High-resolution Sidescan Sonar Research Articles

Mapping seabed biotopes at Hastings Shingle Bank, eastern English Channel. Part 1. Assessment using sidescan sonar

A multi-technique approach was used to map the spatial distribution of seabed biotopes (i.e. physical habitats and their associated benthic assemblages) in the vicinity of Hastings Shingle Bank in the eastern English Channel, part of which is licensed for the extraction of marine aggregates for the construction industry. An area of seabed, approximately 12×4 km in size, was surveyed using a high-resolution sidescan sonar system, and a mosaic of the output was produced, covering 100% of the survey area. The area was then divided into acoustically distinct regions based on the sidescan sonar data, and the benthic communities and sediment types within each of the regions were ground-truthed using a Hamon grab fitted with a video camera, and using a heavy duty 2-m beam trawl. Additional information concerning the seabed was obtained through the application of video and photographic techniques. Sediments within the survey area ranged from cobbles and coarse gravels on the Shingle Bank, to various grades of sands to the north and south. Analysis of faunal data revealed the presence of statistically distinct biological assemblages within each acoustic region. Using all available data, four discrete biotopes were identified and their spatial distribution mapped across the survey area.

Spatial variations in a condensed interval between estuarine and open-marine settings: Holocene Hudson River estuary and adjacent continental shelf

An interval of stratigraphic condensation extending for 300 km from the fluvially dom- inated Hudson River estuary to the adjacent continental shelf reveals stratal relationships within an unconformity-related depositional sequence that are commonly difficult to re- solve in seismic reflection profiles and outcrop. High-resolution side-scan sonar and ba- thymetry, more than 100 sediment cores ;2 m long, and radioisotope ( 14 C, 137 Cs) age control show that much of the valley was filled by ca. 3 to 1 ka. The present rate of sediment accumulation averages 1 mm/yr, corresponding with a sea-level rise of ;1.2 mm/ yr relative to local bedrock. Condensation is manifested today by sedimentary bypass in most parts of the estuary and by the trapping of available sediment (1.2-5.6 3 10 5 t/yr (metric tons)) along narrow reaches and primarily in the vicinity of the estuarine turbidity maximum, a part of the estuary located upstream of the salinity intrusion ;25 km from the mouth (3.0 3 10 5 t/yr). Shelf condensation is due to sediment starvation. The con- densed interval merges updip with a nascent sequence boundary as the estuary reaches its final filling phase and downdip with the sequence boundary that developed at the Last Glacial Maximum. Delta progradation may take place as available shelf accommodation is filled, but such sediments are expected to be removed once sea level begins to fall. This sedimentation pattern, in which a condensed interval merges with different sequence boundaries, is consistent with the stratigraphic record of the Atlantic margin back to the Paleogene and may be typical of sediment-starved margins.

An automatic approach to the detection and extraction of mine features in sidescan sonar

Mine detection and classification using high-resolution sidescan sonar is a critical technology for mine counter measures (MCM). As opposed to the majority of techniques which require large training data sets, this paper presents unsupervised models for both the detection and the shadow extraction phases of an automated classification system. The detection phase is carried out using an unsupervised Markov random field (MRF) model where the required model parameters are estimated from the original image. Using a priori spatial information on the physical size and geometric signature of mines in sidescan sonar, a detection-orientated MRF model is developed which directly segments the image into regions of shadow, seabottom-reverberation, and object-highlight. After detection, features are extracted so that the object can be classified. A novel co-operating statistical snake (CSS) model is presented which extracts the highlight and shadow of the object. The CSS model again utilizes available a priori information on the spatial relationship between the highlight and shadow, allowing accurate segmentation of the object's shadow to be achieved.

Hydrodynamic significance of variable ripple morphology across deep-water barchan dunes in the Faroe–Shetland Channel

A combination of high-resolution (30 kHz) sidescan sonar and seafloor photography has been used to image barchan dunes and sand ripples in the Faroe–Shetland Channel. The barchan dunes display body lengths and horn-to-horn widths of up to 120 m, and the horns point downcurrent to the southwest. The seafloor surrounding the dunes is variable, with gravel patches, sand with gravel streaks, and uniform rippled sand all observed. The pattern of sand ripples across the dune surface is strongly controlled by the dune morphology. On the lower stoss slopes of the dunes, straight-crested transverse ripples gradually merge upslope into sinuous-crested transverse ripples. Most of the upper dune surface is dominated by short-crested linguoid ripples. This sequence represents an increase in height above seafloor and increasing flow velocity. The overall pattern of ripples across the dune surface indicates that the bottom current flow is modified by the dune morphology, with flow concentrated over the dune crest and along the inner edge of the horns. It is also possible to estimate flow velocity across the dune field, using published analogues and short-term current meter observations. Generally, published examples of submarine barchans suggest a flow velocity of 40–80 cm/s. This range of values is in agreement with near-bed current meter measurements in the study area, which indicate peak flow velocities of 50–60 cm/s. However, in the longer term higher flow speeds may be possible, especially considering the presence of erosional furrows adjacent to the dune field.

Side scan sonar image and geologic interpretation of the Ría de Pontevedra seafloor (Galicia, NW Spain)

High-resolution side scan sonar has been used for mapping the seafloor of the Ria de Pontevedra. Four backscatter patterns have been mapped within the Ria: (1) Pattern with isolated reflections, correlated with granite and metamorphic outcrops and located close to the coastal prominence and Ons and Onza Islands. (2) Pattern of strong reflectivity usually located around the basement outcrops and near the coastline and produced by coarse-grained sediment. (3) Pattern of weak backscatter is correlated with fine sand to mud and comprising large areas in the central and deep part of the Ria, where the bottom currents are weak. It is generally featureless, except where pockmarks and anthropogenic features are present. (4) Patches of strong and weak backscatter are located in the boundary between coarse and fine-grained sediments and they are due to the effect of strong bottom currents. The presence of megaripples associated to both patterns of strong reflectivity and sedimentary patches indicate bedload transport of sediment during high energy conditions (storms). Side scan sonar records and supplementary bathymetry, bottom samples and hydrodynamic data reveal that the distribution of seafloor sediment is strongly related to oceanographic processes and the particular morphology and topography of the Ria.

Non-transform offsets along the Mid-Atlantic Ridge south of the Azores (38°N–34°N): ultramafic exposures and hosting of hydrothermal vents

Ten contiguous non-transform offsets (NTOs) along the Mid-Atlantic Ridge (MAR) south of the Azores (between 38°N and 35°40′N) have been studied in detail using swath bathymetric, acoustic backscatter and deep-tow high-resolution sidescan sonar (TOBI) data. In contrast with discontinuities studied elsewhere at slow-spreading ridges, these left-lateral NTOs are consistently broader and larger, with complex structural fabrics accommodating the offset. They are characterized by a range of elevated and faulted massifs detached from their segment flanks, with an irregular acoustic backscatter pattern. Some of these massifs have been explored and sampled recently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks, and sometimes associated with high-temperature hydrothermal venting. Water column surveys adjacent to these massifs show high CH 4 and low TDM (total dissolvable manganese) concentrations, possibly resulting from the process of serpentinization of ultramafic rocks. The correlation between the shallow dome-like shaped massifs and the high concentrations of CH 4 (associated with low levels of Mn) is of particular interest to predict the outcrop of ultramafic rocks within the NTOs where no geological data are available. The exposure of the ultramafic massifs within the NTOs is favored by low magmatic supply and low-angle detachment faulting occurring at segment ends. The pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurrence of high-temperature vent sites.

Mapping marine habitats with high resolution sidescan sonar

The application of marine geophysics and GIS techniques to the characterization of benthic habitats has increased the ability of fisheries managers to assess distribution and habitat types beyond common practices. We report upon a 150 kHz sidescan sonar survey offshore of Kruzof Island, Alaska undertaken to characterize rockfish ( Sebastes) habitat. Using GIS, MapGrafix and Map ∗Factory we determined the percentage of seafloor cover that exists in our survey area. Bathymetry in the study area was determined with sidescan interferometry. All XYZ data were gridded using Surfer and plotted in shaded relief, bathymetric contour, and 3-dimensional formats. Contoured bathymetry was used as an over-lay in MapGrafix. Small sub-areas were extracted from the bathymetric data for closer study, and gridded in Surfer. Areas of the mosaic where backscatter patterns were not distinct were verified with hand samples and video collected with the submersible Delta. The use of submersibles for verification of interpreted lithologies and surface textures enables a high degree of accuracy for the interpretations. Lithotypes were lumped into larger groups based on morphology and fish associations with different morphologies verified using the submersible. The accuracy of digital maps from high-resolution sidescan sonar data allows a close quantification of the areal extents of these important features, directing the application of management strategies to critical areas.

The Canary Debris Flow: source area morphology and failure mechanisms

The morphology of the source area of the Canary Debris Flow has been mapped using both GLORIA reconnaissance and TOBI high‐resolution sidescan sonar systems. West of ≈19°W, the seafloor is characterized by a strongly lineated downslope‐trending fabric. This fabric can be interpreted as being caused by streams of debris separated by longitudinal shears. Multiple flow pulses are indicated by a series of asymmetrical lateral ridges which mark the northern boundary of the flow. East of ≈19°W, GLORIA data show only a weak fabric of irregular patches and alongslope lineaments. The TOBI data show the patches to be coherent sediment blocks up to 10 km across, surrounded by debris flow material. These are interpreted as in situ areas of seafloor sediment which have survived the slope failure and debris flow event rather than transported fragments of a failed sediment slab. TOBI data from the best developed area of alongslope lineaments show a series of small faults downstepping to the west. This area of seafloor is interpreted as one of partial sediment failure, where the failure process became ‘frozen’ before total mobilization of the seafloor sediments could occur. The overall morphology of the failure area indicates removal of a slab‐like body of sediment, although we cannot distinguish between retrogressive and slab‐slide failure mechanisms. If the latter mechanism is applicable, fragmentation of the failing ‘slab’ must have commenced concurrently with the onset of downslope transport. Immediately upslope from the debris flow source area, a seafloor of characteristic rough blocky texture is interpreted as the surface of a debris avalanche derived from the slopes of the island of El Hierro. The debris flow and avalanche appear to be simultaneous events, with failure of the slope sediments occurring while the avalanche deposits were still mobile enough to fill and disguise the topographic expression of the debris flow headwall. Loading of the slope sediments by the debris avalanche most probably triggered the Canary Debris Flow.

Origins of mud clasts and suspensions on the seabed in Hong Kong

Photographs taken using a seabed profiling camera indicate that the seabed sediments in water depths of 25–30 m in the eastern waters of Hong Kong comprise layers of mud clasts often associated with mud suspensions. The mud clasts and suspensions occur at both a mud disposal site and on natural seabed. At the mud disposal site, mud clasts are often large (commonly >0.05 m across), overlie unbioturbated placed dredged sediment and are commonly associated with extensive mud suspensions. By contrast, mud clasts on the natural seabed are typically less than 0.02 m across, are occasionally associated with suspensions and overlie eroded, intensely bioturbated relict deposits. The generation of the mud clasts and the mud suspensions is attributed to the disposal and reworking of dredged sediment by mass flows and density currents at the disposal site, and to the erosion and redistribution of in-situ muds by trawling equipment on the natural seabed. Hydraulic modelling suggests that waves and currents periodically rework the allochthonous mud clasts at the disposal site and the autochthonous clasts on the open seabed, together with the mud suspensions in both areas, although erosion of in-situ seabed only occurs during severe storms. A seabed profiling camera, in combination with high-resolution side-scan sonar, has been used to estimate the extent and type of seabed impacts arising from mud disposal and trawling.

Pockmarks and gas-charged sediments in the eastern Skagerrak

Extensive patches of shallow gas-charged sediments have previously been documented in the Eastern Skagerrak (the strait between Norway and Denmark). Recently obtained high-resolution side scan sonar and sub-bottom profiler data presented here document several surface and sub-surface features at the border and inside zones of such gas-charged shallow sediments. Three examples are presented and discussed: a low-profile, 3 m-deep pockmark and gas-charged sediments; a low profile, 6 m-deep pockmark associated with a rugged sub-surface reflector; large, up to 25 m deep and 500 m wide pockmarks. From similarities with previously described features it is concluded that the pockmarks are formed by gas eruption and seepage and that the rugged subsurface reflector could possibly represent the upper surface of a diapirically deformed unit of gas-charged plastic clays. Therefore, pockmarks occur preferentially when this rugged surface is close to the seabottom, thus allowing gas-emanation.

Large pockmarks, gas-charged sediments and possible clay diapirs in the Skagerrak

Extensive patches of shallow gas-charged sediments have previously been documented in the eastern Skagerrak (the strait between Norway and Denmark). Recently obtained high resolution side-scan sonar and sub-bottom profiler data presented here document several surface and sub-surface features at the border and inside zones of such gas-charged shallow sediments. Four examples are presented and discussed: (1) a low profile, 3 m deep pockmark and gas-charged sediments; (2) a low profile, 6 m deep pockmark associated with a rugged sub-surface reflector; (3) large, up to 24 m deep and 500 m wide pockmarks; and (4) patches of exposed material from the rugged sub-surface reflector. From similarities with previously described features and also with features found in the Kattegat, further south, it is concluded that the pockmarks are formed by gas eruption and seepage and that the rugged sub-surface reflector could possibly represent the upper surface of a diapirically deformed unit of gas-charged plastic clays.

Complex Carbonate and Clastic Stratigraphy of the Inner Shelf Off West-Central Florida: ABSTRACT

The near surface stratigraphy (< 30 m) of the inner shelf off the west coast of Florida was investigated using high-resolution seismic, side-scan sonar, and continuous underwater video camera coverage. The simultaneous operation of all three systems provided a unique opportunity to calibrate acoustic data with actual video images of the sea floor in a geologically complex area characterized by limestone dissolution structures, hard-bottom exposures, and overlain by a limited supply of terrigenous clastics. Three principle bottom types, grass, sand, and hard-bottom mapped using video and side-scan sonographs, show a correlation with two subsurface stratigraphic zones. The nearshore subsurface zone extending to 6-7 m water depth is characterized by flat or rolling strata and sinkholes that increase in size (200-1,200 m in diameter) and become more numerous further offshore. This zone is truncated by a major erosional unconformity overlain by a thin (<3 m) sequence of Holocene sediment, which together form a terrace upon which the Anclote Key barrier island formed. The offshore subsurface zone (7-11 m water depth) exhibits irregular and discontinuous high-amplitude flat or inclined reflections and few sinkholes. Offshore, extensive hard-bottom exposures are common with discontinuous sediment that occur as lenses or sand waves. The complex stratigraphymore » of the west Florida shelf includes outcropping Neogene limestones that have undergone dissolution during sea level lowstands. Carbonates and clastics dispersed during multiple sea level changes overlie the Neogene limestones. Dissolution styles and erosional unconformities produced bedrock topography and now control modern geological and biological processes.« less

Side-Scan Sonar and Submersible Observations: New Techniques for Gleaning More Information from Sea-Floor Outcrops: ABSTRACT

Advances in high resolution side-scan sonar imaging technology are so effective at imaging sea-floor geology that they have greatly improved the efficiency of a bottom sampling program The traditional sea-floor geology methodology of shooting a high-resolution seismic survey and sampling along the seismic grid was considered successful if outcrops were sampled on 20% of the attempts. A submersible was used sparingly because of the inability to consistently locate sea-floor outcrops. Side-scan sonar images have increased the sampling success ratio to 70-95% and allow the cost-effective use of a submersible even in areas of sparse sea-floor outcrops. In offshore basins this new technology has been used in consolidated and semiconsolidated rock terranes. When combined with observations from a two-man submersible, SCUBA traverses, seismic data, and traditional sea-floor bottom sampling techniques, enough data are provided to develop an integrated sea-floor geologic interpretation. On individual prospects, side-scan sonar has aided the establishment of critical dip in poor seismic data areas, located seeps and tar mounds, and determined erosional breaching of a prospect. Over a mature producing field, side-scan sonar has influenced the search for field extension by documenting the orientation and location of critical trapping cross faults. These relatively inexpensive techniques can providemore » critical data in any marine basin where rocks crop out on the sea floor.« less

Bottom Morphology and Shallow Subbottom Sedimentary Structures on U.S. Continental Shelf Between Cape Hatteras, North Carolina, and Jacksonville, Florida: ABSTRACT

During the period July 1978 to 1980, a survey of the U.S. South Atlantic shelf was made to determine the occurrence and distribution of biologically productive bottom areas and bottom and shallow subbottom geologic features of potential hazard to petroleum exploration and production. Methods of data acquisition included the use of high-resolution seismic systems, sidescan sonar, towed television, and minisub. Shelf sediments are largely composed of 2 to 5 m of Quaternary sands and silty clays with mixtures of shell that were deposited in fluvial, paralic, and shallow-marine environments. Greater sediment thicknesses (up to 40 m) are present in buried river channels, tidal inlets, and cut-and-fill structures associated with meandering tidal streams. Surficial sediments exhibit a wide range of bed-form type and maturity. Ripples of less than 0.5 m wavelength and megaripples of 0.5 to 1 m wavelength are common across the shelf. Crest orientation is generally north-south. Algal growth along the crests of many of the smaller bed forms as well as abundant bioturbation and tracks indicate bottom stability except during storms. Sand waves up to 100 m long are common, particularly within the 25-m isobath and are present in discrete fields 3 km or more wide with north-south crest orientation. They are less common, less sharply defined offshore and usually underlie smaller bed forms of different orientation. These features may have resulted from infrequent major storms and, because of rising sea level, may now be below the effect of most s orm-generated waves. Although the mobility of these large bed forms is inferred by the presence of buried reefs within some of the sand bodies, the time and rate of movement are unknown. Many areas of the shelf exhibit irregular, low-relief bed forms that appear to be in advance stages of deterioration from intense bioturbation. Several distinct, acoustcally hard reflectors present in the shallow subbottom over much of the shelf represent indurated strata which on outcrop provide substrate for the development of reef and hardground communities. These temperate bioherms are primarily populated by sponges and octocorals and, on the basis of topographic expression, are classified as low-relief hardgrounds (> 0.5 m), moderate-relief reefs (up to 2 m), and high-relief reefs (up to 15 m). Low-relief hardgrounds are the most widely distributed of the three types but are the least productive in terms of biomass. Moderate- and high-relief reefs occur most commonly off North and South Carolina and along most of the shelf break. Substrate ranges from Pleistocene to Miocene in age and locally contains significant perc ntages of phosphate grains. The total area of shelf occupied by reefs and hardgrounds is estimated to be less than 10%. Outcrop patterns reflect both local bottom processes and regional structural framework. In the middle to outer shelf off the Georgia coast, a relatively narrow zone of large channels and numerous cut-and-fill structures occur in Quaternary deposits just landward of a shallow buried Miocene high. These features are very similar to sedimentary structures associated with present-day barrier islands, tidal inlets, and estuaries and strongly suggest a sea-level stillstand near the end of the last Pleistocene regression or early in the present transgression. End_of_Article - Last_Page 1663------------