Side-scan Images Research Articles

Sediment budget analysis in light of high-resolution echo sounding of seabed topography

Coastal erosion is a long-standing serious issue in Japan and extensive lengths of coast lines have been protected with seawalls, armour units and detached breakwaters. Extreme cases of coastal protection can render sandy beaches totally isolated from otherwise available sediment-supplying systems. The authors took up Ogata Beach on the Joetsu coast as such and applied a high-resolution side scan imaging with bathymetry to it, revealing marked morphological changes underwater. What is most noteworthy is that the areas of significant erosion extended offshore beyond the so-called closure depth.

Automated acoustic seabed classification of multibeam images of Stanton Banks

Dividing sidescan images into regions that have similar seabeds is often done by expert interpretation. Automated classification systems are becoming more widely used. This paper describes techniques, based on image amplitudes and texture, that lead to useful and practical automated segmentation of multibeam images. Seabed (or riverbed or lakebed) type affects amplitudes and texture, but so do system operating details and survey geometry. Effects of the last two must be compensated to isolate the effects of seabed type. Images from multibeam surveys are accompanied by bathymetric data from which grazing angles of all sonar footprints can be calculated. By compiling tables of amplitude against range and grazing angle, systematic changes in amplitude with these two variables can be removed consistently. Classification, based on a large number of features, is done in image space to avoid artifacts common in mosaics. Unsupervised segmentation requires clustering, in which records are divided into their natural classes. An objective clustering method using simulated annealing assigns points to classes based on their Bayesian distances from cluster centres. Stanton Banks is a rocky area 100 km north of County Donegal, Ireland, that rises about 100 m above the ocean floor at 180 m. Multibeam images and data from an 80-km 2 survey were classified into regions of acoustic similarity. Assigning labels of physical properties to these regions requires non-acoustic ground truth, which was obtained from a series of 105 photographs. Photographic geological assignments were found to correlate well with the acoustic classes.

Superellipse Fitting for the Recovery and Classification of Mine-Like Shapes in Sidescan Sonar Images

Mine-like object classification from sidescan sonar images is of great interest for mine counter measure (MCM) operations. Because the shadow cast by an object is often the most distinct feature of a sidescan image, a standard procedure is to perform classification based on features extracted from the shadow. The classification can then be performed by extracting features from the shadow and comparing this to training data to determine the object. In this paper, a superellipse fitting approach to classifying mine-like objects in sidescan sonar images is presented. Superellipses provide a compact and efficient way of representing different mine-like shapes. Through variation of a simple parameter of the superellipse function different shapes such as ellipses, rhomboids, and rectangles can be easily generated. This paper proposes a classification of the shape based directly on a parameter of the superellipse known as the squareness parameter. The first step in this procedure extracts the contour of the shadow given by an unsupervised Markovian segmentation algorithm. Afterwards, a superellipse is fitted by minimizing the Euclidean distance between points on the shadow contour and the superellipse. As the term being minimized is nonlinear, a closed-form solution is not available. Hence, the parameters of the superellipse are estimated by the Nelder-Mead simplex technique. The method was then applied to sidescan data to assess its ability to recover and classify objects. This resulted in a recovery rate of 70% (34 of the 48 mine-like objects) and a classification rate of better than 80% (39 of the 48 mine-like objects).

Multibeam bathymetry and sidescan imaging of the Rivera Transform–Moctezuma Spreading Segment junction, northern East Pacific Rise: New constraints on Rivera–Pacific relative plate motion

To better understand the recent motion of the Pacific plate relative to the Rivera plate and to better define the limitations of the existing Rivera–Pacific plate motion models for accurately predicting this motion, total-field magnetic data, multibeam bathymetric data and sidescan sonar images were collected during the BART and FAMEX campaigns of the N/O L'Atalante conducted in April and May 2002 in the area surrounding the Moctezuma Spreading Segment of the East Pacific Rise, located offshore of Manzanillo, Mexico, at 106°16′W, between 17.8°N and 18.5°N. Among the main results are: (1) the principle transform displacement zone of the Rivera Transform is narrow and well defined east of 107˚15′W and these azimuths should be used preferentially when deriving new plate motion models, and (2) spreading rates along the Moctezuma Spreading Segment should not be used in plate motion studies as either seafloor spreading has been accommodated at more than one location since the initiation of seafloor spreading in the area of the Moctezuma Spreading Segment, or this spreading center is not a Rivera–Pacific plate boundary as has been previously assumed. Comparison of observed transform azimuths with those predicted by the best-fit poles of six previous models of Rivera–Pacific relative motion indicate that, in the study area, a significant systematic bias is present in the predictions of Rivera–Pacific motion. Although the exact source of this bias remains unclear, this bias indicates the need to derive a new Rivera–Pacific relative plate motion model.

Glacial geomorphology of the Central Arctic Ocean: the Chukchi Borderland and the Lomonosov Ridge

AbstractThe last decade of geophysical seafloor mapping in the Arctic Ocean from nuclear submarines and icebreakers reveals a wide variety of glaciogenic geomorphic features at water depths reaching 1000 m. These findings provide new and intriguing insights into the Quaternary glacial history of the Northern Hemisphere. Here we integrate multi‐ and single beam bathymetric data, chirp sonar profiles and sidescan images from the Chukchi Borderland and Lomonosov Ridge to perform a comparative morphological seafloor study. This investigation aims to elucidate the nature and provenance of ice masses that impacted the Arctic Ocean sea floor during the Quaternary. Mapped glaciogenic bedforms include iceberg keel scours, most abundant at water depths shallower than ∼350–400 m, flutes and megascale glacial lineations extending as deep as ∼1000 m below the present sea level, small drumlin‐like features and morainic ridges and grounding‐zone wedges. The combination of these features indicates that very large glacial ice masses extended into the central Arctic Ocean from surrounding North American and Eurasian ice sheets several times during the Quaternary. Ice shelves occupied large parts of the Arctic Ocean during glacial maxima and ice rises were formed over the Chukchi Borderland and portions of the Lomonosov Ridge. More geophysical and sediment core data combined with modeling experiments are needed to reconstruct the timing and patterns of these events. Copyright © 2008 John Wiley & Sons, Ltd.

Sidescan Sonar Segmentation Using Texture Descriptors and Active Contours

This paper is concerned with the application of active contour methods to unsupervised binary segmentation of high-resolution sonar images. First, texture features are extracted from a sidescan image containing two distinct regions. A region-based active contour model of Chan et al. [J. Vis. Commun. Image represent, vol. 11, pp. 130-141,2000] is then applied to the vector-valued image extracted from the original data. Our implementation includes a new automatic feature selection step used to readjust the weights attached to each feature in the curve evolution equation that drives the segmentation. Results are shown on simulated and real data. The influence of the algorithm parameters and contour initialization are also analyzed.

Multiresolution 3-D Reconstruction From Side-Scan Sonar Images

In this paper, a new method for the estimation of seabed elevation maps from side-scan sonar images is presented. The side-scan image formation process is represented by a Lambertian diffuse model, which is then inverted by a multiresolution optimization procedure inspired by expectation-maximization to account for the characteristics of the imaged seafloor region. On convergence of the model, approximations for seabed reflectivity, side-scan beam pattern, and seabed altitude are obtained. The performance of the system is evaluated against a real structure of known dimensions. Reconstruction results for images acquired by different sonar sensors are presented. Applications to augmented reality for the simulation of targets in sonar imagery are also discussed.

Rapid mass wasting following nearshore submarine volcanism on Kilauea volcano, Hawaii

The rapid mass wasting of shallow submarine basalts was documented during SCUBA dives along the flanks of Kilauea volcano, Hawaii during the Kii lava entry of the current eruption (19°20.5′N, 154°59.8′W). Lava entered the ocean at this site from mid-February to late March 1990, with several pauses. Dives on 19–20 March 1990 confirmed the widespread formation of lava pillows at this site over a water depth range of 20–40 m, and visual observations suggested that the resulting volcanic deposits were generally stable, despite the steep (∼40°) incline of the seafloor. (The pre-eruptive nearshore seafloor slope was ∼14°.) However, dives on 2 April 1990 revealed that nearly all submarine volcanic features had been subject to mass wasting, as the offshore area had been transformed into a debris field composed of material ranging in size from fine sand to boulder fragments. This generally featureless seascape extended uniformly to beyond the visual range of divers (∼60 m water depth). High-resolution multibeam bathymetry and sidescan imaging indicate that steeply sloped coarse sediment extends down the flanks of Kilauea in this area to abyssal depths, implying a linkage between nearshore submarine volcanism and deep-water deposits.

Sediment mobility in the Pomeranian Bight (Baltic Sea): a case study based on sidescan-sonar images and hydrodynamic modelling

Sidescan-sonar surveys were performed on a 2×4 km area of seafloor in the southern Baltic Sea (Pomeranian Bight) in 1996 and 1998. Overlapping sub-areas of the individual surveys showing characteristic details were processed into geographically referenced mosaics. Sediment types were identified from echo characteristics and by comparison with granulometric data. The sea bottom covered by the mosaics consists predominantly of sand, with subordinate lag sediments with stones and small ripple fields consisting of coarse sand to fine gravel. A comparison of the two mosaics did not reveal any significant changes of the sea bottom over the 2-year period. Characteristic sedimentary features remained almost unchanged over this period on detailed sonar images of smaller sub-areas. Substantial transport of sandy sediments can thus be excluded in the course of the observation period. Grid files of advective velocity components and orbital velocity of wave motion of a three-dimensional hydrodynamical model for the period from September 1996 to October 1997 were used to estimate the current regime in the study area for the interval between the two sidescan surveys. Comparing critical velocities for the dominant sediment types with the results of the numerical bottom current simulations and the observations from sidescan images, it is apparent that strong current events during the modelled time interval were still too weak to resuspend and transport sand of any grain size, even though maximum current velocities of 30 cm/s at the seafloor were modelled. Only a few patches of newly accumulated (acoustically soft) material (mud, fluff and/or soft plant remnants), with a horizontal extension of about 10 m at a terrain step feature, were recognised in the 1998 mosaic. Our results imply that sand deposits in the southern Baltic Sea can remain stationary over time periods of several years, and that the transport of organic material, nutrients and associated pollutants to depositional areas in deeper water is predominantly accomplished by the movement of material finer than sand. A significant portion of this fine material is evidently transported in the bottom boundary layer under conditions of moderate hydrodynamical forcing.

Mapping seabed biotopes at two spatial scales in the eastern English Channel. Part 2. Comparison of two acoustic ground discrimination systems

Spatial surveys of marine benthic habitats and biota based on the interpretation of acoustic data were carried out at two sites in the eastern English Channel each representing different scales of geographic area and intensity of survey. A small area (4×12 km) crossing the Hastings Shingle Bank was surveyed at a relatively high intensity (track spacing 400 m) and was nested within a larger area between Hastings and Dungeness (12×40 km), which was surveyed at a lower intensity (track spacing 2 km). Surveys were conducted with two acoustic ground discrimination systems (AGDS), RoxAnn and QTC-VIEW and the primary purpose of the investigation was to compare the performance of the two AGDS using a common approach to analysis of the different data outputs (E1 and E2 for RoxAnn and the Q eigenvectors from QTC-VIEW). Exploratory data analysis using variography indicated that interpolation between tracks was justified for the smaller site to create a complete coverage, but was limited to the creation of a digital image of the track data for the larger area. Grab and video sample data were available for supervised classification of the AGDS data and interpreted sidescan images for comparison with unsupervised classification. Both AGDS gave similar outputs, although RoxAnn consistently gave slightly better levels of performance than QTC-VIEW as measured using error matrices. Although the investigation was not designed to compare the performance of AGDS and sidescan, the outputs from AGDS were similar to the visual interpretation of the sidescan sonar data. It was concluded that despite the inherent limitations of AGDS, they may be suitable for providing distribution maps at a broad scale that can give a context for the interpretation of finer scale survey of smaller, nested areas.

Pseudospectral Time-Domain Modeling of Non-Rayleigh Reverberation: Synthesis and Statistical Analysis of a Sidescan Sonar Image of Sand Ripples

The pseudospectral time-domain (PSTD) method is an accurate and efficient scheme for solving the acoustic wave equation numerically. It provides a good basis for a general sonar simulation model because all of the fundamental processes on which sonar depend occur as natural consequences of solving the wave equation. Propagation, interference, and spreading and absorption losses are intrinsic to this solution; reflection and scattering are governed by the distribution of materials within the simulated environment. These processes are analogous to those in the physical system being modeled. The method generates the full spatial and temporal evolution of the acoustic field for a specified model environment. This paper presents the application of a PSTD model to the simulation of a sidescan sonar system operating in deep water. Synthetic sidescan images of sand ripples are simulated using a directional fractal surface as the model sea bed. Different forms of time-varying gain are applied to the received signals to investigate its effects on the statistics of the resulting images. These images are visually realistic and have significantly non-Rayleigh histograms. Rayleigh distribution, Rayleigh mixtures with up to four modes, and K distribution fits to these histograms demonstrate that the form of the applied time-varying gain has a substantial effect both on the derived distribution parameters and on the probability that the data are drawn from that distribution. They also demonstrate that, with a greater chi-square test probability and with fewer fitting parameters, the K distribution provides the more appropriate description of the reverberation from the simulated sand-ripple sea bed generated with the model sonar system.

Automatic seabed classification by the analysis of sidescan sonar and bathymetric imagery

The authors present a technique for making use of both sidescan amplitude and bathymetric data provided from sidescan bathymetric sonars for the classification of underwater seabeds. Sidescan amplitude is corrected for physical factors and used to plot ‘processed’ sidescan images. Both amplitude and textural features are derived from these images. Textural features are obtained using 2-D discrete wavelet transforms. Bathymetric images are used to derive features indicating seafloor variability. These features are combined together and the most relevant ones are selected by feature selection algorithms. If grab samples are available, the areas around them are used as training data in a supervised approach. The backpropagation elimination algorithm is used on the training dataset to select relevant features. If training data are not available, an unsupervised approach can be used. The dimensions of the whole dataset are reduced using principal component analysis in this case, and the principal components are used as features. In both cases, clustering techniques are used to cluster the data into sediment classes. The classified points are then plotted against their GIS position in the survey. Classification results correlate with grab sample types from the areas considered (in the supervised case) and with expert observation of sidescan images, where training data is not available.

Geophysical signatures of mud mounds at hydrocarbon seeps on the Louisiana continental slope, northern Gulf of Mexico

Mud mounds are common features at cold seeps in many areas of the world. They are abundant in the northern Gulf of Mexico, where they occur at the numerous hydrocarbon seeps on the continental margin. Although the mounds are widely known, their geophysical signatures and geology have not been widely studied in detail. In this study, we examined mounds in two areas of the upper Louisiana slope using geophysical data of several types: 11–12 kHz side-scan sonar, 3.5 kHz echo-sounder profiles, near-bottom 2–12 kHz chirp sonar profiles, and three-dimensional (3D) multichannel seismic data. In side-scan images the mud mounds appear as ovoid high backscatter spots associated with larger, irregular high backscatter zones along faults and fault complexes. Three different sonar responses were noted: (1) a more-or-less uniform appearance, (2) a central higher backscatter spot surrounded with a lower return halo (bulls-eye), and (3) a more-or-less uniform high backscatter area with an acoustic dead-spot near its center. Near-bottom chirp sonar records indicate high lateral variability in seafloor reflection characteristics, implying that the long-range acoustic signature of a given mound results from the integration of seafloor characteristics. In these records, anomalous seafloor echo types include strong seafloor reflectors (hardbottom) or buried hardbottom, acoustic wipe-out (attenuation), and acoustic turbidity (prolonged echoes). Cores from acoustically anomalous seafloor areas indicate that a variety of seep-related material has been introduced into the near-seafloor sediments. The prevalence of strong bottom return signatures implies that the formation of hard grounds or crusts is an integral part of hydrocarbon seep evolution and suggests that much of the ovoid high backscatter signatures may result from such features. Multichannel seismic reflection data from several of the active mounds show columnar subsurface chaotic reflection zones and strong reflections in sediments directly underneath. Seafloor seismic amplitude maps from these mounds are either generally uniform, with a positive or negative anomaly, or display a ring of higher reflectivity around the mound periphery. Positive or negative anomalies may depend on the degree of seafloor lithification and sediment gas charging. The ring feature is thought to result from the preferential concentration of seep-debris in the mound periphery relative to the center. Some mounds, which show no seismic reflection anomaly from the seafloor, appear inactive. Altogether, observations from this study are consistent with the mud mounds being formed by sedimentary volcanism. The data imply they are Holocene features that have a life-cycle that includes hardbottom formation, which progressively crusts over the fluid vent, ending its activity.

The origin of deep-water, coral-topped mounds in the northern Rockall Trough, Northeast Atlantic

Mounds associated with the cold water coral Lophelia pertusa are widespread in the North Atlantic, although the factors controlling their distribution are not well understood. Here we examine a group of small, coral-topped mounds (the Darwin mounds) which occur at 1000 m water depth in the northern Rockall Trough, northwest of the UK. Individual mounds are up to 75 m in diameter and 5 m high, although some ‘mound-like’ targets seen on sidescan sonar have little or even negative relief. Some mounds are associated with ‘tail-like’ features, imaged as elongate patches of moderate backscatter up to 500 m long, elongated parallel to prevailing bottom currents. High-resolution sidescan images and seabed photographs show hundreds of coral colonies, each a metre or so across, on each individual mound. Many other organisms, mainly suspension feeders, occur in association with the coral. Piston cores from the mounds contain predominantly quartz sand with only scattered coral fragments, showing that bioclastic material is not a major contributor to mound building. A field of seabed pockmarks occurs immediately south of the Darwin mounds. On sidescan sonar data, pockmarks are low relief, circular depressions, typically around 50 m in diameter. The seafloor around the pockmarks consists of uniform, heavily-burrowed, muddy sediments and no specific biological communities, nor any sedimentological or photographic evidence for active seepage, were observed. The distribution of mounds and pockmarks suggests a gradual transition from mounds in the north to pockmarks in the south. This, combined with the lack of bioclastic material in the mound sediments, suggests that both mounds and pockmarks are created by fluid escape from below the seafloor. Mounds occur where fluids carry subsurface sand to the surface, where it forms mounds because bottom currents are not strong enough to disperse it. Pockmarks form where muddy material is eroded by fluid escape but dispersed by bottom currents. Despite the origin of mounds through fluid escape, we suggest that it is the elevated mound topography, rather than any fluid escape, that is advantageous to the corals. This is supported: (1) by the wide variety of suspension-feeding organisms that occur on the mounds, since all of these are unlikely to have a specialised seepage-related lifestyle, and (2) because corals and their associated community do not occur around pockmarks, where seepage has also occurred but elevated topography is absent.

Swath mapping the base of the arctic ice canopy

Satellite data have demonstrated that the areal extent of the Arctic ice canopy has decreased over the past few decades, however, whether sea ice thickness in the Arctic Ocean has substantially decreased remains controversial because of limited information available for the base of the ice canopy. Studies of sea ice thickness to date have involved analysis of ice draft data acquired by single-beam ice profiling sonars on US and British Navy submarines or single-beam moored sonars. During 1998 and 1999, a seafloor-mapping sonar mounted on the hull of the US Navy nuclear submarine USS Hawkbill serendipitously acquired sonar returns from the base of the arctic ice canopy despite design features intended to minimize such returns. Standard topographic processing of the data revealed coherent signals collected between the outgoing sonar pulse and the first returned bottom echoes. Existing processing software was modified to produce swath maps of the information collected from the beginning of ping transmission until the seafloor echoes were detected. These images demonstrate that the recorded water-column signals are different on the port and starboard sides of the system and that individual features can be traced from one side of the submarine to the other. Further software modification allowed us to produce the first wide-swath (2–6 km) sidescan images of the base of the Arctic ice canopy. Although the resultant maps cannot resolve the debate regarding changes in Arctic sea ice thickness, the approach will have far-reaching impact on the design of future systems, whether based on submarines or unmanned vehicles. Our analyses demonstrate that system performance is regulated by the ratio between sonar operating depth and height of the sonar above the seafloor. Where the ratio approaches 1, echoes from the seabed interfere with echoes from the ice canopy, preventing the creation of ice maps. When the sonar is operated below 135 m, signal attenuation results in ice maps with little useful information. However, where the sonar operational depth is less than 135 m and the total water depth ranges between 500 and 1200 m, our approach produces swaths up to 6 km wide exhibiting detailed lineations similar to those observed in higher frequency upward-looking sidescan sonar data.

Pulsational gravity-driven sediment transport on two energetic shelves

Field observations of shoreface benthic processes made in the Middle Atlantic Bight off Duck, N.C. and on the Eel River shelf off northern California have yielded several types of evidence that gravity-induced downslope transport of suspended sediment may constitute an important, but episodic, mode of across-shelf transport during storms. Observed manifestations of the inferred gravity flows include side scan images showing abrupt interfaces between fine sediment “blankets” and coarser sands, rapid bed accretion during the waning phases of storms and short-lived periods of accelerated seaward sediment flux following groups of higher waves. To explore the possibility that gravity-driven transport may be episodically significant in energetic shoreface and shelf environments, we applied a relatively simple analytical model that considers the effects of wave and current turbulent friction in modulating the downslope response of turbidity-induced gravity currents as well as in suspending sediment to produce negative buoyancy. The theory was applied to data on benthic flows from three phases of a storm event off Duck in October 1994. Results indicate that the gradient Richardson number, Ri, remained appreciably below the critical value of 0.25 during average conditions and during high wave groups. During lulls, however, Ri briefly became supercritical within a few tens of centimeters above the bed implying short-lived turbulence suppression. On the Eel River shelf, direct observations show that, within the thin ((O)10 cm) muddy layer, Ri values also fluctuated between subcritical (<1/4) during groups and supercritical (>1/4) during lulls again implying that stratification suppressed production of turbulence during lulls. Downslope transport was observed during both high and low wave packets but accelerated during lulls as in the Duck case. We conclude that gravity-induced downslope transport of sediment suspended by waves is likely to be an important mode of across-shelf transport on the inner shelf of energetic sandy environments and the mid-shelf of energetic muddy environments. Such transports are most likely pulsational, with high turbidities and buoyancy anomalies being created by the high stresses associated with groups of large waves and moderately rapid but brief downslope movement occurring when bed stresses are temporarily relaxed during the lulls between high groups. This mechanism operates out of phase with group-forced long waves.

Acoustic profiles and images of the Palos Verdes margin: implications concerning deposition from the White's Point outfall

Subbottom profiles and sidescan-sonar images collected on and around the Palos Verdes Shelf show a surficial deposit interpreted to contain effluent from the White's Point diffusers, as well as showing several geologic features that affect the deposit's distribution. The effluent-affected deposit is visible in high-resolution subbottom profiles on the shelf and the adjacent San Pedro basin slope to water depths of 170 m. It has a maximum thickness of 75 cm and was mapped acoustically over an area of 10.8 km 2, which encompasses a volume of about 3.2 million m 3. The deposit's basal reflector is acoustically distinct over most of the mapped area, implying that the deposit has not been extensively mixed across its base, perhaps being relatively free of reworking since its initial deposition. Nearshore, the basal reflector is weak and fades away toward land, which could result from syndepositional intermixing of coarse native sediment (particularly from the Portuguese Bend landslide) with effluent in the high-energy nearshore zone, or postdepositionally by physical (wave) or biological mixing across the interface. The geometry of the deposit implies that effluent is dispersed primarily in a northwesterly and seaward direction from the diffusers. Dispersal across the shelf break is in some places strongly affected by topography, particularly by submarine canyons. The deposit overlies stratified and unstratified Quaternary sediment, up to 30 m thick, that in turn overlies the irregular erosional surface of deformed Miocene bedrock that crops out in places on the shelf and upper basin slope. The effluent-affected deposit rests on potentially unstable landslide deposits on the San Pedro basin slope. The acoustic profiles and side-scan images show evidence for active and inactive vents, probably of hot water and gas, some of which are within the boundary of the effluent-affected sediment deposit and could disrupt it if seepage occurs.

A side-scan sonar image of a glacially-overdeepened sea loch, upper Loch Etive, Argyll

Synopsis Loch Etive in Argyll on the west coast of Scotland, is a 30 km long, up to 145 m deep sea loch of glacial origin. A unique side-scan sonar image with new accompanying bathymetric data of the upper basin of the loch has been obtained, covering an area of 19.5 km 2 . These data illustrate the morphology of the loch floor and the depositional processes that have been active in the loch since the end of the last (Younger Dryas stadial) glaciation, approximately 10 ka bp . The new bathymetry reveals the complex physiography of the loch with the deepest (&gt; 100 m) regions confined to the SW with slope angles of 5–15°. Towards the head of the loch water depths are shallower, with &lt; 50 m common and the loch floor is also smoother with slope angles of 2–5°. Using the side-scan image, areas of high and low acoustic backscatter have been identified. The high backscatter areas correspond to regions of exposed outcrops of in situ granite, with localized glacial erratics on the loch floor, and the low backscatter areas to deeper-water basins containing fine-grained sediments. Other features observed are submarine cliffs, downslope creep of water-laden sediments on the steeper loch slopes, and outwash submarine fans from rivers.

Cenozoic sediment distribution and tectonic movements in the Faroe region

A complex history of Cenozoic vertical movements in the Faroe region has been revealed from interpretation of geophysical and geological data, mainly offshore reflection seismic data, side-scan images, shallow cores, and onshore mapping. The history comprises several phases of tectonic disturbances observed at different scales. On the eastern margin of the Faroe Platform a late Eocene–early Oligocene phase of doming of the Faroe Platform has caused a postdepositional tilting of Eocene strata along the southern margin of the platform; a mid-Miocene phase of compressional tectonics is evidenced on seismic transects as gentle anticlines and associated reverse faults; and possible Pliocene uplift of the Faroe Islands is indicated by a progradational wedge of sediments deposited on the eastern Faroe Platform. At the continental margin/slope north of the Faroe Platform, reflection seismic data imaging the postbasalt sedimentary strata indicate three distinct tectonic events phases in the Eocene–Oligocene, Miocene and Pliocene, respectively. In contrast to the Faroe Platform the Faroe–Shetland Channel was characterised by more or less continuous subsidence dominated throughout the Cenozoic. During the Eocene, sediments deposited in the Faroe–Shetland Channel was mostly derived from a source area on the British shelf.

Lithoherms on the Florida–Hatteras slope

Sections of the Florida–Hatteras Slope were investigated during dives of the Johnson Sea-Link I submersible and US Navy's submarine NR-1. Numerous calcareous coral-capped lithoherms, often exceeding 40 m in elevation, were encountered in the subphotic depths below 440 m. One huge lithoherm complex was explored that forms a long, slope-parallel ridge perched on the seaward edge of mid-slope terrace. This lithoherm belt is ∼400 m wide, more than 4.4 km long and ∼150 m high on its down slope flank. Direct observations and side-scan imaging indicate that the long axes of individual ridge crest mounds trend perpendicular to the regional current flow. Away from the ridge individual lithoherms trend more parallel with regional current flow and occur without obvious bedrock control. All of these structures have very steeply sloping flanks (frequently 30–60°) where cemented crusts are exposed by erosion. Thickets of living ahermatypic coral coat the crest and up-stream flanks of these topographic features. Living deep-water coral have modern 14C ages (700±80 year bp) and trap older (3250±100 year bp) sediment rich in fine-grained aragonite. Elsewhere, older deep-water coral rubble (20,230±230 year bp) traps younger sediment (13,760±140 year bp). The trapped sediments contain >50% aragonite fines (<62.5 μm). The friable crusts exposed on these flanks are generally similar in composition to the sediments that accumulate on the crests of these structures, they are older (14C ages of 17,770±330 and 32,710±570 year bp) and contain less aragonite (∼20%) and more high-magnesium calcite. Apparently, lithoherms grow upwards by trapping fine sediments within ahermatypic coral thickets on their crests and upstream flanks. Cemented crusts may be a consequence of earlier paleoceanographic conditions or a result of gradual and ongoing cementation and diagenesis. The large size, platform edge position, and linear trend of this deep-water lithoherm complex are strikingly similar to a shallow marginal reef system.