Side-scan Sonar Images Research Articles

Simulation and analysis of synthetic sidescan sonar images

The sidescan sonar process produces a representation in image form of the seabed; however, no closed-form analytic expression or direct modelling technique exists to represent this complex process. The basic principle is presented for a model for the simulation of sidescan sonar, which produces as its output a qualitative image of the form generated by the actual sidescan. The model considers the underlying physical processes including the effects of the refractive water medium, the scattering from the seabed and the transducer motion and directivity characteristics. The statistical and visual techniques that have been applied to attempt to overcome the resulting problems of testing and verifying the images output by the sonar simulation model are then discussed.

Fault scarp identification in side-scan sonar and bathymetry images from the Mid-Atlantic Ridge using wavelet-based digital filters

Digital filters designed using wavelet theory are applied to high resolution deep-towed side-scan sonar data from the median valley walls, crestal mountains, and flanks of the Mid-Atlantic Ridge at 29°10′ N. With proper tuning, the digital filters are able to identify the location, orientation, length, and width of highly reflective linear features in sonar images. These features are presumed to represent the acoustic backscatter from axis-facing normal faults. The fault locations obtained from the digital filters are well correlated with visual geologic interpretation of the images. The side-scan sonar images are also compared with swath bathymetry from the same area. The digitally filtered bathymetry images contain nine of the eleven faults identified by eye in the detailed geologic interpretation of the side-scan data. Faults with widths (measured perpendicular to their strike) of less than about 150 m are missed in the bathymetry analysis due to the coarser resolution of these data. This digital image processing technique demonstrates the potential of wavelet-based analysis to reduce subjectivity and labor involved in mapping and analyzing topographic features in side-scan sonar and bathymetric image data.

Intraplate en echelon volcanic ridges in the South Pacific west of the Easter microplate

Satellite altimetric data for the south-central Pacific ocean show linear geoid anomalies extending from Pitcairn island (130°W-24°S) to the western border of the Easter microplate (118°W-26°S). Previously, these anomalies have been interpreted as a submarine volcanic chain more than 1300 km long, inclusive of Henderson island, Ducie island and Crough seamount, and constructed on young (< 30 Ma) oceanic lithosphere. Recent bathymetric data and side-scan sonar images (GLORIA) from the eastern part of this volcanic chain suggest that the anomalies correspond to a succession of elongate ridges arranged in a right-stepping en echelon pattern along a fracture zone. At least five volcanic ridges were recognized during the SO80a cruise (1992) between 123°W and 118°W at 25°S, trending 275°–280° (parallel to plate motion). The ridges range from 70 to 200 km long, 20 to 35 km wide and 1100 to 2900 m high. Their volumes, ∼ 600 to 1500 km 3, are equivalent to those of large intraplate submarine volcanoes from the Polynesian hotspots. Sediment thickness and the degree of alteration of samples increase progressively from east to west, suggesting that the chain grew from a volcanic centre, located near 118°W–25°S, close to the West Rift that forms the western border of the Easter microplate. The ridges are built along a fracture zone of the Pacific plate, trending N100°–110°, which shifts the magnetic anomalies more than 300 km to the left. The origin of this fracture zone is not well assessed but it strikes in the continuity of the present-day Orongo transform fault forming the southwestern boundary between the Easter and Pacific plates, east of 114°50′W. The crests of the volcanic ridges are oriented N055°–075°, at angles of 30°–50° with respect to the fracture zone. The arrangement of both features could be compared to that of right-stepping opening cracks (volcanic ridges) along a sinistral shear zone (transform fault). An alternative model is based on the orientation of the abyssal hills which is almost perpendicular to that of the ridges, especially in the eastern part of the volcanic alignment. The origin of the en echelon ridges could also be related to tension and fracturing in the youngest lithosphere along leaky transform faults.

Formation of large sulfide mineral deposits along fast spreading ridges. Example from off-axial deposits at 12°43′N on the East Pacific Rise

Submersible investigations with Nautile near the 13°N hydrothermal field on the EPR were made in 1992. We selected diving areas based on deep towed side scan sonar imaging. Close to a pit-crater at the top of a young seamount 2 km east of the axial ridge we discovered one of the largest (70 m high and 200 m in diameter) sulfide mounds formed at a fast spreading ridge (12 cm/yr). Despite an overlap in composition between axial and off-axial deposits the general trend reflects real differences in compositions. Off-axis deposit differs from axial deposits by the scarcity of zinc sulfides and cobalt and selenium enrichment. Sulfur isotope variation is minimal for the off-axis seamounts and suggests that sulfides precipitated from unmodifed end-member hydrothermal solutions. Axial sulfides have a broader range due to reduction of seawater sulfate at mixing fronts between hydrothermal and seawater solutions along the axial graben fault system. Lead isotopic compositions of sulfides are in two overlapping but distinct fields indicating different Pb sources and separate convective systems for the seamount and the axial deposits. Our observations suggest that the plumbing system is stable and centred on a shallow localised magma chamber under the seamount. This configuration is by far more efficient than unstable axial processes to create, during several hydrothermal episodes, major sulfide deposits near a fast spreading ridge. Off-axis seamounts are localised areas with high magmatic budget able to drive and focus hydrothermal systems at the same place for a long time. Other known occurrences of sulfides related to seamounts are at less than 11 km from the ridge axis. Thus we suggest that off-axial volcanoes close to the ridge are first order targets to discover active or inactive large deposits along fast- to medium-spreading ridges.

Mapping oil seeps on the sea floor by Gloria side-scan sonar images—A case study from the northern Gulf of Mexico

An oil seep site in the northern Gulf of Mexico is characterized by high backscattering levels on the GLORIA (Geological Long-Range Inclined Asdic) side-scan sonar images against a background of low backscattering. The high backscattering from the oil-seep area are most likely caused by a combination of small-scale roughness and porosity reduction due to the precipitation of CaCO3 formed during biodegradation of the oil-seep. Geostatistical methods have been applied to analyze side-scan images from both oil seep and nonseep areas. The results show that GLORIA images from oil seep areas can be distinguished from nonseep areas in terms of local histograms, variograms, and textural patterns. Pixels from seep and nonseep areas cluster into distinct groups on a textural feature plot. GLORIA side-scan images could be used as a reconnaissance tool to delineate oil and gas seep sites on the seafloor and thereby, reduce the dry-hole risk of petroleum exploration in deep-water frontier areas.

Giant sediment drifts on the continental rise west of the Antarctic Peninsula

Multichannel seismic reflection profiles from the continental rise west of the Antarctic Peninsula between 63° and 69°S show the growth of eight very large mound-shaped sedimentary bodies. MCS profiles and long-range side-scan sonar (GLORIA) images show the sea floor between mounds is traversed by channels originating in a dendritic pattern near the base of the continental slope. The mounds are interpreted as sediment drifts, constructed mainly from the fine-grained components of turbidity currents originating on the continental slope, entrained in a nepheloid layer within the ambient southwesterly bottom currents and redeposited downcurrent.

Sidescan sonar images of mud volcanoes from the Mediterranean Ridge: possible causes of variations in backscatter intensity

A survey of a large field of mud diapirs on the Mediterranean Ridge south of Crete was carried out using both OKEAN (9.5 kHz) and MAK-1M (30 kHz) sidescan sonar systems during Leg-2 of the Third Training Through Research cruise of R/V Gelendzhik in June and July, 1993. High backscatter intensities were recorded by both systems to different degrees over the mud diapirs and their immediate surroundings. Causes of variations in the backscatter intensity are considered to be the spatial distribution of mud breccia associated with the mud domes (both the burial depth of the mud breccia beneath hemipelagic drape and its areal extent) and its physical properties, along with contributions from seafloor roughness and the topography of the mud domes. Significant sources of volume scattering of the sidescan sonar sound signal are considered to be mud breccia inhomogeneities represented by millimetric/centimetric clasts of various lithologies and free gas bubbles distributed through the mud matrix. A good general relationship is established between these sources and the observed backscatter variations through analysis of ground truth data from core samples, with 5 kHz deep-tow subbottom profiler records and seafloor video observations as well as the sidescan sonar records; but a fully quantitative assessment of this relationship would require acoustic modelling and additional ground truth data.

Structural features of mud volcanoes and the fold system of the Mediterranean Ridge, south of Crete

New information on the geometry of the mud volcanoes, folds and faults located in the central part of the Mediterranean Ridge is provided from long range and deep-tow sidescan sonar images, high resolution seismic and low frequency echosounder profiles, and gravity cores obtained on the R/V Gelendzhik during the 1993 UNESCO-ESF Training Through Research cruise. Symmetrical gentle folds, with a mean wavelength of 750 m, deform predominantly Pliocene-Quaternary sediments, but also perhaps as old as Messinian. In most areas of the Mediterranean Ridge, the north-northeast directed subduction of the African plate below the Eurasian plate at the Hellenic arc produces folds with hinge lines subparallel to the trend of the ridge. However, the hinge lines of the folds curve around the southern part of the area with highest concentration of mud volcanoes and produce an irregular U-shape fold belt. Some of the folds show an intrusive nucleus and, in some of them, mud breccia appears to have flowed from the flanks of the folds into the troughs. The mud volcanoes consist of domes of interlayered pelagic sediments and mud breccias containing sediments and rock fragments of Late Aptian to Pleistocene ages. Mud breccia flows are mainly extruded radially from vents, although rare fissure extrusions are also observed. The mud volcanoes have an irregular to elliptical shape, with diameters up to 16 km. The distribution of mud volcanoes in the area is irregular, but they are more frequent in an area called the Olimpi Field. Faults in the uppermost part of the Mediterranean Ridge are rare. Most of the faults are normal and subparallel to fold limbs. In addition, N20°E and N100°E subvertical faults are found controlling the shape of the mud volcanoes.

Magnitude and timing of episodic sea-level rise during the last deglaciation

Research Article| September 01, 1996 Magnitude and timing of episodic sea-level rise during the last deglaciation Stanley D. Locker; Stanley D. Locker 1Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701 Search for other works by this author on: GSW Google Scholar Albert C. Hine; Albert C. Hine 1Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701 Search for other works by this author on: GSW Google Scholar Lenore P. Tedesco; Lenore P. Tedesco 2Department of Geology, Indiana University/Purdue University, Indianapolis, Indiana 46202 Search for other works by this author on: GSW Google Scholar Eugene A. Shinn Eugene A. Shinn 3U.S. Geological Survey Center for Coastal Geology and Regional Marine Studies, St. Petersburg, Florida 33701 Search for other works by this author on: GSW Google Scholar Author and Article Information Stanley D. Locker 1Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701 Albert C. Hine 1Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701 Lenore P. Tedesco 2Department of Geology, Indiana University/Purdue University, Indianapolis, Indiana 46202 Eugene A. Shinn 3U.S. Geological Survey Center for Coastal Geology and Regional Marine Studies, St. Petersburg, Florida 33701 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (9): 827–830. https://doi.org/10.1130/0091-7613(1996)024<0827:MATOES>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Stanley D. Locker, Albert C. Hine, Lenore P. Tedesco, Eugene A. Shinn; Magnitude and timing of episodic sea-level rise during the last deglaciation. Geology 1996;; 24 (9): 827–830. doi: https://doi.org/10.1130/0091-7613(1996)024<0827:MATOES>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A succession of elevated ridge deposits on the south Florida margin was mapped using high-resolution seismic and side-scan sonar imaging in water depths ranging from 50 to 124 m. The ridges are interpreted to be subtidal shoal complexes and paleoshorelines (eolian dune or beach) formed during the last sea-level transgression. Oolitic and skeletal grainstones and mixed skeletal-peloidal-ooid packstones were recovered using a research submersible. All of the grains are of shallow-water or intertidal origin, and both marine and nonmarine cements were identified.Formation and preservation of these features are attributed to episodic and rapid changes in the rate of the deglacial sea-level rise at the onset of the termination 1A δ18O excursion. This high-resolution record of sea-level change appears to be related to deglacial processes operating on submillennial time scales and supports increasing evidence of rapid episodic fluctuations in ice volume, climate, and ocean-circulation patterns during glacial-interglacial transitions. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seabed classification through multifractal analysis of sidescan sonar imagery

The authors present a technique for the classification and analysis of seabed sediments from sidescan sonar images, the origins of which lie in the body of fractal theory. Six seabed types were analysed, namely clay, mud, sand, gravel, stones and rock. These data sets have previously been analysed by several authors who have used techniques based on the power spectrum. The method proposed in the paper allows frequency information to be obtained but without the use of large windows which are generally required for frequency domain measurements. Results are presented for the classification of individual ground truthed sediments and the segmentation of composite images containing these sediments. Correct classification rates of greater then 99% have been obtained and good segmentation accuracy achieved.

Seafloor map generation for autonomous underwater vehicle navigation

Elevation map generation is an essential component of any autonomous underwater vehicle designed to navigate close to the seafloor because elevation maps are used for obstacle avoidance, path planning and self localization. We present an algorithm for the reconstruction of elevation maps of the seafloor from side-scan sonar backscatter images and sparse bathymetric points co-registered within the image. Given the trajectory for the underwater vehicle, the reconstruction is corrected for the attitude of the side-scan sonar during the image generation process. To perform reconstruction, an arbitrary but computable scattering model is assumed for the seafloor backscatter. The algorithm uses the sparse bathymetric data to generate an initial estimate for the elevation map which is then iteratively refined to fit the backscatter image by minimizing a global error functional. Concurrently, the parameters of the scattering model are determined on a coarse grid in the image by fitting the assumed scattering model to the backscatter data. The reconstruction is corrected for the movement of the sensor by initially doing local reconstructions in sensor coordinates and then transforming the local reconstructions to a global coordinate system using vehicle attitude and performing the reconstruction again. We demonstrate the effectiveness of our algorithm on synthetic and real data sets. Our algorithm is shown to decrease the average elevation error when compared to real bathymetry from 4.6 meters for the initial surface estimate to 1.6 meters for the final surface estimate from a survey taken of the Juan de Fuca Ridge.

Catastrophic collapse of the volcanic island of Hierro 15 ka ago and the history of landslides in the Canary Islands

Research Article| March 01, 1996 Catastrophic collapse of the volcanic island of Hierro 15 ka ago and the history of landslides in the Canary Islands D. G. Masson D. G. Masson 1Southampton Oceanography Centre, European Way, Empress Dock, Southampton SO14 3ZH, United Kingdom Search for other works by this author on: GSW Google Scholar Geology (1996) 24 (3): 231–234. https://doi.org/10.1130/0091-7613(1996)024<0231:CCOTVI>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. G. Masson; Catastrophic collapse of the volcanic island of Hierro 15 ka ago and the history of landslides in the Canary Islands. Geology 1996;; 24 (3): 231–234. doi: https://doi.org/10.1130/0091-7613(1996)024<0231:CCOTVI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Landslides play an important role in the evolution of many volcanic islands, producing huge fields of blocky volcanic debris on their submarine slopes. Sidescan sonar images presented in this paper provide evidence for a large debris avalanche, El Golfo, on the northern flank of Hierro Island in the Canary Islands. Angular blocks, as much as 1.2 km across and 200 m high, cover the debris avalanche surface. El Golfo avalanche is related to both the Canary debris flow and a volcaniclastic turbidite found in the Madeira abyssal plain 600 km west of the Canaries. Dating of the turbidite and the failure scarp onshore indicates that the failure probably occurred between 13 and 17 ka. There appears to be a general correlation between volcaniclastic turbidites in the abyssal-plain sequence and landslides in the Canaries during the past 750 ka. Tentatively, this correlation suggests that seven major landslides have affected the Canaries in that time. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Initiation and evolution of boundary-wall faults along the Mid-Atlantic Ridge, 25–29°N

Abstract The major faults that bound the median valley on slow spreading ridges form a staircase that migrates upwards and outwards from the valley floor like a slow escalator. Each boundary wall fault is generated at the margin of the median valley floor at about 2 km from the spreading axis. Boundary wall faults that appear to be single scarps on the relatively low resolution Seabeam multibeam echosounder maps are shown by high resolution TOBI sidescan sonar images in some cases to be single faults, but in others to be complex anastomosing fault zones. The major faults in such zones are often scalloped in plan view. There is no systematic relation between fault zone geometry and segment type. We conclude that during the growth of boundary wall faults the strain is first accommodated over a wide area, in which an array of small faults dissects the volcanic surface. However, with time, and with continued extension, the strain becomes organized into increasingly narrower zones of deformation until a major boundary wall fault develops. Faults grow by propagation and linkage with other nearby faults. The early stages of linkage include tilted ramps, but at later stages brittle faults cut across the ramps, and eventually become part of a scalloped fault trace. In some places, linkage is through zones of small-scale diffuse faulting, rather than through a single fault. Fault capture and linkage can occur over lateral distances (perpendicular to the mean azimuth of the faults) of up to 1.5 km. This deformation takes place within a fault growth window fixed relative to the spreading axis, through which the lithosphere passes as seafloor spreading continues. Most of the growth of each boundary fault takes place before the next fault is initiated. We conclude that the location and size of the narrow fault growth window is controlled by the changing mechanical properties of the ocean lithosphere as it spreads away from the axis. Its inner boundary represents the position at which a normal fault reaches the sea floor after originating at the base of the lithosphere at the spreading axis. Its outer boundary is interpreted as the point at which the strength of the boundary wall fault, increasing as the lithosphere ages and thickens, becomes greater than the strength of the lithosphere at the spreading axis, when a new fault is generated. The process of fault generation at mid-ocean ridges has relevance to fault generation in other extensional environments.

Aftershocks and Faults of the Hyogo-ken Nanbu Earthquake beneath Akashi Strait.

We carried out a seismic marine survey around Akashi Strait employing reflection and side-scan sonar imaging methods and an ocean bottom seismograph (OBS) array in order to closely examine the aftershocks in the immediate vicinity of the rupture initiation area of the 1995 Hyogo-ken Nanbu earthquake. A total of 71 aftershocks and some mechanisms were determined using OBS data for a period of 1 month with land temporary station data starting 12 days after the main shock. We identified a number of seismologically characteristic clusters to understand the heterogeneity that seem to exist around Akashi Strait. We determined that two major faults, the Nojima Fault and the Rokko fault system, are right-stepping within the strait cause en-echelon faultings in between. The Nojima Fault seems to terminate by deflecting westward in the strait. Focal mechanisms of these events are generally concordant with an E-W compression stress field, which is the tectonic trend. Events beneath the northern part of Awaji Island occur as a cluster dipping eastward, but mechanisms vary.

Recurrent landslides events on the submarine flank of Piton de la Fournaise volcano (Reunion Island)

Abstract New studies confirm the existence of large landslide events related to both the Piton des Neiges and the Piton de la Fournaise volcanic systems. Like many other island basaltic shield volcanoes, Réunion Island has had a complex evolution combining construction processes and recurrent destructive events. Landslide deposits probably form an important part of its internal structure. Sliding events and related tsunamis are one of the major hazards of this kind of volcanic activity, in an intraplate oceanic context. The submarine flank of Piton de la Fournaise has been extensively studied using Seabeam bathymetry and high resolution side-scan sonar images. The eastern submarine flank is entirely covered by 550 km 3 of landslide deposits, which are mainly subaerially erupted basaltic lavas, transported and fractured in large-scale mass wasting. Volumetric considerations imply recurrent partial destruction of the edifice, by landsliding, during its construction. For each sector collapse the fundamental process is the sliding of large blocks, up to several kilometres in length. Fractions of the slide blocks break up and evolve into debris avalanches. Analogue experiments have been carried out to tentatively explain the geometry of these periodic large landslides of the unbuttressed flank. The morphology and the deformation in the landslides of the model are compared to the natural system of Piton de la Fournaise.

Mid-Atlantic Ridge volcanism from deep-towed side-scan sonar images, 25 °–29 °N

We present deep-towed side-scan sonar mosaics of the inner valley floor of eight spreading segments at the slow-spreading Mid-Atlantic Ridge between 25 ° and 29 °N. An analysis of these images, which well-resolve features a few tens of meters in size, confirms that the multitude of small seamounts, with diameters between 0.5 and 3 km identified on the inner valley floor from previously collected multibeam bathymetry data, are volcanically constructed. Moreover, these images reveal that these volcanoes have distinct surface morphologies not evident in the coarser resolution multibeam bathymetry maps: 83% of the seamounts have a hummocky (bulbous) morphology; the other 17% have a smooth morphology. In addition to near-circular seamounts, small (1–2 km long) volcanic ridges are abundant in our study regions, and are not, in general, seen in the bathymetry maps. We combine these new morphological data with existing models for the construction of the shallow oceanic crust to obtain a better understanding of the melt delivery system that builds the distinctive seafloor topography at the slow-spreading Mid-Atlantic Ridge.

Geology of the Wrightsville Beach, North Carolina shoreface: Implications for the concept of shoreface profile of equilibrium

Nearly 300 km of 3.5 kHz subbottom profile and 100 kHz sidescan-sonar data, a suite of over 100 short (~2 m) percussion cores and vibracores have been collected on the shoreface and inner continental shelf off Wrightsville Beach, North Carolina. Sidescan-sonar images were analyzed for acoustic backscatter to delineate the surface sediment distribution. Groundtruth data for the sidescan-sonar interpretations were provided by surface grab samples.Cross-shore sediment transport by combined waves and currents is the predominant sedimentologic signature on this shoreface. The shoreface is dominated by a shore-normal system of rippled scour depressions that begin in 3–4 m water depth and extend to the base of the shoreface about 1 km offshore, at 10 m depth. The depressions are 40–100 m wide, and up to 1 m deep. They are floored by coarse, rippled shell hash and gravel; some are separated by rock-underlain fine sand ridges. On the inner shelf, the bathymetric and sedimentary fabrics become shore-oblique, due to a series of relict ridges with 1–2 m of relief. The ridges are coarse on their landward sides and covered on their seaward flanks by thin veneers of fine sand.Field evidence from the Wrightsville Beach shoreface demonstrates that a shoreface equilibrium profile as defined by Dean (1991) and others does not exist here. For example: (1) the grain size varies widely and inconsistently over the profile; (2) shoreface profile shape is controlled predominantly by underlying geology, including Tertiary limestone outcrops and Oligocene silts; and (3) sediment transport patterns cannot be explained by simple diffusion due to wave energy gradients, and that transport occurs seaward of the assumed engineering “closure depth” of 8.5 m. This has several implications for the application of equilibrium profile-based numerical models used to investigate coastal processes and design coastal engineering projects at Wrightsville Beach. The most important practical implication is that a number of assumptions required by existing analytical and numerical models (e.g., Dean, 1991; genesis; sbeach) used for the design of shore protection projects and large-scale coastal modeling over decadal time scales cannot be met.

Geomorphology and sedimentary processes of a modern slope-type fan delta (Fujikawa fan delta), suruga trough, Japan

The Fujikawa fan delta is a slope-type fan delta developed on the arc-arc collision boundary between the Izu-Bonin and Honshu arcs in the East Asian monsoon-related humid-temperature climate region. It comprises a stream-dominated alluvial fan and a large subaqueous portion that progrades into deep water (1300 m in water depth). Based on a detailed subaqueous morphology obtained from remote-sensing imagery, including IZANAGI, deep-towing side-scan sonar and deep-towing video camera images, the subaqueous portion is subdivided into delta front, upper slope, lower slope and prodelta. The lower slope, comprising three elongate convex-upward bulges, occupies the largest portion, and is the main sedimentation site in the Fujikawa fan delta. A single feeder channel or canyon is absent, instead, numerous chutes are developed along the length of the bulge. Related to the diversion of the river course on the alluvial fan, the elongate subaqueous bulge fed directly to the river is formed, then switched. Hence, the subaqueous fan shape of the Fujikawa fan delta is mainly controlled by the pre-existing basin topography and the diversion pattern of the river course on the alluvial fan.

Detection of barrels and waste disposal sites on the seafloor using spatial variability analysis on sidescan sonar and bathymetry images

The availability and use of remotely sensed digital image data have increased rapidly over the last two decades. One application where remotely sensed image data are a critical component is environmental analysis and monitoring. In our case it involves using sidescan sonar images to detect and map the location of 55‐gal drums that were disposed of near the Farallon Islands (about 50 nautical miles west of San Francisco). About 48,000 drums containing low‐level radioactive waste were dumped in water depths ranging from 100 to 2500 m covering 1400 km2. The high number of drums littering the seafloor and the rather large area involved makes remotely sensed image data one of the few promising tools to detect and map drum locations. In our analysis, sidescan sonar images collected by the SeaMarc IA and having about 1‐m pixel resolution were used. Spatial variability analysis (SVA) and other digital processing procedures were applied to the sidescan sonar images to automatically detect and map the location of t...

Geometric corrections on sidescan sonar images based on bathymetry. Application with SeaMARC II and Sea Beam data

Geometric corrections on sidescan sonar images based on bathymetry. Application with SeaMARC II and Sea Beam data