Multibeam Bathymetry Surveys Research Articles

Research on the Optimization of Multibeam Bathymetry Survey Line Layout Based on Simulated Annealing Algorithm

Multibeam bathymetry systems are widely used in water depth measurement, and their principle is based on the propagation characteristics of sound waves in water. The system has multiple advantages, such as high resolution, high accuracy, wide measurement range, automated mapping, and high efficiency. This study is based on the propagation characteristics of sound waves in water and utilizes a multibeam bathymetry system for water depth measurement, focusing on key issues such as coverage width, survey path, and seawater depth. Based on a two-dimensional plane model, the direction of the emitted beams from the measurement vessel and the relationship with seafloor slope angles are considered. The key parameter calculation model of the multibeam bathymetry system is established using the sine theorem and cosine theorem. To enhance the analysis accuracy, the calculation model is extended to three dimensions, incorporating more factors that have a significant impact on the measurement, such as the slope angle along the survey lines and changes in water depth. In practical applications, the survey lines are set to run in a north-south direction, and a greedy algorithm is used to ensure complete coverage of the measurements. Taking into account the complex seafloor topography, the fitted lines are used to represent the seafloor slopes. The multi-objective programming model is transformed into an unconstrained single-objective programming problem for further optimization. By solving the problem using simulated annealing algorithm, the optimized solution provides specific values for the spacing between adjacent survey lines, total track length, missed coverage rate of the surveyed area, and duplicate measurement coverage, enabling the optimal utilization of multibeam bathymetry systems in oceanic surveys.

Improved techniques to resolve the water column sound speed structure for multibeam ray-tracing

Multibeam bathymetry surveys conducted in highly stratified environments are routinely affected by sound speed errors. The approach proposed here aims to combine measured with synthetic sound speed profiles in order to minimize sound speed errors. The adequacy of synthetic profiles derived from a regional hydrodynamic model and spatiotemporal interpolation is therefore investigated. Ray-tracing comparisons at the 65° launch angle between measured and synthetic profiles demonstrates that the expected depth bias for modeled profiles will be in excess of 1% of water depth. Spatiotemporally interpolating hourly sampled profiles decreases the depth bias up to a factor of three for higher beam angles. Residual sounding depth biases are observed and ascribed to a residual surface sound speed error.

Investigating the subsurface in a shallow water environment using array and single-station ambient vibration techniques

SUMMARYSingle-station and array ambient vibration techniques are widely used in onshore environments, in particular to retrieve the subsurface structure and shear-wave velocity profiles. We apply these techniques offshore in Lake Lucerne (Switzerland) using single-station and array Ocean Bottom Seismometer (OBS) data. This lake has experienced tsunamigenic subaquatic slope failures in the past and still has sediment-charged slopes that might fail in the presence of a seismic or aseismic trigger. The application of traditional onshore methods offshore brings additional challenges related to the processing of recorded data. To overcome these challenges, we perform multibeam bathymetry surveys to precisely locate the OBS on the lake floor and airgun shootings to determine the orientation of the horizontal components of the seismometer and to correct the time drift of the recorder. Then we obtain surface-wave phase velocity dispersion curves of Scholte and Love waves, and Scholte wave ellipticity curves at six subaquatic slopes. After the estimation of the dispersion curves, we deal with their modal identification using mode attribution analysis. The shear-wave velocity and thickness of the sedimentary layers at the investigated slopes are inferred using a transdimensional Bayesian inversion algorithm. The resolved velocity profiles show very low shear-wave velocities in shallow lake sediments and allow us to improve the understanding of the local stratigraphy. This research contributes to the assessment of stability and tsunamigenic potential of subaquatic slopes in Lake Lucerne.

Morphological evolution of a barchan dune migrating past an offshore wind farm foundation

AbstractAs the number of manmade structures installed on the seafloor is increasing rapidly, we seek to understand the impact of these immobile obstacles on marine geomorphological processes, such as the evolution of bedforms. A 5.8 m diameter monopile foundation was installed at the case study offshore windfarm approximately 30 m ahead of an approaching barchan (crescent‐shaped) dune. The impact of the monopile on the dune's evolution was analysed using six multibeam bathymetry surveys spanning 20 years. To substantiate this analysis, coupled three‐dimensional numerical modelling of flows and sediment was conducted in which the scour inducing bed shear stresses were calculated from the modelled turbulent kinetic energy. Following the installation of the monopile, the mid‐section of the dune accelerated and stretched in the direction of the wake of the monopile. Four years after the monopile's installation the rest of the dune had caught up, flattening out the slip face within half the dune's length downstream of the monopile. Due to the modified flow field, the dune was scoured deeply at the base of the monopile to a depth of 6.8 m (supported by the model results that predicted a scour depth exceeding 2 m over a period of just a few days). The surveyed volume of material scoured amounted to 8% of the total dune volume. Whilst the process of scouring occurs at a timescale of days to weeks, the dune migrated on average by 25 m/yr. The difference in the timescale of the two processes allowed the scouring to occur through the full thickness of the dune. The scoured dune profile recovered rapidly once the dune migrated downstream of the monopile. This article demonstrates how large geomorphological features can intercept and migrate past a monopile foundation without long‐lasting impacts on the integrity of the feature or the foundation. © 2020 John Wiley & Sons, Ltd.

Elastic Flexure of Young, Overlapping Basaltic Lava Flows Offshore the Galápagos and Hawaiian Islands: Observations, Modeling, and Thermal/Chronological Analysis

AbstractA recent multibeam bathymetry survey in the Galápagos archipelago revealed a novel observation between the islands of Santa Cruz and Santiago: a steep flexural moat formed between two volcanic flows, nearly 50 m deep and about 500 m wide. Submarine observations and elastic plate modeling are consistent with this feature forming after one inflationary flow overlapped another young flow, deflecting the underlying lava flow in what appears to be a classical elastic flexural pattern. We present a flexural analysis of this moat feature using a thin‐plate approximation to determine the effective elastic thickness of the underlying flow. Moreover, we propose that this elastic thickness serves as a proxy for the thickness of the cooling, solidifying upper crust of the lava flow at the time of deflection, a time‐dependent thermal property. This approach offers a chronological tool to put bounds on the elapsed time between two overlapping lava flows. We applied this method to the Galápagos moat to constrain the time history of its formation. Additionally, we identified and modeled seven additional flexural moat candidates in the Hawaiian archipelago and in the Galápagos using multibeam bathymetry. Elastic/thermal history analysis of these features yields relative time intervals between submarine lava flows ranging from a few years to about 600 years, which suggests that there is a critical time interval during which elastic deflection of the underlying lava flow may become “frozen in.” This methodology may provide helpful constraints on the temporal relations between overlapping lava flows wherever they form in relatively rapid succession.

Morphological signature of gully development by rapid slide retrogression in a layered coarse-grained delta foreslope

Abstract Coarse-grained deltas are often characterized by steep foreslopes (often more than 10°) that are traversed by delta-front channels. The channels thus erode into relatively steeply inclined bedding. In this context, the slopes flanking the channels can be steeper than the friction angle since they include a component of dip related to the delta-front slope as well as the channel-related erosion slope. In this study, part of the Busu River delta (Papua New Guinea) was imaged using a high-resolution multibeam bathymetry survey over an area where the angle of the slopes flanking the channels locally reaches 50°. A detailed analysis of the delta slope morphology has revealed an additional source of instability due to erosion within the main channels. In some places, erosion cuts into the channel flank forming a local knickpoint inclined in a direction approaching that of the bed dip. The cut can then initiate breaching or static liquefaction failure from that point up to the crest of the interfluve resulting in a V-shaped gully.

Discrete characterization of abyssal hills: Unraveling temporal variations in crustal accretion processes at the 10°30′N segment, East Pacific Rise

The morphology of abyssal hills provides important information on the crustal accretion processes acting at mid ocean ridges. Discretely characterizing abyssal-hill shape is challenging, and therefore, the study of abyssal-hill morphology is often approached by examining the average properties of relatively wide regions containing multiple hills. However, this averaging process removes any spatial trends in abyssal-hill shape that may reflect temporal variations in crustal accretion processes. Here we present an automated approach for analyzing the shape of individual abyssal hills, which we detect using the ridgelet transform method of Downey and Clayton (2007). We first analyze seafloor morphologies, as revealed in multibeam bathymetry surveys, across 16 mid-ocean ridge segments with varying spreading rates. The results of this analysis display the known negative correlation between the width and height of abyssal hills and the spreading rate, thus validating our approach. Our results also show that the fraction of inward-facing abyssal hills exhibits a simple linear trend across all spreading rates, suggesting a property that could be used to determine the spreading rate at which the seafloor formed. We then apply our technique to study a flow line transect collected across the fast-spreading 10°30′N segment of the northern East Pacific Rise, revealing temporal changes in the shape of the abyssal hills formed during the last 3.8 Myr. The youngest part of the flow line coincides with a location where basaltic glasses were previously sampled. The abyssal-hill morphology and MgO content of the glass samples share mutual trends (over 105-106 yrs), suggesting that abyssal-hill morphology is sensitive to the rate at which magma is supplied to the ridge axis. Finally, a ∼1 Myr cyclic variation in seafloor morphology is observed. This periodicity is attributed to temporal changes in magma supply at the ridge axis, possibly related to upper mantle dynamics or the presence of chemical heterogeneities.

Analysis of hummocky bedforms offshore Fire Island, New York, generated by superstorm Sandy

Hummocky bedforms are mainly studied in ancient sedimentary outcrops; they are interpreted to be associated with storms, an inference that is based on paleohydraulic conditions derived from sedimentary deposits. However, the modern hummocky bedform catalog is sparse, with few associations of hydraulic conditions resulting in their deposition and preservation. In this study we investigate small-scale (< 20 m wide) surficial bedforms superimposed on top of large-scale (200–3000 m wide) westward-migrating shoreface-attached sand ridges and sorted bedforms, using multibeam bathymetry surveys acquired two months after “superstorm” Sandy offshore of Fire Island, NY. Geostatistical analysis of two post-storm surveys, one along the western half of the island dominated by 1–3 km-wide sand ridges, and the other to the east, dominated by 0.2–1.0 km-wide sorted bedforms, is utilized in conjunction with robust visualization techniques to classify the surficial bedforms into medium dunes (7.4–13.6 m wavelength, 2.2–4.6 cm high), hummocky bedforms (4.9–9.4 m wavelength, 2.6–6.7 cm high) and 2.5-dimensional dunes (4.7–6.8 m wavelength, 3.9–7.2 cm high). Storm-driven hydrodynamics that drive the migration of the large-scale shoreface-attached bedforms effect the grain size distribution and hence the distribution of small-scale bedforms across both sand ridges and sorted bedforms. The small-scale bedforms are interpreted to have formed under wave-dominated conditions, based on the near-zero asymmetry index. The nature and character of hummocky bedforms examined in this study improves our three-dimensional perspective of this sedimentary structure, providing tools for better identification of the structure from the past geological record, and interpretation of hydraulic processes attributed to its origin.

Comprehensive investigation of submarine slide zones and mass movements at the northern continental slope of South China Sea

Multi-beam bathymetry and seismic sequence surveys in the northern slope of the South China Sea reveal detailed geomorphology and seismic stratigraphy characteristics of canyons, gullies, and mass movements. Modern canyons and gullies are roughly elongated NNW–SSW with U-shaped cross sections at water depths of 400–1000 m. Mass movements include slide complexes, slide scars, and debris/turbidity flows. Slide complexes and slide scars are oriented in the NE–SW direction and cover an area of about 1790 and 926 km2, respectively. The debris/turbidity flows developed along the lower slope. A detailed facies analysis suggests that four seismic facies exist, and the late Cenozoic stratigraphy above the acoustic basement can be roughly subdivided into three sequences separated by regional unconformities in the study area. The occurrence of gas hydrates is marked by seismic velocity anomalies, bottom-simulating reflectors, gas chimneys, and pockmarks in the study area. Seismic observations suggest that modern canyons and mass movements formed around the transition between the last glacial period and the current interglacial period. The possible existence and dissociation of gas hydrates and the regional tectonic setting may trigger instability and mass movements on the seafloor. Canyons may be the final result of gas hydrate dissociation. Our study aims to contribute new information that is applicable to engineering construction required for deep-water petroleum exploration and gas hydrate surveys along any marginal sea.

Coastal hazard due to submarine canyons in active insular volcanoes: examples from Lipari Island (southern Tyrrhenian Sea)

The recent high-resolution multibeam bathymetry surveys around Lipari Island allowed to evidence several submarine canyons, whose head often cut back up to very shallow water and at a few tens of meters far from the coast. These canyons are mainly located in the eastern and southern side of the island and are characterized by an ongoing retrogressive (landward) erosion, that also controlled the shape and the evolution of the coastline. The canyon heads are formed by minor slide scars. By coupling slide scar morphometry and simple numerical model we have been able to roughly estimate the potential tsunami wave amplitudes generated by related slope failures. Moreover, the retrogressive erosion of canyon heads can be claimed as a cause of the enhanced subsidence reported in the last few thousand years in the eastern part of Lipari, where the main villages are located. Based on these evidence, we propose a first assessment of the coastal hazard due to marine retrogressive activity in the largest and most densely populated island of the Aeolian Archipelago.

Abrupt emergence of a large pockmark field in the German Bight, southeastern North Sea

A series of multibeam bathymetry surveys revealed the emergence of a large pockmark field in the southeastern North Sea. Covering an area of around 915 km2, up to 1,200 pockmarks per square kilometer have been identified. The time of emergence can be confined to 3 months in autumn 2015, suggesting a very dynamic genesis. The gas source and the trigger for the simultaneous outbreak remain speculative. Subseafloor structures and high methane concentrations of up to 30 μmol/l in sediment pore water samples suggest a source of shallow biogenic methane from the decomposition of postglacial deposits in a paleo river valley. Storm waves are suggested as the final trigger for the eruption of the gas. Due to the shallow water depths and energetic conditions at the presumed time of eruption, a large fraction of the released gas must have been emitted to the atmosphere. Conservative estimates amount to 5 kt of methane, equivalent to 67% of the annual release from the entire North Sea. These observations most probably describe a reoccurring phenomenon in shallow shelf seas, which may have been overlooked before because of the transient nature of shallow water bedforms and technology limitations of high resolution bathymetric mapping.

Submarine canyon-head morphologies and inferred sediment transport processes in the Alías-Almanzora canyon system (SW Mediterranean): On the role of the sediment supply

Canyon head regions are key areas for understanding the shelf-to-canyon sedimentary dynamics and assessing the predominant hydrodynamic and sedimentary processes shaping their morphology. High-resolution multibeam bathymetry surveys conducted on the shelf, upper slope and along the main thalwegs of the Alías-Almanzora submarine canyon system were used to recognize the specific morphological features of the various canyon heads and infer the main sediment transport mechanism operating through them. Off the Almanzora River, different canyon head morphologies were observed at 65–90m water depth, incising the outer prodeltaic deposits. A direct connection from the river supply is evidenced by the presence of a large radial field of sediment waves on the prodelta, presumably related to hyperpycnal flows during flash flood events, and their continuation towards trains of cyclic steps that merge into the Almanzora canyon head. Processes linked to seepage and groundwater sapping also seems apparent in the prodeltaic deposits, which results in pockmark alignments and formation of elongated narrow canyon heads. At the southern limit of the Almanzora prodelta, another type of canyon head results from the formation and merging of linear gullies. These have been interpreted as the morphological expression of the distal off-shelf transport of flood-related hyperycnal flows. However, most of the canyon heads of the main tributaries of this canyon system (i.e., Garrucha, Cabrera and Alías) are found at much shallower water depths (7–20m). They generally show a continuation in land with rivers or intermittent creeks (known as “ramblas”) that also experience flash flood events, but some of them are disconnected from any river source. These canyon heads cut into the infralittoral prograding wedge, displaying feeder channels that tend to oriented against the direction of the prevalent long-shore current. Some canyon head tributaries show sediment waves along their axis interpreted as the result of hyperpycnal flood events or storm-induced turbidity flows. This study focusses on the diversity of sediment transport processes that can co-exist in the various canyon tributaries of a given submarine canyon system, stressing on the importance of the canyon heads location with respect to the principal sedimentary source, which ultimately determines their main geomorphological traits.

Magmatic and tectonic extension at the Chile Ridge: Evidence for mantle controls on ridge segmentation

AbstractWe use data from an extensive multibeam bathymetry survey of the Chile Ridge to study tectonomagmatic processes at the ridge axis. Specifically, we investigate how abyssal hills evolve from axial faults, how variations in magmatic extension influence morphology and faulting along the spreading axis, and how these variations correlate with ridge segmentation. The bathymetry data are used to estimate the fraction of plate separation accommodated by normal faulting, and the remaining fraction of extension, M, is attributed primarily to magmatic accretion. Results show that M ranges from 0.85 to 0.96, systematically increasing from first‐order and second‐order ridge segment offsets toward segment centers as the depth of ridge axis shoals relative to the flanking highs of the axial valley. Fault spacing, however, does not correlate with ridge geometry, morphology, or M along the Chile Ridge, which suggests the observed increase in tectonic strain toward segment ends is achieved through increased slip on approximately equally spaced faults. Variations in M along the segments follow variations in petrologic indicators of mantle melt fraction, both showing a preferred length scale of 50 ± 20 km that persists even along much longer ridge segments. In comparison, mean M and axial relief fail to show significant correlations with distance offsetting the segments. These two findings suggest a form of magmatic segmentation that is partially decoupled from the geometry of the plate boundary. We hypothesize this magmatic segmentation arises from cells of buoyantly upwelling mantle that influence tectonic segmentation from the mantle, up.

An overview of the Sediment Characterization Experiment 2015 survey cruise

The acoustics of fine grained marine sediments, how they impact shallow water acoustic propagation, and the use of inverse and statistical inference techniques for sediment characterization are being studied in this multi-year, multi-institutional project, which is focused on an area of the Southern New England continental shelf known as the New England Mud Patch. An overview of the first survey cruise associated with this project, which took place in July and August of 2015, will be presented. Measurements during that cruise included sub-bottom profiling and multi-beam bathymetry surveys of the experimental area, as well as CTD and gravity core, box core, and multicore sampling. A limited number of acoustic propagation measurements were made along with ship-board laboratory measurements of recovered sediment sound speed and attenuation measurements. A preliminary assessment of the presence and effect of ocean bottom biology was conducted. In this talk, an overview of the survey cruise will be presented. ...

Continental shelf landscapes of the southeastern United States since the last interglacial

The wide, sediment-starved continental shelf and modern coastal areas of the southeastern United States retain well-preserved but scattered remnants of a submerged paleolandscape. This paper presents a conceptual model of stratigraphic deposition and landscape formation since the last interglacial on the continental shelf of South Carolina, with portions of North Carolina, Georgia, and Florida (USA). Data for this study include multibeam bathymetry surveys, sidescan sonar mosaics, high-resolution subbottom profiles, and ground-truth surveys from −250m to the modern tidewater region.Four bathymetric zones are recognized with eleven landforms and landform indicators. The described zones range in depths from the modern shoreline, across the shelf, and over the shelf edge to −250m MSL. Relative sea level curves are presented for the area and discussed in conjunction with cultural and climatic events.The potential for preservation of Paleoamerican sites is high at the shelf edge between −130m and −45m, with Archaic and later occupations likely in depths of less than −25m. Prominent vantage points for Paleoamericans (>11kya) would have existed at the shelf edge, and tidewater resources would have been available nearby for a period of almost 6ka. Rapid transgression rates (>60km/ka) after the sea level rose over the shelf edge make preservation of tidewater sites less likely on the outer and middle shelf. Searches for the earliest Paleoamericans should focus on promontories at the edge of the shelf and along future discoveries of paleoincisions on the shelf. Mapping and delineating this paleolandscape and associated unconsolidated sedimentary deposits interspersed with rocky plains and ledges will continue to be a priority to marine archeologists, coastal managers, fishery scientists, and marine spatial planners over the next several decades.

Sediment transport patterns in the San Francisco Bay Coastal System from cross-validation of bedform asymmetry and modeled residual flux

Abstract The morphology of ~ 45,000 bedforms from 13 multibeam bathymetry surveys was used as a proxy for identifying net bedload sediment transport directions and pathways throughout the San Francisco Bay estuary and adjacent outer coast. The spatially-averaged shape asymmetry of the bedforms reveals distinct pathways of ebb and flood transport. Additionally, the region-wide, ebb-oriented asymmetry of 5% suggests net seaward-directed transport within the estuarine-coastal system, with significant seaward asymmetry at the mouth of San Francisco Bay (11%), through the northern reaches of the Bay (7–8%), and among the largest bedforms (21% for λ > 50 m). This general indication for the net transport of sand to the open coast strongly suggests that anthropogenic removal of sediment from the estuary, particularly along clearly defined seaward transport pathways, will limit the supply of sand to chronically eroding, open-coast beaches. The bedform asymmetry measurements significantly agree (up to ~ 76%) with modeled annual residual transport directions derived from a hydrodynamically-calibrated numerical model, and the orientation of adjacent, flow-sculpted seafloor features such as mega-flute structures, providing a comprehensive validation of the technique. The methods described in this paper to determine well-defined, cross-validated sediment transport pathways can be applied to estuarine-coastal systems globally where bedforms are present. The results can inform and improve regional sediment management practices to more efficiently utilize often limited sediment resources and mitigate current and future sediment supply-related impacts.

Response of megabenthic assemblages to different scales of habitat heterogeneity on the Mauritanian slope

The topographically complex deep seabed on the Mauritanian slope, from 990 to 1460m water depth, was imaged with video in an extensive quantitative survey of 17,199m2 of seafloor using a Remote Operated Vehicle (ROV). This study investigated the influence of habitat heterogeneity at two scales on the megafaunal assemblages observed by ROV. Changes in megafaunal assemblages on the Mauritanian slope were assessed at a broad scale, within depth zones, and at a finer scale, in response to changes in local geomorphology associated with submarine landslides. Geomorphology was determined by classification of habitat parameters (slope, aspect, bathymetric position, curvature, fractal dimension and ruggedness) derived from an autonomous underwater vehicle-based multibeam bathymetry survey. Habitat parameters were classified by Iterative Self Organizing Clustering into six major geomorphological groups, four of which were assessed in the ROV video survey. A total of 29 megafaunal taxa were observed along the entire survey, with an overall average faunal density of 0.344 indm−2. Megafaunal assemblage density, species richness and evenness varied significantly across the depth range of the survey in the most common geomorphological zone (sedimentary plains of low slope and complexity). Characteristic species inhabited the shallow areas (asteroid, ophiuroid, anemone, small macrourid), intermediate areas (Benthothuria funabris, black cerianthid, squat lobster) and deeper areas (the holothurians Enypniastes eximia and Elipidia echinata). Megafaunal density, species richness and evenness were not significantly different between geomorphogical groups within one depth zone (1300–1400m). However, the steepest zone, on the edge of a major headwall feature, had four unique taxa (Parapagurus pilosimanus, a comatulid crinoid, a gorgonian and its associated ophiuroid).

Bathymetric constraints on the tectonic and volcanic evolution of Deception Island Volcano, South Shetland Islands

AbstractDeception Island is the largest volcano in the actively extending Bransfield Basin, a marginal basin situated behind the extinct South Shetland Islands arc. Deception Island has been well studied but its submerged flanks have not. A multibeam bathymetry survey was conducted around the island in 2005. Data from the flooded caldera show no evidence for recent localized resurgence. The gently-sloped bottom of the caldera basin is consistent with either a broad zone of resurgence on its east side associated with trap door deformation or with higher rates of sediment supply from the east side of the island. Around the island, numerous tectonic and volcanic features on the volcano's east and west flanks are nearly all aligned with the regional strike (~060°) of the Bransfield rift and there is very little evidence for the other fault populations that have been identified on the island. We infer that models that link the ongoing tectonic development of Deception Island to complex regional tectonics are less likely than models in which the dominant regional extension in Bransfield Strait is modulated by the local effects of caldera collapse and possibly a small right-lateral transfer zone offsetting the primary extension axes in the Central and Western Bransfield Basins.

Late Quaternary history of the Nouméa lagoon (New Caledonia, South West Pacific) as depicted by seismic stratigraphy and multibeam bathymetry: A modern model of tropical rimmed shelf

The barrier reef tract of New Caledonia is the second largest in the world enclosing lagoons of variable width. The Late Quaternary evolution of these barrier reefs and lagoons is poorly known. A recent high resolution seismic and multibeam bathymetry survey was carried out in the south-western lagoon from shoreline to open ocean across the barrier reef pass systems to perform sequence stratigraphy and to better understand the Quaternary evolution of the lagoon. Two depositional sequences bounded by type-1 erosional surfaces can be identified. The bounding surfaces delineate the bottom of two generations of incised valleys. System tracts comprising a complete succession of regressive, lowstand, transgressive and highstand tracts were observed only in the upper sequence. Chronostratigraphic and sedimentological interpretations of the seismic data are based on correlations with the chronology defined in the nearby cores drilled through the reefs and in the gravity cores previously recovered in the inner lagoon. Interpretation of seismic data is also carried out on the basis of correlation with multibeam bathymetric data. Our results indicate that the history of the New Caledonian lagoon probably does not exceed 200ka marked by two or three brief episodes each of immersion of about ten thousand years. The bathymetric data reveal that during successive low sea level stands, the lagoon was dependent upon erosional fluvial networks. The deep fluvial valleys were connected to the continental slope through the passes. Seismic records reveal that upstream palaeo-channels are not connected to the modern coastal rivers. In contrast, many aggrading deposits, including numerous stacking channels, and extending across the proximal part of the outer lagoon, can be identified. Examination of drainage network morphology suggests a progressive tilt of the outer lagoon. Such evolution reveals that the Nouméa lagoon which is one of the widest in the world is an exceptional modern example of a rimmed shelf providing data to analyze the past counterparts.

Multibeam bathymetric investigations of the morphology of sand ridges and associated bedforms and their relation to storm processes, Sable Island Bank, Scotian Shelf

Storm-formed sand ridges are important morphologic features occurring on many continental shelves of the world. Our understanding of the morphology, migration, and the controlling processes over this type of sand ridges under different environmental settings is still limited. In the present study, an integrated approach of combining multibeam bathymetry survey, seabed sampling, sidescan/seismic surveys and seabed instrumentation measurements was used to characterize the surficial geology and morphology of offshore sand ridges on Sable Island Bank, Scotian Shelf, and to evaluate the relationship between sand ridge morphology, migration and the storm processes on this storm-dominated outer shelf bank. Multibeam data obtained at selected sites demonstrated the series of NE–SW oriented sand ridges with an average height of 4.3 m and wavelength of 1.5 km. Sand ridges attain maximum spacing, height and steepness in the intermediate depths. Sand ridge size and steepness also decrease from west to east and this decrease is correlated with decreasing dynamic condition, sediment grain size, and depth. Sand ridge profiles show weak easterly asymmetry. This morphological asymmetry augments the sediment texture and grain size asymmetry to suggest eastward long-term sand ridge migration and net sediment transport in the study area. Sand ridge asymmetry also shows strong local variation and the number of sand ridges asymmetric to the west is nearly equal to that asymmetric to the east. The likely cause is the nearly equal number of major storms passing to the south versus to the north of the Island so that the westerly and easterly peak storm-driven currents associated with these storm path scenarios affect equally the sand ridges on Sable Island Bank. A suite of smaller bedforms were found superimposed on the sand ridges: sand waves widely superimposed on both flanks of sand ridges, mixed 2D and 3D megaripples commonly occurring in the sand ridge troughs and on the lower western flanks, and large-wave ripples ubiquitously found in the troughs of all megaripples. Surficial sediment is coarsest (coarse sand with shell fragments) in the sand ridge trough and becomes progressively finer eastward toward the sand ridge crest and down the eastern flank, while best sorting occurs near the sand ridge crest. Box cores collected across sand ridges reveal characteristic storm event bed structure that reflects the development and migration of bedforms with the evolution of storms.