Water Column Imaging Research Articles

Methane seeps on the U.S. Atlantic margin: An updated inventory and interpretative framework

Since the discovery of >570 methane flares on the northern U.S. Atlantic margin between Cape Hatteras and Georges Bank in the last decade, the acquisition of thousands of kilometers of additional water column imaging data has provided greater coverage at water depths between the outer continental shelf and the lower continental slope. The additional high-resolution data reveal >1400 gas flares, but the removal of probable duplicates from the combined database of new flares and those recognized in 2014 yields ∼1139 unique sites. Most of these sites occur in clusters of 5 or more seeps, leaving about 275 unique locations (including 47 clusters) for seepage along the margin. As a function of depth, seep distribution is heavily skewed toward the upper continental slope at water depths shallower than 400 m on the southern New England margin and ∼ 550 m in the Mid-Atlantic Bight, with additional seeps clustered at ∼1100 m and just deeper than ∼1400 m in both sectors. Despite little ongoing tectonic deformation or active faulting on this passive margin, a variety of processes driven from below the seafloor (e.g., migration of fluids along faults or through permeable strata, seepage above diapirs or other pre-existing structures) and from above (e.g., erosion, sapping, unroofing) contribute to the development of seeps in different settings along the margin. In addition, the prevalence of seeps on promontories overlooking shelf-breaking canyons may be directly related to the three-dimensional nature of the hydrate stability zone in these locations. As a function of depth, the parts of the slope at the contemporary landward limit of gas hydrate stability are devoid of seeps, and the upper slope zones with the most concentrated seepage were not within the gas hydrate stability zone even during the Last Glacial Maximum. Thus, if the large number of upper slope seeps is at least partially sourced in gas hydrate degradation, the gas emitted at these seeps must have migrated there from greater depths on the continental slope.

Experimental study of underwater operation scene with target perception framework

This paper presents a target perception framework aimed at enhancing diver safety and facilitating underwater operations by extracting critical information from underwater scenes. The framework employs a layered processing approach, which encompasses water column imaging, constant false alarm rate detection, and local feature analysis. To simulate the diver's underwater environment, we conducted experiments with three distinct fields of view: fixed down-looking, fixed front-looking, and mobile side-looking perspectives. Our experimental findings demonstrate the framework's ability to accurately differentiate between false targets, stationary targets, and moving targets within the underwater scenes, as well as to capture the motion trajectories of dynamic targets. Furthermore, the application of 3D reconstruction techniques to underwater scene data enables the generation of approximate stereoscopic representations of divers and bubble groups.

Barkley Canyon Gas Hydrates: A Synthesis Based on Two Decades of Seafloor Observation and Remote Sensing

Barkley Canyon is one of the few known sites worldwide with the occurrence of thermogenic gas seepage and formation of structure-II and structure-H gas hydrate mounds on the seafloor. This site is the location of continuous seafloor monitoring as part of the Ocean Networks Canada (ONC) cabled observatory off the west coast off Vancouver Island, British Columbia, Canada. We combine repeat remotely operated vehicle (ROV) seafloor video observations, mapping with an autonomous underwater vehicle (AUV), ship-, ROV-, and AUV-based identification of gas flares, as well as seismic and Chirp data to investigate the distribution of fluid migration pathways. Geologically, the site with the prominent gas hydrate mounds and associated fluid seepage is covering an area of ∼0.15 km2 and is situated on a remnant of a rotated fault block that had slipped off the steep flanks of the north-east facing canyon wall. The gas hydrate mounds, nearly constant in dimension over the entire observation period, are associated with gas and oil seepage and surrounded by debris of chemosynthetic communities and authigenic carbonate. The formation of gas hydrate at and near the seafloor requires additional accommodation space created by forming blisters at the seafloor that displace the regular sediments. An additional zone located centrally on the rotated fault block with more diffuse seepage (∼0.02 km2 in extent) has been identified with no visible mounds, but with bacterial mats, small carbonate concretions, and clam beds. Gas venting is seen acoustically in the water column up to a depth of ∼300 m. However, acoustic water-column imaging during coring and ROV dives showed rising gas bubbles to much shallower depth, even <50 m, likely a result of degassing of rising oil droplets, which themselves cannot be seen acoustically. Combining all observations, the location of the gas hydrate mounds is controlled by a combination of fault-focused fluid migration from a deeper reservoir and fluid seepage along more permeable strata within the rotated slope block. Fluids must be provided continuously to allow the sustained presence of the gas hydrate mounds at the seafloor.

Compression of Multibeam Echosounders Bathymetry and Water Column Data

Over the past decade, Multibeam Echosounders (MBES) have become one of the most used techniques in sea exploration. Modern MBES are capable of acquiring both bathymetric information on the seafloor and the reflectivity of the seafloor and water column. Water column imaging MBES surveys acquire significant amounts of data with rates that can exceed several GB/h depending on the ping rate. These large file sizes obtained from recording the full water column backscatter make remote transmission difficult if not prohibitive with current technology and bandwidth limitations. In this paper, we propose an algorithm to decorrelate water column and bathymetry data, focusing on the KMALL format released by Kongsberg Maritime in 2019. The pre-processing stage is integrated into FAPEC, a data compressor originally designed for space missions. Here, we test the algorithm with three different datasets: two of them provided by Kongsberg Maritime and one dataset from the Gulf of Mexico provided by Fugro USA Marine. We show that FAPEC achieves good compression ratios at high speeds using the pre-processing stage proposed in this paper. We also show the advantages of FAPEC over other lossless compressors as well as the quality of the reconstructed water column image after lossy compression at different levels. Lastly, we test the performance of the pre-processing stage, without the constraint of an entropy encoder, by means of the histograms of the original samples and the prediction errors.

Focused Fluid Flow, Shallow Gas Hydrate, and Cold Seep in the Qiongdongnan Basin, Northwestern South China Sea

The 3D seismic data acquired in the central Qiongdongnan Basin, northwestern South China Sea, reveal the presence of shallow gas hydrate, free gas, and focused fluid flow in the study area, which are indicated by multiple seismic anomalies, including bottom simulating reflectors, polarity reverses, pulldowns, minor faults, and gas chimneys intensively emplaced within the shallow strata. A new cold seep is also discovered at approximately 1520 m water depths with an ~40 m wide crater in the west part of the study area. Water column imaging, seafloor observation, and sampling using the remotely operated vehicle “Haima” demonstrate ongoing gas seepages and shallow gas hydrates at this site. Thermogenic gas in the study area migrates from the deep reservoir through the gas hydrate stability zone along deep faults and gas chimneys, forms shallow gas hydrate and free gas, and sustains localized gas seepage within this cold seep. The results provide insight into the relationship between shallow gas hydrate accumulation and deep hydrocarbon generation and migration and simultaneously have important implications for hydrocarbon explorations in the Qiongdongnan Basin, northwestern South China Sea.

Greenhouse gas emissions from marine decommissioned hydrocarbon wells: leakage detection, monitoring and mitigation strategies

Hydrocarbon gas emissions from with decommissioned wells are an underreported source of greenhouse gas emissions in oil and gas provinces. The associated emissions may partly counteract efforts to mitigate greenhouse gas emissions from fossil fuel infrastructure. We have developed an approach for assessing methane leakage from marine decommissioned wells based on a combination of existing regional industrial seismic and newly acquired hydroacoustic water column imaging data from the Central North Sea. Here, we present hydroacoustic data which show that 28 out of 43 investigated wells release gas from the seafloor into the water column. This gas release largely depends on the presence of shallow gas accumulations and their distance to the wells. The released gas is likely primarily biogenic methane from shallow sources. In the upper 1,000 m below the seabed, gas migration is likely focused along drilling-induced fractures around the borehole or through non-sealing barriers. Combining available direct measurements for methane release from marine decommissioned wells with our leakage analysis suggests that gas release from investigated decommissioned hydrocarbon wells is a major source of methane in the North Sea (0.9-3.7 [95% confidence interval = 0.7-4.2] kt yr−1 of CH4 for 1,792 wells in the UK sector of the Central North Sea). This means hydrocarbon gas emissions associated with marine hydrocarbon wells are not significant for the global greenhouse gas budget, but have to be considered when compiling regional methane budgets.

Improving 3D water column seismic imaging using the Common Reflection Surface method

Water column processing has gained attention in recent years since a seismic model of a water column could assist marine data processors to correctly image the sub-seafloor geology, which is the target of primary interest. In addition to seismic processing, water column imaging has gained interest in the physical oceanography community for improved study of oceanographic processes. However, seismic water column processing is challenging since the internal reflections of the ocean are inherently weak and are often masked by noise. In this work, we adopt the common reflection surface stack technique in order to improve the imaging of ocean water layers. The common reflection surface stack is a robust data preconditioning and stacking technique in seismic processing that relies on the kinematic wavefront attributes of seismic waves. The method is applied to a multichannel 3D data set collected for oil and gas exploration in the deep-water Gulf of Mexico. The method greatly improves inline sections but does not significantly enhance crosslines and horizontal slices, which are more sensitive to both the acquisition geometry and the temporal variability of ocean water masses.

A method of sidelobe effect suppression for multibeam water column images based on an adaptive soft threshold

Recording abundant back scatter information in the water, multibeam sonar water column imaging (WCI) has become one of the most effective means for the acoustic remote sensing investigation of water targets and has been applied and developed rapidly for sounding, fishery, oceanography and archaeology applications as well as in other industries in recent years. Due to the signal interference caused by the sidelobes, there is a great deal of noise in non-target areas of WCI, which reduces the accuracy and efficiency of interpretation of WCI and hinders the automatic detection and recognition of underwater objects. In this paper, an adaptive soft threshold denoising algorithm for WCI is proposed. The WCI data are divided into background areas and target-noise mixing areas by analysing the mathematical features of all angle sequences; thus, the target, noise, and sidelobe artefacts are separated using adjustable threshold parameters and by suppressing noise and sidelobe artefacts to obtain a clearer image. Lastly, the measured data are used for verification. The results indicate that the algorithm has a certain reference value for sidelobe suppression of multibeam WCI.

Investigation of Possible Shallow Gas Accumulations Associated with Pockmarks on the Otago Slope Southeast of New Zealand

High-resolution ‘boomer’ seismic data, together with an industrial 3D seismic dataset indicate the possible presence of shallow gas in the vicinity of the Waitaki Canyon, SE of New Zealand’s South Island. Crescent-shaped seafloor depressions are abundant on the marginal extent of the Otago canyon system in water depths between 500 and 1100 m. Fluids seeping through the seabed, potentially forming pockmarks, play a crucial role in seabed ecological systems and can be used to investigate the distribution of hydrocarbons in underlying geological units. Recently acquired 3D seismic data in the Canterbury frontier basin exhibit the first indications of shallow gas in the region. Two areas with shallow high-amplitude reversed phase reflections were identified and targeted for a high-resolution 2D seismic survey. The 2D data, acquired by the University of Otago in 2017, show reduced amplitudes beneath the bright spots (as also observed in the 3D data). Above the bright spots, bathymetry data show crescent-shaped seafloor depressions, which were most likely modified by northward flowing currents. Both recent and buried seafloor depressions along the shelf exhibit the same northward facing crescent form that we associate with water current modification. Water column imaging data reveal no evidence for present-day seepage of gas through the seabed. An ongoing programme of 2D seismic and bathymetry data collection is underway. High-density velocity analysis and amplitude variations with offset (AVO) will be assessed around the bright spots using several in- and crosslines from the 3D data.

Forty years of progress in multibeam echosounder technology for ocean investigation

Along four decades, multibeam echosounders (MBES) have continuously progressed in their technology and application fields, and are today the favorite active sonar tool of several communities for ocean investigation. Measurement capabilities extended progressively from seafloor bathymetry to interface imagery and reflectometry, and to water column imaging and target quantification; the concerned domains spread from hydrography and seafloor-mapping to geosciences, offshore industry, biology and fisheries, coastal engineering, habitat mapping, environmental science and monitoring. The paper proposes an overview of this history, based on the experience of the author’s oceanography institute since 1977. Technologically MBES first extended the classical single-beam echosounder toward a narrow fan of elementary beams, then to a wide-coverage design similar to sidecan sonars, to multiple swaths, and to various geometries of 3-D insonification. Various aspects of MBES technology evolution are presented: frequencies and powers; array configuration and beam features; electronics; sounding detection methods; reflectometry techniques. The progress in performance level includes: coverage extent, bathymetry accuracy, resolution, and sampling density; reflectometry reliability; water column data specificities. The importance of ancillary sensors and on-carrier platform installation is emphasized. Recent trends and possible future evolutions are finally presented, as well as some facts related to MBES environmental impact.

Ongoing methane discharge at well site 22/4b (North Sea) and discovery of a spiral vortex bubble plume motion

First direct evidence for ongoing gas seepage activity on the abandoned well site 22/4b (Northern North Sea, 57°55′ N, 01°38′ E) and discovery of neighboring seepage activity is provided from observations since 2005. A manned submersible dive in 2006 discovered several extraordinary intense seepage sites within a 60 m wide and 20 m deep crater cut into the flat 96 m deep seafloor. Capture and (isotope) chemical analyses of the gas bubbles near the seafloor revealed in situ concentrations of methane between 88 and 90%Vol. with δ13C–CH4 values around −74‰ VPDB, indicating a biogenic origin. Bulk methane concentrations throughout the water column were assessed by 120 Niskin water samples showing up to 400.000 nM CH4 in the crater at depth. In contrast, concentrations above the thermocline were orders of magnitude lower, with a median value of 20 nM. A dye tracer injection into the gas seeps revealed upwelling bubble and water motion with gas plume rise velocities up to ∼1 ms−1 (determined near the seabed). However, the dissolved dye did not pass the thermocline, but returned down to the seabed. Measurements of direct bubble-mediated atmospheric flux revealed low values of 0.7 ± 0.3 kty−1, much less than current state-of-the-art bubble dissolution models would predict for such a strong and upwelling in situ gas bubble flux at shallow water depths (i.e. ∼100 m).Acoustic multibeam water column imaging data indicate a pronounced 200 m lateral intrusion at the thermocline together with high methane concentration at this layer. A partly downward-orientated bubble plume motion is also visible in the acoustic data with potential short-circuiting in accordance to the dye experiment. This observation could partly explain the observed trapping of most of the released gas below the well-established thermocline in the North Sea. Moreover, 3D analyses of the multibeam water column data reveal that the upwelling plume transforms into a spiral expanding vortex while rising through the water column. Such a spiral vortex motion has never been reported before for marine gas seepage and might represent an important process with strong implication on plume dynamics, dissolution behavior, gas escape to the atmosphere, and is considered very important for respective modeling approaches.

Tectonic and sedimentary controls on widespread gas emissions in the Sea of Marmara: Results from systematic, shipborne multibeam echo sounder water column imaging

AbstractUnderstanding of the evolution of fluid‐fault interactions during earthquake cycles is a challenge that acoustic gas emission studies can contribute. A survey of the Sea of Marmara using a shipborne, multibeam echo sounder, with water column records, provided an accurate spatial distribution of offshore seeps. Gas emissions are spatially controlled by a combination of factors, including fault and fracture networks in connection to the Main Marmara Fault system and inherited faults, the nature and thickness of sediments (e.g., occurrence of impermeable or gas‐bearing sediments and landslides), and the connectivity between the seafloor and gas sources, particularly in relation to the Eocene Thrace Basin. The relationship between seepage and fault activity is not linear, as active faults do not necessarily conduct gas, and scarps corresponding to deactivated fault strands may continue to channel fluids. Within sedimentary basins, gas is not expelled at the seafloor unless faulting, deformation, or erosional processes affect the sediments. On topographic highs, gas flares occur along the main fault scarps but are also associated with sediment deformation. The occurrence of gas emissions appears to be correlated with the distribution of microseismicity. The relative absence of earthquake‐induced ground shaking along parts of the Istanbul‐Silivri and Princes Islands segments is likely the primary factor responsible for the comparative lack of gas emissions along these fault segments. The spatiotemporal distribution of gas seeps may thus provide a complementary way to constrain earthquake geohazards by focusing the study on some key fault segments, e.g., the northern part of the locked Princes Islands segment.

Water column imaging with multibeam echo-sounding in the mid-Okinawa Trough: Implications for distribution of deep-sea hydrothermal vent sites and the cause of acoustic water column anomaly

Water column imaging with multibeam echo-sounding in the mid-Okinawa Trough: Implications for distribution of deep-sea hydrothermal vent sites and the cause of acoustic water column anomaly

Flare imaging with multibeam systems: Data processing for bubble detection at seeps

Multibeam sonar surveys have been conducted since their invention in the 1970s; however, mainly reflections from the seafloor were considered so far. More recently, water column imaging with multibeam is becoming of increasing interest for fisheries, buoy, mooring, or gas detection in the water column. Using ELAC SEABEAM 1000 data, we propose a technique to detect gas bubbles (flares) although this system is originally not designed to record water column data. The described data processing represents a case study and can be easily adapted to other multibeam systems. Multibeam data sets from the Black Sea and the North Sea show reflections of gas bubbles that form flares in the water column. At least for reasonably intense gas escape the detection of bubbles is feasible. The multibeam technique yields exact determination of the source position and information about the dimension of the gas cloud in the water. Compared to conventional flare imaging by single‐beam echo sounders, the wide swath angle of multibeam systems allows the mapping of large areas in much shorter time.