Search

Small‐scale heterogeneities within the seafloor such as glacial boulders, concretions, or unexploded ordnance are of major scientific and economic interest. Because of their small size, such objects are hardly imageable by conventional seismic methods since they produce only faint diffractions. Despite the growing interest, reliable and efficient object detection methods are still not established. So, a marine acquisition system to image objects in the size range 0.3–4.1 m in the near‐surface has been designed and workflows have been developed. Source signals with a central frequency of ∼1,000 Hz are necessary to generate diffractions for the object size range. Performing beam pattern analyses, a rigid tow frame with the dimensions 8 × 3 m and attached hydrophones was designed. A synthetic aperture approach is realized to improve the resolution. Reflection events are suppressed in Normal‐Move‐Out corrected shot gathers by the muting of high singular values to enhance the diffractions. The diffractions are imaged with a beamforming algorithm with a high efficiency, as a total swath angle of about 80° in across‐track direction is covered. The processing of synthetic data sets allows an optimization and validation of the workflow and sensitivity analyses. Ground truthing is achieved with 1–2 m large boulders on the seafloor in 22 m depth, which were identified in a Multi‐Beam Echo Sounder data set. Although diffractions are only weak events in seismic data sets, 3D object detection with seismic beamforming has been found to be an efficient and reliable method to perform derisking for offshore infrastructure installations, drillings, or geologic interpretation. Plain Language Summary: Buried glacial boulders and other small, nonferrous objects like unexploded ordnance are hardly detected with standard geophysical methods. However, those objects are of special relevance being indicators for glacial processes and posing risks for infrastructure installations. So, we designed a marine acquisition system and a processing scheme to exploit the seldom used diffracted wavefield of a seismic record. The system includes a sound source emitting an acoustic signal with a dominant frequency of 1,000 Hz to the water, a rigid tow frame of 8 × 3 m with attached underwater sound recorders and 3D positioning. As a first step, the digital data processing aims to enhance the diffractions, which are the waves scattered by small‐scale objects. Second, beamforming is used to detect the objects building on the constructive interference of the sound recorded and combined at calculated time shifts. As a swath of the seafloor greater than twice the depth of the water can be investigated in a single pass, our proposed method is very efficient. In this study, we show the development and validation of the method with computer models. Also, the practical feasibility is proven with the detection of 1–2 m large boulders on the seafloor of the North Sea in about 20 m depth. Key Points: Ultra‐High Resolution seismic data beamforming is used to perform efficient 3D diffraction imaging in the near‐surface including ground truthingBoulders and other small‐scale objects in the sub‐seafloor can be detected by diffraction imagingDensely spaced receivers on a rigid frame for accurate positioning and an Ultra‐High Resolution seismic source are proposed for acquisition [ABSTRACT FROM AUTHOR]

More than 100 circular, elongate or crescent‐shaped very large seafloor depressions were found on the southern Namibian continental margin in water depths between 900 and 1700 m. These depressions show extraordinary sizes of up to 3 km diameter and 180 m depth. Multichannel seismic and bathymetric data show the influence of bottom currents on their morphology through prograding infills on the upcurrent and erosion on the downcurrent flanks, thus creating a crescent morphology in the largest depressions. Although their seafloor morphology resembles pockmarks related to seafloor fluid seepage, no indications for an active fluid flow system were observed in the area. Some depressions show extensive underlying cut‐and‐fill sequences that could be traced down to slope failure structures and deposits at depth. A correlation to ODP drill site 1085 indicates a continuous development of seafloor depressions for ∼10 Ma, documented by successive stages of infilling, erosion and size variations. Analysis of the seismic reflection data suggests that these depressions formed through the interaction of bottom currents with existing seafloor morphology. The equilibrium seafloor of the continental margin was initially disturbed by mass‐wasting events, common occurrences in the region. Subsequently, ongoing sedimentation in their vicinity was affected by ocean bottom currents, leading to the preservation and constant re‐shaping of seafloor depressions until recent times. This study thus highlights the ability of continental slope sedimentation under bottom‐current influence to produce unique seafloor features over long geological time scales. Key Points: Giant seafloor depressions are formed by an interaction of bottom currents and pre‐existing seafloor topographInitial seafloor depressions are initiated by slope failure processes [ABSTRACT FROM AUTHOR]

A variety of gravel‐ to cobble‐sized rocks, recovered from the Mar del Plata (MdP) Canyon area (Western South Atlantic at 38°S) and interpreted as ice‐rafted debris, represent the first evidence that large icebergs have floated in the Falkland (Malvinas) Current from the southern polar high latitudes far northward. Detailed petrographic analyses identified the Antarctic Peninsula, sub‐Antarctic islands in the Scotia Sea, and Tierra del Fuego as plausible source areas. The drift process could have started as early as at the beginning of the last deglaciation, according to an age obtained from a cold‐water coral fragment associated with one of the dropstones. At the end of the Last Glacial Maximum, large icebergs have been supplied to the Antarctic Circumpolar Current, captured by those ocean current branches that circumvent the Falkland (Malvinas) Islands and entered the Argentine Margin. When the iceberg fleets approached the Brazil‐Falkland (Malvinas) Confluence Zone with its steep latitudinal temperature gradient, the icebergs got oceanographically trapped and melted off rapidly. The sediment load sinking down to the seafloor formed a dropstone blanket particularly where the MdP Canyon had incised into the continental slope. Here, mass‐flow processes, induced by local slope instability, and along‐slope sediment resorting, due to the erosional effects of strong and persistent contouritic bottom currents, favored local enrichment in dropstones in the form of a loose, coarse sediment drape inside morphological depressions. The bottom current velocity would be locally strong enough to rework this sediment, leaving coarse rafted debris as a lag deposit. Plain Language Summary: Icebergs can transport sand‐ to cobble‐sized sediment while drifting with the ocean currents over large distances away from the high latitudes. When seawater temperature forces the ice to melt, this "ice‐rafted debris" will sink down as "dropstones" to the seabed. Offshore dropstone deposits are common in polar regions but rarely found in lower latitudes because icebergs barely survive if the ocean water is not extremely cold. We found evidence of dropstone deposits along the Argentine Margin at the relatively low latitude of 38°S. They rest inside morphological seafloor depressions at the continental slope in association with the Mar del Plata Canyon. Here, also the oceanic Brazil‐Falkland (Malvinas) Confluence Zone is located, which acts as a barrier for northward drifting icebergs. The presence of icebergs in the Argentine Sea was already known; we, however, describe the first finding of rocks that clearly had originated from the Antarctic Peninsula, sub‐Antarctic islands in the Scotia Sea, and Tierra del Fuego, the southern tip of the South American continent. These dropstones are often found together with fragments of cold‐water corals, suggesting that the imported rocks have locally served as hard substrate for coral colonies to establish. Key Points: Rocks recovered from the Mar del Plata Canyon area, 38°S in the Western South Atlantic, are interpreted as ice‐rafted debrisAntarctic Peninsula, sub‐Antarctic islands in the Scotia Sea and Tierra del Fuego are identified as plausible source areasDropstones lie as a coarse sediment drape inside morphological depressions [ABSTRACT FROM AUTHOR]

Abstract: Seafloor seepage is a widespread phenomenon within salt‐influenced basins as the deformation provides pathways for hydrocarbons to reach the seafloor. However, only minor attention has been given to the distal parts of such systems where the impact of salt tectonic deformation is relatively unpronounced. The stress put on the sedimentary column by moving salt on a continental margin may influence fluid flow systems even outside of the salt province. This stress may lead to overpressure formation within reservoirs and determine the orientation of overpressure‐induced fractures. Seepage in the Congo Fan has been discovered in such a distal position at the Regab pockmark, about 35 km west of the salt front, and its geology and biology have been studied extensively in recent years. We present high‐resolution multichannel seismic data from the Regab pockmark that reveal the underlying migration pathways from a buried channel flank 300 m below seafloor to the seafloor via hydraulic fractures in the sealing overburden. Local doming of the reservoir and the remobilization and uplift of sedimentary strata along the migration pathways are interpreted as the result of overpressure within the reservoir. The orientation of the hydraulic fractures is WSW‐ENE, and the fracture outline corresponds to the area of most intense seepage activity within the seafloor pockmark. Along with a similar orientation of other fractures in the vicinity, we propose that this alignment is due to the stress imposed on the sedimentary column in the fan by the seaward moving salt and rafting sedimentary packages of the salt province further east. [ABSTRACT FROM AUTHOR]

Abstract: We present a comprehensive regional bathymetric data compilation for the southwest Indian Ocean (swIOBC) covering the area from 4°S to 40°S and 20°E to 45°E with a spatial resolution of 250 m. For this, we used multibeam and singlebeam data as well as data from global bathymetric data compilations. We generated the swIOBC using an iterative approach of manual data cleaning and gridding, accounting for different data qualities and seamless integration of all different kinds of data. In comparison to existing bathymetric charts of this region, the new swIOBC benefits from nearly four times as many data‐constrained grid cells and a higher resolution, and thus reveals formerly unseen seabed features. In the central Mozambique Basin a surprising variety of landscapes were discovered. They document a deep reaching influence of the Mozambique Current eddies. Details of the N‐S trending Zambezi Channel could be imaged in the central Mozambique Basin. [ABSTRACT FROM AUTHOR]

The Sorokin Trough (Black Sea) is characterized by diapiric structures formed in a compressional tectonic regime that facilitate fluid migration to the seafloor. We present acoustic data in order to image details of mud volcanoes associated with the diapirs. Three types of mud volcanoes were distinguished: cone-shaped, flat-topped, and collapsed structures. All mud volcanoes, except for the Kazakov mud volcano, are located above shallow mud diapirs and diapiric ridges. Beyond the known near-surface occurrence of gas hydrates, bottom simulating reflectors are not seen on our seismic records, but pronounced lateral amplitude variations and bright spots may indicate the presence of gas hydrates and free gas. [ABSTRACT FROM AUTHOR]

Meteor cruise M52/1 documented the presence of gas hydrates in sediments from mud volcanoes in the Sorokin Trough of the Black Sea. In a mud flow on the Odessa mud volcano, a carbonate crust currently forms in association with anaerobic methane oxidation. Dvurechenskii mud volcano (DMV), a flat-topped “mud pie”-type structure, appeared to be very active. Pore water in sediments of DMV is enriched in several constituents, such as ammonium and chloride, which seem to originate at depth. High sediment temperatures of up to 16.5 °C in close contact to the ambient bottom water of 9 °C also suggest strong advective transport of material from greater depth. Steep temperature gradients indicate a high fluid and/or mud flux within DMV, which is confirmed by the shape of the pore water profiles. Active fluid expulsion sites are evidenced by direct seafloor observation, and a potential flux of methane from the sediment to the bottom water is indicated by water-column methane measurements. [ABSTRACT FROM AUTHOR]

Hydrothermal circulation within the sea floor, through lithosphere older than one million years (Myr), is responsible for 30% of the energy released from plate cooling, and for 70% of the global heat flow anomaly (the difference between observed thermal output and that predicted by conductive cooling models). Hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 Myr old. But in view of the relatively impermeable sediments that cover most ridge flanks, it has been difficult to explain how these fluids transport heat from the crust to the ocean. Here we present results of swath mapping, heat flow, geochemistry and seismic surveys from the young eastern flank of the Juan de Fuca ridge, which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km. Our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops. We find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies. [ABSTRACT FROM AUTHOR]

Continental shelves off major river deltas or estuaries act as an interface between terrestrial environments and marine depo centers like the Amazon Fan. In order to understand sedimentary processes on the Amazon sub-aquatic delta and outer shelf, Late-Quaternary erosional and depositional structures have been investigated with an ultra high-resolution seismic survey system (4 kHz). The wavy morphology of the outer shelf implies the presence of tidal sand ridges or meandering channels. In the latter case, the local presence of channel fill deposits suggests several cut-off loops (ox bows). The inter-channel areas are consequently interpreted as levees, which reach their maximum height of 10 m in the area between the Amazon river mouth and the Amazon canyon. The morphology of terraces in front of the sub-aquatic delta reflect erosional processes which presumably occurred during accelerated sea level rise at the beginning and the end of the Younger Dryas. A carbonate platform is present in the northwestern survey area close to the shelf break. Its water depth of 120–130 m implies an evolution during the last glacial maximum. The asymmetric shape of the Cabo Norte Shoal on the topset of the sub-aquatic delta is typical of bottom current produced shoals. The steeper northwestern flank lies in the lee position of the northwest flowing coastal current. Southeast of the shoal the <10 m thick uppermost sequence of presumably Holocene age consists of lobes that dip with the same apparent angle as the foreset towards the offlap break. Within the area 20 km northwest of the shoal the uppermost sequence forms a smooth depression and may be erosionally truncated at the seafloor. Here, the prograding direction includes a northwest component. A shale diapir, which rises from a transparent underlying sequence, marks the transition to an area where the upper sequence is not resolved in the data. [ABSTRACT FROM AUTHOR]

A high-resolution seismic survey was carried out at the accretionary prism on the continental slope off Vancouver Island, Canada. Two GI-Gun data sets with different source frequency ranges of 50–150 and 100–500 Hz were combined with 4 kHz narrow-beam echosounding data (Parasound). The data allow spatial correlation between a gas hydrate bottom simulating reflector (BSR) and distinct areas of high near-sea-floor reflectivity. An integrated interpretation of the multi-frequency data set provides insight into the regional distribution of tectonically induced fluid migration and gas hydrate formation in the vicinity of ODP Leg 146 Sites 889 and 890. The BSR at the base of the gas hydrate stability field is observed within accreted and deformed sediments, but appears to be absent within bedded slope basin deposits. It is suggested that these basin deposits inhibit vertical fluid flow and prevent the formation of a BSR, whereas the hydraulic conductivity of the accreted sediments is sufficiently high to allow for pervasive gas migration. An elevation of the BSR beneath the flanks of a topographic high is interpreted as an indicator for local upflow of warm fluids along permeable pathways within outcropping accreted sediments. Parasound data reveal discontinuous zones of high reflectivity at or directly beneath the sea floor, which may indicate local cementation of surface sediments. In combination with GI-Gun data, the occurrence of these reflective areas can be related to the location of slope sedimentary basins acting as hydraulic seals. It is proposed that the seals sometimes fail along faults extending beneath the BSR, leading to focused upflow of methane-bearing fluid and the formation of carbonate pavements at the sea floor. [ABSTRACT FROM AUTHOR]

An ultra-high-resolution seismic study of the eastern Bengal Shelf with the parametric narrow-beam echosounder Parasound allows the interpretation of late Quaternary depositional patterns in terms of seismic stratigraphy. Accommodation space was still present on the outer shelf during the last lowstand, where a prograding delta developed in the western survey area. Oolitic beach ridges were later formed on top of this lowstand delta. Farther east, large parts of the shelf were exposed to subaerial erosion and a river system extended seaward across the area. A subaqueous highstand delta prograded southwards following the maximum transgression about 7,000 years ago. Its foreset beds exhibit acoustic voids very likely generated by sediment liquefaction, possibly caused by episodic energetic events such as major cyclones and/or earthquakes. Bottomset sediments extend seaward close to the shelf break in the west, whereas no Holocene sediments cover the outer shelf in the east. [ABSTRACT FROM AUTHOR]