability of fault displacement rates over short (< 2 ka ) timescales suggests that fault activity rates measured from geodetic surveys may not be adequately sampling the fault activity, and longer timescale estimates of displacement rate, such as are presented here, are more reliable an indicator of seismic risk and earthquake recurrence intervals.
The acoustic properties of marine sediment containing free gas bubbles are considerably more comp... more The acoustic properties of marine sediment containing free gas bubbles are considerably more complex, and therefore more poorly understood, than the acoustic properties of saturated sediment. A more detailed understanding of gassy sediments would therefore be advantageous to a wide range of marine users, ranging from marine surveyors to those responsible for homelands defence. To provide a simplified system through which the acoustic properties of gassy sediments can be investigated, laboratory experiments have been performed. Samples of both gassy mud and a synthetic bubbly gel mimic were examined. Results for the compressional wave velocity and attenuation coefficient are presented from 30 to 110 kHz. These are compared to the predictions of linear theoretical models for both sediment and water based bubble distributions. In addition, the results of combination frequency techniques are presented. These exploit the non linear interaction between the acoustic field the bubbles, therefore allowing bubble size distributions to be inferred though the analysis of the sum and difference frequency scattered fields. All of the results are compared to bubble size distributions measured independently using a X-Ray CT scanner, which is able to resolve bubbles with radii greater than 20 micrometers.
This paper will present the first true 3D high resolution acoustic imagery of a wreck site buried... more This paper will present the first true 3D high resolution acoustic imagery of a wreck site buried in the marine environment. Using a 3D Chirp system developed at the University of Southampton, a marine seismic survey of the mid-eighteenth century wreck site has been undertaken. The Invincible was a 74 gun warship built by the French in 1744, captured by the British in 1747 and subsequently lost off Portsmouth, UK in February 1758. The wreck was re-discovered by divers in 1979, partially buried on the margins of a mobile sandbank in approximately 8 metres of water. In 2004 the system was surveyed using a 60 channel, rigid framed 3D Chirp (1.5-13 kHz source sweep) system with integral RTK GPS and attitude systems. An area of 160 m x 160 m, centered over the wreck site, was surveyed with a total of 150 Gb data being acquired. The data was processed, using 3D Promax, to produce 25 cm bins with typical 3-6 fold coverage. The stacked traces have been visualized and interpreted using Kingdom Suite software. The final imagery shows at unprecedented resolution the full three-dimensional buried form of the wreck and it's relationship to the surrounding sedimentary sequences, enabling the full evolution of the site to be discussed. Further, the data is compared to previously acquired swath bathymetry and 2D seismic data in order to illustrate the impact of such a device for underwater cultural heritage management.
In situ techniques provide the most reliable method of examining the geoacoustical properties of ... more In situ techniques provide the most reliable method of examining the geoacoustical properties of marine sediments. In the past, individual in situ surveys have only been able to examine compressional waves over a maximum frequency range of 100 Hz to 50 kHz. A new in situ acoustic device, the Sediment Probing Acoustic Detection Equipment, or SPADE, has been developed, which can emit a variety of pulses, e.g. tonal and swept-frequency, over a continuous frequency range of 10 -100 kHz. Data from a recent field trial are analysed to obtain the in situ velocity and attenuation over frequency increments of 5 kHz between 10 -75 kHz. Results imply that scattering is a dominant attenuation mechanism from 10-75 kHz and the media is dispersive for frequencies between 60 and 70 kHz and below 20 kHz. Biot theory cannot accurately model the observed velocity and attenuation.
We present results from decimetre resolution 3D seismic volumes acquired over Late Quaternary mas... more We present results from decimetre resolution 3D seismic volumes acquired over Late Quaternary mass movement deposits in both Lake Windermere, UK, and the Trondheim Harbour area, central Norway. Both deposits were imaged using the 3D Chirp sub-bottom profiler, which combines the known, highly repeatable source waveform of Chirp profilers with the coherent processing and interpretation afforded by true 3D seismic volumes. Reflector morphology from these two volumes are used to identify and map structure on scales of 10s cm to 100s metres. This shows the applicability of the method for the interpretation of failure mechanism, flow morphology and depositional style in these two environments. In Windermere, Younger Dryas deposits have been substantially reworked by the episodic redistribution of sediment from the steep lakesides into the basin. Within the 100 x 400 m 3D seismic volume we identify two small debris flow deposits (1500 m3 and 60,000 m3) and one large (500,000 m3) erosive mass flow deposit. These two depositional mechanisms are distinct. The debris flows have high amplitude, chaotic internal reflections, with a high amplitude reflector representing a lower erosional boundary, discontinuous low amplitude top reflector, and thin out rapidly with distance from the lake margin. The thicker mass flow unit lacks internal structure, and has high amplitude top and base reflectors,. In the Trondheim Harbour we image the down-slope extent of three large slide blocks (which have a net volume > 1 x 106 m3), mobilised by a landslide in 1990, in the 100 x 450 m 3D seismic volume. The morphology of these mass movement deposits is distinct again; demonstrating translational failure along a clear slip plane, leaving well defined slide scars, and forming prominent compressional/extensional structures.
The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall eve... more The town of Matata in the Eastern Bay of Plenty (New Zealand) experienced an extreme rainfall event on the 18 May 2005. This event triggered widespread landslips and large debris flows in the Awatarariki and Waitepuru catchments behind Matata. The Light Detection and Ranging technology (LIDAR) data sets flown prior to and following this event have been differenced and used in conjunction with a detailed field study to identify the distribution of debris and major sediment pathways which, from the Awatarariki catchment, transported at least 350,000 ± 50,000 m3 of debris. Debris flows were initially confined to stream valleys and controlled by the density and hydraulic thrust of the currents, before emerging onto the Awatarariki debris fan where a complex system of unconfined sediment pathways developed. Here, large boulders, clasts, logs and entire homes were deposited as the flows decelerated. Downstream from the debris fan, the pre-existing coastal foredune topography played a significant role in deflecting the more dilute currents that in filled lagoonal swale systems in both directions. The differenced LIDAR data have revealed several sectors characterised by significant variation in clast size, thickness and volume of debris as well as areas where post-debris flow cleanup and grading operations have resulted in man-made levees, sediment dumps, scoured channels and substantial graded areas. The application of differenced LIDAR data to a debris flow event demonstrates the techniques potential as a precise and powerful tool for hazard mapping and assessment.
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Papers by Justin Dix
sediment transport is restricted to the flood tide and during storm conditions, resulting in a triple-lobed scour pit elongated in the flood tide direction. Despite maximum bed-level change, within the zone hydrodynamically influenced by the structure, occurring over a range of 2.5m/yr, total bed-level change over the survey period was only -0.6m, with yearly averages of <0.1m. This long-term (inter-annual) balance between bed-level loss and gain is suggestive of a self-contained system.
sediment transport is restricted to the flood tide and during storm conditions, resulting in a triple-lobed scour pit elongated in the flood tide direction. Despite maximum bed-level change, within the zone hydrodynamically influenced by the structure, occurring over a range of 2.5m/yr, total bed-level change over the survey period was only -0.6m, with yearly averages of <0.1m. This long-term (inter-annual) balance between bed-level loss and gain is suggestive of a self-contained system.