Sedimentary Seabeds Research Articles

Application of acoustic classification in different sedimentary environments: A case study of the Gaoping slope in the southwest coast of Taiwan

• Acoustic remote sensing can be used to determine different types of sedimentary seabeds using quantitative classification. • Three attributes extracted from acoustic waveforms reveal four types of sedimentary seabeds. • The proposed method differentiates sedimentary seabed types with similar grain size distribution. • Core data from the offshore area of southwestern Taiwan validates the method. • This study provides a sensitive and robust quantitative method and reduces the uncertainty of manual identification between different seabeds. The sedimentary seabed properties are direct indicators of its stability, whose measure has several important implications in understanding and forecasting geological events. This study investigates the classification of sedimentary seabed types in different depositional environments using the three attributes: reflection coefficient (RC), similarity index (SI), and amplitude ratio (AR) values extracted from sub-bottom profiling data. Four sedimentary seabed types were evaluated automatically according to the waveforms in the horizontal and vertical directions. To infer the seabed properties, the attribute SI is used to identify the occurrence of homogeneity of sediment transportation in an area, while RC and AR are used to determine the seabed characteristics in terms of the variation of acoustic energy reflected from the sediment. To validate the results of this quantitative analysis method, sixteen seafloor sediment samples collected at different sites in the offshore area of southwestern Taiwan were analyzed. The automatic classification results suggest that the different sub-bottom profiling images are associated with the grain size distributions and compositions, such as the concentrated distribution, sand, silt, and clay percentages, and the mean bulk density. In addition, an inhomogeneous environment with similar grain size distribution can be identified. The proposed seabed classification is an effective method to study seabed characteristics, especially for identifying chaotic sediments.

Optical Illusions of the Lower Atmosphere

Optical Illusions of the Lower Atmosphere Natalie Vestin (bio) Before any sea was the sea, it was the sea over the future. Before any home was a home, it was a seabed, the now-home missing its giant cephalopod and trilobite, dry-home whispering of saline and unicellular precursors of life. It was the autumn of pretending I was under an enormous, ancient ocean. It was the autumn of being reluctant to leave my apartment, of closing myself in. It was the autumn of being very still and very quiet. During the Pre-Cambrian era, an ocean covered Minnesota, which is to say, there was no Minnesota save sedimentary seabed. Though it seems rash to dismiss home so easily as something that might never have been. I carry and cultivate in me a sickness for home that includes its past, its cephalopods, its could-be-life. Is it unreasonable to suspect that the seabed once weighed itself against a sickness for the future, for conifer and erupting cliff and even the red steppes of the taconite mines? The layers of this ocean have names and colors and life they greet with measured opportunity and hostility. Cambrian, Ordovician, Silurian, Cretaceous. Each era had its own ocean, even if ocean were a retreating ghost or patience for the next encroaching sea. During the Cambrian, the ocean over Minnesota brought forth the brachiopods, sea slugs, graptolites, and trilobites, whose fossils still cover the northeastern cliffs. The Ordovician, whose advent the sea creatures may have noticed in the way they register the subtle changes in the day's light or the tides, heralded more complex biology: cephalopods, bryozoans, mollusks, and worms whose circulatory systems promised us our soon-blood and someday-vessels and if-you're-lucky hearts. And then, during the Silurian, the sea over Minnesota, sea full up with life stretching its mistake-laden helical arms into a wealth of descendants, vanished as if it never had been. And then, in the Devonian appeared a sea in the same form but made different for the disappearing. And then, the new sea retreated once more and a newer same sea emerged to haunt its own beds during the Cretaceous, bringing with it ammonoids, oysters, and clams. Imagining the oceans atop and below me during the autumn when I was very still and very quiet allowed a world I couldn't look at directly to be different, false and yet real as a haunting. The ocean became an invisible cloudscape, a before-world saturating the present when summoned and given form by the accumulation of its many names. _______ It was the autumn I started a new job researching the treatment of bacterial infections in critically ill people, though I strayed frequently from documenting research on pathology to studying the behavior of bacteria, the opportunities we give them or the licenses they take that drive our knowledge about them into a trajectory whose aim is extermination. I study the Kingdom of Monera, the mostly microscopic universe composed of one-celled prokaryotes. I study one part of the multiverse. My dad taught me about the possible multiverse when I was a [End Page 134] child, about how many of our lives might be lived in parallel, how a choice or a story can unspool in the fates of multiple selves, though no one can really say how and few wish to think of consequences sown or reaped without choice. When I study bacteria for work, I study them indirectly as the effect they bear on a life. I research treatment, pharmacodynamics, antibiotic troughs, and resistance. And when I learn about bacteria as a part of my ocean only for this autumn, I study the protection they offer each other, their resistance toward destruction, the forms of their communities known through microscopy and light scatter, their bodies and strange breath, their kinship, their homes, and their struggles. Seeking an understanding of Monera means confronting how to define life and how and when to protect or end it. It is a seeking that illuminates the reality that, in this universe, I am never entirely myself but instead a haphazard collection of ocean and microbiota, a form whose...

Distribution patterns of deep-sea fish and benthic invertebrates from trawlable grounds of the Hatton Bank, north-east Atlantic: effects of deep-sea bottom trawling

Distributions of deep-sea fish, benthic invertebrates and the effects of deep-sea bottom trawling were studied based on data collected in 2005 from a joint collaboration survey undertaken between the Spanish Institute of Oceanography and a deep-sea trawler on the Hatton Bank (north-east Atlantic). A total of 163 valid bottom trawl hauls (600–1600 m) were analysed. The main trawlable grounds were located on the sedimentary seabed of the western flank of the bank (Hatton Drift). Grenadiers and smoothheads were predominant in the trawl catches (67% and 11.8% by weight respectively). Both species were abundant along the western flank. Deep-water sharks accounted for 7.4% of weight, and were abundant along the south-eastern slopes. Chimerids, lotids, morids and other deep-sea species were also taken as by-catch. Grenadiers and deep-water sharks dominated the discards. By-catches of cold-water corals were generally associated with the rocky outcrop and were more abundant at the top of the bank. Abundant by-catches of large sponges, characteristic of sponge-dominated biotopes, were taken from the eastern flank.

A probabilistic analysis of random wave-induced liquefaction

Seabed instability caused by soil liquefaction due to build-up of excess pore pressure within the sedimentary seabed represents a serious threat to coastal structures. Models of varying sophistication exist for predicting the liquefaction process but most previous calculations are limited to regular waves while the real waves are random. In this study, a numerical study of liquefaction potential of a sand bed under narrow-band random waves is carried out employing ensemble modelling techniques. The aim of the work is to investigate the effect of random waves on excess pore pressure build-up and liquefaction processes and study the probability distribution of the maximum liquefaction depth. The computational results using a 1D liquefaction model indicate that the random wave-induced liquefaction can be much deeper than that of the corresponding regular waves with the largest individual waves in the random wave time series playing a dominant role in determining the maximum liquefaction depth. It is also found that the time for the maximum liquefaction depth to be reached can vary considerably from one random wave series to another, which suggests that in random waves notable densification may occur within the same timeframe as that for liquefaction.

Morphological characteristics of the Basque continental shelf (Bay of Biscay, northern Spain); their implications for Integrated Coastal Zone Management

This contribution integrates and analyzes data from high-resolution multibeam, seismic profiles, bathymetric LiDAR, and surficial sediment data for the geomorphological seascapes characterisation and process-description of the Basque inner and middle continental shelf (northern Spain). From the data obtained, the Basque shelf is characterised by a heterogeneous seafloor where, on a small spatial scale, different morphologies and sedimentary processes can be observed. Tectonic activity and sea-level changes, together with present processes of sediment supply and climatic conditions, have a critical influence on the present configuration of the continental shelf and the distribution of seafloor types. On the basis of all of the datasets, seafloor classification, bedform analysis, long-term sea-level change-induced seafloor features and anthropogenic features over the seafloor, are described. As a result, three distinct zones have been identified for the Basque shelf, related to the main geomorphological features and seascapes, as summarised below. (i) A western part, with a northwestern orientation, which coincides with the prevailing wave direction. This zone is characterised by a predominantly rocky substrate, except within the mouth of the Nervión estuary, where sandy sediments appear to infill a paleo-channel. (ii) A central part, which is oriented towards the northeast and receives less wave energy. Within this zone, a sedimentary seabed is predominant. Sorted bedforms are well developed and can be detected in water depths in excess of 90 m. (iii) An eastern zone, which is characterised by sedimentary seafloor and shore terraces; here, a flat rocky seafloor is covered by a thin layer of sediments. Throughout the study area, the bathymetry and sedimentary features of the shoreface and inner shelf are controlled mainly by climatic conditions, coastal dynamics and the underlying geological framework. The integration of these data sets, together with their interpretation, is a valuable source of information for Integrated Coastal Zone Management (ICZM) and constitutes a useful tool for implementing various European Directives.

Female southern right whales Eubalaena australis: Are there reproductive benefits associated with their coastal distribution off South Africa?

Predictability in both meso- and microscale distribution of southern right whale Euba- laena australis females off the coast of South Africa is attributed to maternal philopatry, as well as favourable environmental conditions (calm water, generally shallow sloping, sedimentary sea bed) conducive to reduced energy consumption and a lowered risk of injury for both cows and calves. Spa- tial differences in reproductive success were compared between favoured (nursery) and non- favoured (non-nursery) areas and related to environmental differences in those areas. Reproductive success was inferred from analysis of calving intervals (3 yr = successful, 2 and 4 yr = unsuccessful; n= 808 calving events). Resighting rates as well as differences in distribution between experienced cows (3 or more calves) and inexperienced cows (1 or 2 calves) were also analysed. Less experienced cows were found to have significantly more unsuccessful calving intervals than experienced cows. No relationship was found between calving success and spatial distribution using calving interval analysis. Significant spatial differences in stranding rates of neonatal calves strongly suggest higher rates of calf mortality in areas dominated by non-cows, independent of environmental conditions. Social structure within wintering grounds was concluded to be more important than previously thought and potentially as important as environmental conditions for calf survival.

Analysing the impact of bottom trawls on sedimentary seabeds with sediment profile imagery

Sediment profile imagery (SPI) was evaluated for the assessment of otter trawling impacts on the seabed. This technique allows the imaging of the topmost sediment layers in profile, including the sediment–water interface. Two areas in the Aegean Sea were investigated in time series, each with control and impact areas: a commercial fishing lane with soft sediments at approximately 200 m depth and an experimentally trawled lane with harder maerly sediments at approximately 80 m depth. In total, 158 images were taken at the deep ground and 124 at the shallow ground. A number of measurements were taken from each image, leading to estimates of comparative penetration and small-scale seabed surface roughness. In addition, a large number of surface and subsurface attributes were noted in the images to form the basis of a multivariate analysis. Results indicated that penetration and roughness by themselves were not very good indicators, although roughness was a better indicator particularly in coarse sediments. The major reason for this is that the measurements alone (in particular roughness) do not distinguish between biological and anthropogenic disturbance. The multivariate analysis combining the measurements with the attributes was a good indicator in investigating trawling impacts in coarse sediments, where the lack of good penetration can be compensated by the view over the sediment surface, where more attribute-type data can be gathered. The SPI sampling window gives a relatively small imaged sample in comparison to other imaging techniques (side scan sonar, video, etc.) and in a heterogeneous environment, the more the replicates, the more reliable the method will be. A tiered imaging approach is recommended where more than one methodology is used.

On the spatial scale needed for benthos community monitoring in the coastal North Sea

In community monitoring an attempt is made to identify long-term trends by regular sampling of selected sites. Since the benthos is reputed to be fairly sedentary, the spatial resolution is often reduced to single sites. However, members of many benthic invertebrate species have been found drifting across sedimentary seabeds in shallow waters. Transportation by currents may result in changes of their spatial pattern in the sediment, thereby changing local community composition. The quantitative importance of drifting was tested by repeated sampling of a 2-km 2 shallow (10 m) offshore area west of the island of Sylt (North Sea). Within the fortnight period between two samplings the benthic community composition had changed dramatically. Despite fairly calm weather, translocation of organisms by currents exceeded 1 km. In about half of the species, the spatial changes in abundance within these two weeks roughly equalled the average variation between consecutive years. This example suggests that community monitoring needs a wide spatial scale to discriminate long-term temporal changes from short-term variability. Extending the sampling area from 2 to 180 km 2 strongly reduced the variability of abundance estimates. However, only in a few species was the spatial distribution over the sampling sites found on one sampling date a suitable estimator for the spatial pattern found one or two months later, at the same sites. Instead the spatial patterns of the fauna changed strongly during a single month with a spatial scale of re-distribution exceeding several km in some species. At the same time the granulometric sediment composition changed, indicating changes of habitat quality. Hence, sampling of a large area, with random selection of the sampling sites on each sampling date, is suggested to yield the most reliable estimates of population development in the coastal North Sea. However, in view of the expected spatial scale of re-distribution during storm tides and the spatial variability of recruitment, even a 180-km 2 sampling area may be too small.

Vertical coherence of ambient noise in shallow water overlying a fluid seabed

Broadband (200 Hz–20 kHz) measurements of the vertical coherence of ambient noise in two well-surveyed, shallow-water channels with fluid, sedimentary seabeds are reported. The noise at one of the sites, the StrataForm natural laboratory off Eureka, northern California, was occasionally found to be exclusively wind generated, with negligible contributions from surface traffic and biological sources. Under these conditions, the theoretical coherence, computed using the known properties of the sediment at Eureka, closely follows the coherence data up to a frequency of 20 kHz. Subtle effects due to the finite depth of the sources, that is, the bubbles produced by wave breaking, are evident in both the theory and the data. At the second site, in Jellicoe Channel, New Zealand, some 64 km north of Auckland, wind noise again was a major factor, but much of the time local shipping also contributed to the overall noise field. Two sets of coherence data from the New Zealand site are examined, the first taken in near-isovelocity conditions, when the noise was due to wind plus a local ship, and the second recorded in the absence of shipping, when the channel showed a mildly linear, downward refracting profile. In both instances, the coherence data over a 5-kHz band compare well with theoretical curves computed using the known geoacoustic parameters of the sediment in the region.

Vertical coherence of ambient noise in shallow water: Application to broadband inversions

Broadband (200-Hz to 20-kHz) measurements of the vertical coherence of ambient noise have been made in two well-surveyed, shallow-water channels with fluidlike, sedimentary seabeds. The noise at one of the sites, the StrataForm natural laboratory off Eureka, northern California, was occasionally found to be almost exclusively wind generated. Under such conditions, the theoretical coherence, computed using the known properties of the sediment at Eureka, closely matches the data. Subtle effects associated with the depth of the sources (bubbles from breaking waves) are evident above 6 kHz in both the theory and the data. At the second site, in Jellicoe Channel, New Zealand, wind noise was also a major factor but shipping sometimes contributed significantly to the field. Again, theory and data matched well when the known geoacoustic properties of the bottom were used as inputs to the model. Based on these results, it appears that the vertical coherence of ambient noise in shallow water is predictable over a bandwidth as high as 20 kHz, and accordingly has potential as the basis of an inversion technique for obtaining the geoacoustic properties of the sea floor.

Time and angle spreads due to scattering by bottom roughness and sediment volume inhomogeneity

A bistatic scattering model that treats both roughness and sediment volume scattering is used to infer time and angle spreading for sedimentary seabeds. This model is embedded in a simulation code that predicts the received intensity time series taking account of single-bounce bistatic geometry as well as transmitter and receiver directivity. Roughness scattering is treated in the Kirchhoff approximation without invoking the stationary-phase approximation. Sediment volume scattering is treated in the perturbation approximation. Both roughness and volume inhomogeneity are assumed to be random processes with power-law spectra. The relative importance of roughness and sediment volume scattering as spreading mechanisms and the energy loss due to forward scattering are examined for a range of sediment types.

Wave-induced liquefaction potential in a porous seabed in front of a breakwater

A three-dimensional short-crested wave system produced by waves reflecting obliquely from a breakwater is more conducive to soil instability in the porous seabed than the action of a two-dimensional progressive wave. The excess pore pressure induced within the sedimentary seabed can result in liquefaction and trench scouring; hence causing structural failure. General solutions are presented for soil response in a porous seabed induced by a partial short-crested wave formed by angled incident waves and partially reflected waves. The potential of liquefaction under the action of such a wave system is evaluated for the regions in the seabed fronting a breakwater. The effects of several pertinent wave and soil parameters on the liquefaction potential are then examined in detail—these including reflection coefficient, relative water depth, degree of soil saturation and soil permeability.

Porosity, permeability, and shear strength images of the seabed measured by acoustic cross-well tomography experiments

Cross-well tomography of the sedimentary seabeds below man-made islands in Tokyo Bay and Florida Bay was affected by very high attenuation and very high background noise; nevertheless, high resolution (0.2 to 1 m) velocity images were obtained between wells separated by long distances (50 to 250 m). A piezoelectric source in a 60-m-deep well-used long sequences (4095 cycles) of phase-modulated pseudo-random binary codes at high carrier frequencies (up to 10 kHz). A 24-channel hydrophone array in another well received the signal. Inversion of first arrival times by damped least-squares imaged the compressional wave velocities. Assuming the normal consolidation condition, the porosity and shear strength images were predicted from the compressional wave velocity image. The differences in the compressional wave velocity images obtained using different carrier frequencies were used to determine the permeability image of sediments based on the Biot theory. The direct measurements of porosity and shear strength conducted on the cores and the standard penetration test boreholes were used to verify the tomographic predictions, resulting in good agreements. The direct measurements of permeability from pumping tests confirmed the tomographically determined permeability images. [Work supported by KSC and ONR.]