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Nitrogen (N) is the most abundant element in Earth's atmosphere, but is extremely depleted in the silicate Earth. However, it is not clear whether core sequestration or early atmospheric loss was responsible for this depletion. Here we study the effect of core formation on the inventory of nitrogen using laser-heated diamond anvil cells. We find that, due to the simultaneous dissolution of oxygen in the metal, N becomes much less siderophile (iron-loving) at pressures and temperatures up to 104 GPa and 5000 K, a thermodynamic condition relevant to the bottom of the magma ocean in the aftermath of the moon-forming giant impact. Using a core–mantle–atmosphere coevolution model, we show that the impact-induced processes (core formation and/or atmospheric loss) are unlikely to account for the observed N anomaly, which is instead best explained by the accretion of mainly N-poor impactors. The terrestrial volatile pattern requires severe N depletion on precursor bodies, prior to their accretion to the proto-Earth. [ABSTRACT FROM AUTHOR]

Seabed topography is important for marine geophysics and geodesy. However, conventional seabed topography inversion methods based on gravity data are constrained to a linear fitting, neglecting the impact of nonlinear terms. In this paper, we propose an innovative method for seabed topography inversion by establishing fresh mathematical relationships through machine learning techniques based on the Smith and Sandwell (SAS) method. Utilizing global sea depth data from the National Geophysical Data Center (NGDC) and gravity anomaly data from satellite altimetry, our study employs the SAS method for seabed topography inversion. The improved SAS method, enhanced by the Genetic Algorithm-Backpropagation (GA-BP) algorithm, is specifically applied to invert the Huangyan Seamount Chain topography in the South China Sea. The accuracy is evaluated by seabed topography models, including EOTPO1, GEBCO-2023, and S & S V19.1. The results show the GA-BP method significantly reduces residuals and improves the accuracy estimated by the SAS inversion method. The Root Mean Square Error (RMSE) and Mean Absolute Error (MAE) are decreased by 14.98% and 5.07%, respectively. The proposed method has valuable reference significance for future marine exploration endeavours around the world. [ABSTRACT FROM AUTHOR]

The seismic response of seabed is fundamental for researching the seismic response of marine structures. The one-dimensional transient response of an unsaturated seabed was studied in this paper. An analytical solution for a special case with infinite permeability coefficients and a semianalytical solution for a general case with arbitrary permeability coefficients were developed. The advantage of this semianalytical solution is that it can compute the behavior of unsaturated soils with very low permeability coefficients. During the derivation, the boundary conditions were transformed into homogeneous boundary conditions first, and then the eigenvalues and eigenfunctions were obtained. To obtain solutions in the time domain, the eigenfunction expansion method and the precise time-step integration method were employed. Through numerical examples, the reflection and transmission characteristics of three types of compression waves at the fluid–solid interface were elucidated. The impact of permeability coefficients and water saturation on the seismic response of the seabed was investigated through parametric analysis. When the soil is nearing saturation, even a slight change in saturation can lead to a drastic alteration in the seismic response of the nearly saturated seabed. [ABSTRACT FROM AUTHOR]

Using 9 years of full‐depth profiles from 55 Deep Argo floats in the Southwest Pacific Basin collected between 2014 and 2023, we find consistent warm anomalies compared to a long‐term climatology below 2,000 m ranging between 11 ± 2 to 34 ± 2 m°C, most pronounced between 3,500 and 5,000 m. Over this period, a cooling trend is found between 2,000 and 4,000 m and a significant warming trend below 4,000 m with a maximum rate of 4.1 ± 0.31 m°C yr−1 near 5,000 m, with a possible acceleration over the second half of the period. The integrated Steric Sea Level expansion below 2,000 m was 7.9 ± 1 mm compared to the climatology with a trend of 1.3 ± 1.6 mm dec−1 over the Deep Argo era, contributing significantly to the local sea level budget. We assess the ability to close a full Sea Level Budget, further demonstrating the value of a full‐depth Argo array. Plain Language Summary: Cold, dense waters formed near polar regions in both hemispheres, sink to great depths and fill‐up the majority of the world's deep ocean. Compilation of sparse observations of temperature from global ship‐based surveys at roughly 10‐year intervals worldwide have shown that sequestration of excess atmospheric heat into the deep ocean has caused these waters to warm steadily since the 1990's into the Present. Not only does this warming have implications for changes in large scale ocean circulation, but is also associated with warming‐induced sea level rise. Using a new data set collected between 2014 and 2023 from 55 freely drifting robotic floats (Deep Argo) which gather crucial bimonthly temperature and salinity data between the surface ocean and the ocean floor, we find the greatest warming trend at a depth of 5,000 m of 4 ± 0.3 m°C yr−1 and an associated sea level rise rate below 2,000 m of 1.3 ± 1.6 mm dec−1. Deep Argo data being collected in ocean basins worldwide are crucial in providing high resolution data of the warming deep ocean and its implications on global sea level, ocean mixing and large‐scale ocean circulation. Key Points: Nine years of Deep Argo data in the S.W. Pacific reveals continued warming in the abyss while the mid‐depths cooledWaters below 4,000 m show an accelerated warming trend with a maximum overall warming rate of 4.1 ± 0.31 m°C yr−1 at 5,000 mDeep ocean steric expansion contributed 1.3 ± 1.6 mm dec−1 to total the local sea level [ABSTRACT FROM AUTHOR]

Accurate earthquake source parameters are crucial for understanding plate tectonics, yet, it is difficult to determine these parameters precisely for offshore events, especially for outer‐rise earthquakes, as the limited availability of direct P or S wave data sets from land‐based seismic networks and the unsuitability of simplified 1D methods for the complex 3D structures of subducting systems. To overcome these challenges, we employ an efficient hybrid numerical simulation method to model these 3D structural effects on teleseismic P/SH and P‐coda waves and determine the reliable centroid locations and focal mechanisms of outer‐rise normal‐faulting earthquakes in northeastern Japan. Two M6+ events with reliable locations from ocean bottom seismic observations are utilized to calibrate the 3D velocity structure. Our findings indicate that 3D synthetic waveforms are sensitive to both event location, thanks to bathymetry and water reverberation effects, and the shallow portion of the lithospheric structure. With our preferred velocity model, which has Versus ∼16% lower than the global average, event locations are determined with uncertainties of <5 km for horizontal position and <1 km for depth. The refined event locations in a good match between one of the nodal strikes and the high‐resolution bathymetry, enabling the determination of the causative fault plane. Our results reveal that trench‐ward dipping normal faults are more active, with three parallel to the trench as expected, while five are associated with the abyssal hills. The significant velocity reduction in the uppermost lithosphere suggests abundant water migrating through active normal faults, enhancing both mineral alteration and pore density. Plain Language Summary: The subduction of oceanic plates plays a crucial role in recycling earth materials and generating the largest earthquakes and tsunamis. This process causes significant bending of the oceanic plate near the trench, where forces from frictional coupling on the plate interface and far‐field subduction loading act in opposite directions, forming a gentle bulge on the seafloor termed the outer‐rise region. This region is rich in shallow bending‐related earthquakes at shallow depths and is characterized by rough bathymetry bounded by faults. Identifying the relationship between these earthquakes and their hosting faults is critical for understanding the seismic and tsunami hazards and fault development in the outer rise. However, building up this link is impeded by large uncertainties of earthquake source parameters. Here, we employ an advanced numerical wavefield simulation approach to precisely handle the structural complexity near 8 ∼ M6 outer‐rise earthquakes, which allows use of far‐field seismic waveform data to constrain source parameters. Reliable earthquake locations and focal mechanisms, along with high‐resolution bathymetry, enable us to identify the causative faults. Dramatically low velocity structure of the subducting oceanic plate is also revealed through this process, which suggests a fluid‐rich outer rise region that efficiently recycles water into the deeper earth. Key Points: Centroid location and focal mechanism of outer‐rise earthquakes were reliably determined through 3D modeling of teleseismic body wavesAlong with high‐resolution bathymetry, reliable source parameters allow us to identify the ruptured faults hosting these normal events3D waveform modeling required a Versus ∼16% lower than the global average, suggesting a fluid‐rich oceanic lithosphere at the outer rise [ABSTRACT FROM AUTHOR]

We investigated the effects of the propagation path and site amplification of shallow tremors along the Nankai Trough. Using far‐field S‐wave propagation from intraslab earthquake data, the amplification factors at the DONET stations were 5–40 times against an inland outcrop rock site. Thick (∼5 km) sedimentary layers with VS of 0.6–2 km/s beneath DONET stations have been confirmed by seismological studies. To investigate the effects of thick sedimentary layers, we synthesized seismograms of shallow tremors and intraslab earthquakes at seafloor stations. The ratios of the maximum amplitudes from the synthetic intraslab seismograms between models with and without thick sedimentary layers were 1–2. This means that thin lower‐velocity (<0.6 km/s) sediments just below the stations primarily control the estimated large amplifications. Conversely, at near‐source (≤20 km) distances, 1‐order amplifications of seismic energies for a shallow tremor source can occur due to thick sedimentary layers. Multiple S‐wave reflections between the seafloor and plate interface are contaminated in tremor envelopes; consequently, seismic energy and duration are overestimated. If a shallow tremor occurs within underthrust sediments, the overestimation becomes stronger because of the invalid rigidity assumptions around the source region. After 1‐order corrections of seismic energies of shallow tremors along the Nankai Trough, the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. Hence, the physical mechanisms governing seismic slow earthquakes can be the same, irrespective of the region and source depth. Plain Language Summary: The deployment of campaigns and permanent ocean bottom seismometers (OBSs) has enabled us to investigate the activity and physical properties of offshore seismic phenomena. Our knowledge of offshore subsurface structures is still limited; consequently, many studies have used conventional analysis methods with the simplest assumptions. Using observed and synthetic seismograms near the Nankai Trough, we found a limitation in the conventional analysis method applied to OBS data. Thick sedimentary layers, which have been confirmed by seismological studies along the Nankai Trough just below the OBSs, cause an approximately 1‐order overestimation of source parameters for seismic phenomena occurring around the shallow plate boundary. This overestimation may have occurred during the seismic energy estimation of shallow slow earthquakes in Hikurangi, Costa Rica, and Mexico. After correcting for the effects of thick sedimentary layers, we found that the scaled energies of seismic slow earthquakes were 10−10–10−9 irrespective of the region and source depth. This suggests that the physical mechanisms governing seismic slow earthquakes can be the same, regardless of region and source depth. Key Points: Effects of path and site on the seismic energy estimation of slow earthquakes at shallow plate boundaries were investigatedThe assumption of far‐field body waves without thick sediments causes an overestimation of seismic energies for shallow tremorsScaled energies of seismic slow earthquakes at both shallow and large depths range from 10−10 to 10−9 [ABSTRACT FROM AUTHOR]

A study has been conducted to analyse the impact of an ice sheet on the flexural-gravity wave force acting on a neutrally buoyant, spherical object immersed in a sea in accordance with the linear wave hypothesis. The bottom of the sea is presumed to be protected by a surface composed of some sort of flexibility, and the top is enclosed by an ice sheet. The solution to a diffraction problem is established using the multipole method. The estimated vertical and horizontal flexural-gravity wave forces are visualized in relation to different immersion depths of the spherical body, depths of the fluid and various flexural rigidities of the ice plate and the flexible seabed. The approach adopted here is likely to be of immense significance in the construction of various spherical gadgets like a seabed pressure detector device for tsunami recognition or communication in crisis for a submarine in earthquake zones. [ABSTRACT FROM AUTHOR]

The structure of King's Trough and its surroundings (King's mesostructural cluster), located on the eastern flank of the Mid-Atlantic Ridge in the North Atlantic, is described in this paper. This work is based on geological and geophysical data obtained during the 55th expedition of the R/V Akademik Nikolaj Strakhov. Six provinces were identified within the southeastern part of King's Trough, based on the results of bathymetric survey. Each province has its own morphostructural feature resulting from multistage tectonic and volcanic processes, which alternate and conjugate with each other in time. According to seismoacoustic profiling data, three main types of seismic facies have been identified: (a) pelagic complexes; (b) deposits of turbidite flows; and (c) chaotic facies of gravity origin. It is shown that the anomalous magnetic field of the study area is the superposition of linear and isometric anomalies. The first were formed during the formation of the oceanic crust in the axial zone of spreading. The second are associated with volcanic massifs formed under intraplate conditions. The obtained data confirm the assumption that the formation of King's Trough was preceded by the formation of an elongated arched rise, which became a scene of intense intraplate volcanism that increased from southeast to northwest. This stage was followed by subsidence of the axial part of the rise with the formation of King's Trough and the Peake and Freen Troughs. [ABSTRACT FROM AUTHOR]

This research work evaluates the wave trapping efficiency of the periodic array of two-layer pile-rock breakwaters (TPRBs) placed far away from vertical rigid wall. The analytical model is developed to study the effect of TPRBs on the variation of wave reflection and reduction of fluid force experienced by wall based on the classical potential flow and porous media flow theory. The effect of the uneven seabed on wave trapping by TPRBs is added to show its significance, and the porous structure consists of a pair of layers with individual porosities. The seaside and leeside protection walls are introduced to secure the two-layer rock from vertical and horizontal failures. Thus, the continuity of velocity and pressure is applied in the vertical and horizontal directions. The eigenfunction expansion method based on orthogonal mode-coupling relation is used to solve the unknown coefficients. The effect of incident wave properties and TPRBs physical properties on wave trapping is reported as a function of breakwater width, gap region, and trapping chamber length. The correctness of the present study results is verified with the available results. The study strongly suggests that the variation of wave reflection by TPRBs is a trivariate function, which would depend upon the width, spacing, and porosity of the TPRBs. Around 30–40% reduction of wave reflection is obtained after introducing the TPRBs. [ABSTRACT FROM AUTHOR]

The present numerical investigation evaluates the significance of seabed undulations on the efficiency of an oscillating water column (OWC) device. The proposed physical problem is formulated in a two-dimensional Cartesian coordinate system under the framework of linearized potential flow theory. The numerical model based on the dual-boundary integral equation method (DBEM) is employed to solve the boundary value problem (BVP), and the study presents the hydrodynamic coefficients of the OWC device in the presence of a composite wavy seabed. Various effects such as the effect of seabed undulations, OWC configuration, chamber spacing, partial rotation of lip-wall, and lip-wall draft on the system radiation coefficients (i.e., wave energy capturing efficiency, radiation susceptance, and conductance) and wall force coefficient is presented against the relative wave frequency. The numerical results indicate that the efficiency of OWC is a trivariate function, which depends upon incident wave frequency, lip-wall rotation, and chamber spacing. The comparative study between various types of OWC devices (i.e., lip-wall configurations) is reported against relative wave frequency in the presence of bottom undulations. The peak performance of OWC is plausible using the resonance mechanism concept when the chamber spacing is moderate and lip-wall is either seaside horizontal or seaside partially inclined. [ABSTRACT FROM AUTHOR]

Broadband signals interacting with deep ocean fine-grained sediment are crucial in shaping the acoustic field of the geometric shadow zone. These signals travel through both the seabed reflected path and the refracted path. In this article, a sequential inversion scheme is employed to estimate the geoacoustic parameters in abyssal clay sediments. This inversion is based on seabed reflection loss data at different frequencies, as well as travel time difference data between refractions and reflections obtained from the South China Sea Experiment in 2018. Depth-dependent profiles of geoacoustic parameters are formulated using Bernstein polynomials. The polynomial coefficients and their posterior probability density functions are efficiently estimated using the adaptive simplex simulated annealing method and an approximate variational inference technique known as Variational Bayesian Monte Carlo. This technique demonstrates superior efficiency and comparable accuracy to Markov Chain Monte Carlo sampling. The inversion results indicate that the abyssal clay sediments in this area exhibit a positive sound speed gradient and relatively low attenuation, both with high probabilities. The deduced seabed model accurately predicts the transmission loss, aligning well with the experimental data. [ABSTRACT FROM AUTHOR]

By zeroing in on the term 'heritage' in the most important international legal instrument on oceans – the 1982 Convention on the Law of the Sea (LOSC), this article explores how the concept of the Common Heritage of [Hu]mankind and maritime heritage are produced and deployed in the global legal project of ocean-ordering and the implications thereof. The Common Heritage of [Hu]mankind concept embodies and enacts a dominant resource-centric framing of the ocean. This framing prioritises and valorises the material and economic value of the deep seabed, thereby transmuting it into an oceanless exploitable common. The LOSC, with minimal cultural preoccupation, also conceptualises and confines maritime heritage within a narrow land-based and material-centric understanding of heritage that often prioritises state control and claims. This is because the LOSC is constrained and shaped both by the metaphysical, geographical, and legal tendency of ocean parcelisation and by the authorised heritage discourse that conditions how heritage is understood and practiced. We conclude by suggesting taking the Middle Passage as an analytical provocation, that is to recognise the mutually constitutive and constituting relationship between the Middle Passage and the (Atlantic) Ocean, to enable critical reflections and alternative perspectives on heritage-making in ocean environment. [ABSTRACT FROM AUTHOR]

The potential application of conducting Scholte‐wave analysis using electroseismic pressure fields excited by an electric current source due to the electrokinetic effect in fluid‐saturated porous seabed sediments is investigated. First, we develop a numerical modelling algorithm by combining the Luco–Apsel–Chen generalized reflection and transmission method with the peak‐trough averaging method to simulate the electroseismic wave fields in stratified fluid/porous media. The modelling results show that the electroseismic pressure signals recorded on the seafloor are mainly composed of evanescent electroseismic waves, and Scholte waves are the dominant wave pattern. Their amplitudes are generally within the order of magnitudes capable of being detected by current seismic instruments. Then, the modified frequency–Bessel transform method is extended to extract the Scholte‐wave dispersion curves from electroseismic pressure fields. Results demonstrate that Scholte‐wave dispersion curves extracted from electroseismic records are superior to those extracted from conventional seismic wave fields excited by an airgun source under the same source–receiver geometry because they contain many overtones and are almost free from the interferences of dispersive guided waves. Furthermore, the Scholte‐wave dispersion inversion results obtained by employing the Levenberg–Marquardt method show that the shear‐wave velocity model inverted by Scholte‐wave dispersion curves extracted from the electroseismic pressure field is more accurate than those obtained by dispersion curves extracted from the seismic wave fields with the guided‐wave removal. The above results indicate that the electroseismic Scholte‐wave analysis method has the potential to evaluate the shear‐wave velocities of shallow‐water seabed sediments. [ABSTRACT FROM AUTHOR]

Full-waveform inversion (FWI) has emerged as the state-of-the art high resolution seismic imaging technique, both in seismology for global and regional scale imaging and in the industry for exploration purposes. While gaining in popularity, FWI, at an operational level, remains a heavy computational process involving the repeated solution of large-scale 3-D wave propagation problems. For this reason it is a common practice to focus the interpretation of the results on the final estimated model. This is forgetting FWI is an ill-posed inverse problem in a high dimensional space for which the solution is intrinsically non-unique. This is the reason why being able to qualify and quantify the uncertainty attached to a model estimated by FWI is key. To this end, we propose to extend at an operational level the concepts introduced in a previous study related to the coupling between ensemble Kalman filters (EnKFs) and FWI. These concepts had been developed for 2-D frequency-domain FWI. We extend it here to the case of 3-D time-domain FWI, relying on a source subsampling strategy to assimilate progressively the data within the Kalman filter. We apply our strategy to an ocean bottom cable field data set from the North Sea to illustrate its feasibility. We explore the convergence of the filter in terms of number of elements, and extract variance and covariance information showing which part of the model are well constrained and which are not. Analysing the variance helps to gain insight on how well the final estimated model is constrained by the whole FWI workflow. The variance maps appears as the superposition of a smooth trend related to the geometrical spreading and a high resolution trend related to reflectors. Mapping lines of the covariance (or correlation matrix) to the model space helps to gain insight on the local resolution. Through a wave propagation analysis, we are also able to relate variance peaks in the model space to variance peaks in the data space. Compared to other posterior-covariance approximation scheme, our combination between EnKF and FWI is intrinsically scalable, making it a good candidate for exploiting the recent exascale high performance computing machines. [ABSTRACT FROM AUTHOR]

Full-waveform inversion (FWI) of seismic data provides quantitative constraints on subsurface structures. Despite its widespread success, FWI of data around the critical angle is challenging because of the abrupt change in amplitude and phase at the critical angle and the complex waveforms, especially in the presence of a sharp velocity contrast, such as at the Moho transition zone (MTZ). Furthermore, the interference of refracted lower crustal (Pg) and upper mantle (Pn) arrivals with the critically reflected Moho (PmP) arrivals in crustal and mantle studies makes the application of conventional FWI based on linearized model updates difficult. To address such a complex relationship between the model and data, one should use an inversion method based on a Bayesian formulation. Here, we propose to use a Hamiltonian Monte Carlo (HMC) method for FWI of wide-angle seismic data. HMC is a non-linear inversion technique where model updates follow the Hamiltonian mechanics while using the gradient information present in the probability distribution, making it similar to iterative gradient techniques like FWI. It also involves procedures for generating distant models for sampling the posterior distribution, making it a Bayesian method. We test the performance and applicability of HMC based elastic FWI by inverting the non-linear part of the synthetic seismic data from a three-layer and a complex velocity model, followed by the inversion of wide-angle seismic data recorded by two ocean bottom seismometers over a 70 Ma old oceanic crustal segment in the equatorial Atlantic Ocean. The inversion results from both synthetic and real data suggest that HMC based FWI is an appropriate method for inverting the non-linear part of seismic data for crustal studies. [ABSTRACT FROM AUTHOR]

Multicomponent seismic exploration offers advantages in precisely describing complicated geological structures compared with conventional PP-wave exploration. For multicomponent ocean bottom node (OBN) seismic data, due to the complication of the seismic-wave propagation path and the diffuse scattering of wavefield energy in rugged seabeds, the reflections show distortion and crossover characteristics, resulting in the distortion of underlying strata in migration sections. Furthermore, the common midpoint stacking approach based on the uniform datum is no longer applicable because of the large elevation differences between sources and receivers. In this study, we propose a PP- and PS-waves prestack time migration (PSTM) method for OBN acquisition with rugged seabeds. The proposed method can significantly improve the imaging quality by correcting the downward P-wave traveltime in seawater and adopting an optimized migration aperture from the formation dip-angle model. Moreover, we develop feasible strategies to establish formation dip-angle and PP- and PS-migration velocity models. The typical feature of the improvements we make on PSTM is that the non-hyperbolic moveouts of both PP- and PS-waves on common image gathers induced by the non-uniform elevations of shot and receiver points and the asymmetry ray path of PS-wave are corrected to be quasi-hyperbolic. Numerical tests and field data applications demonstrate that our method can effectively image underlying structures with rugged seabeds and has potential in multicomponent OBN exploration. [ABSTRACT FROM AUTHOR]

Accurate seismic wave modeling of viscoelastic anisotropic medium is a fundamental tool for seismic data processing, interpretation and full waveform inversion. Also, free water surface, topographic relief and irregular seabed are often encountered in practical seismic surveys. Thus, basing on the General Maxwell Body, we proposed a generalized matrix form of the velocity-stress seismic wave equation, which becomes valid for composite viscoelastic anisotropic media and satisfies the boundary conditions in presence of topographic free surfaces and irregular fluid–solid interfaces. We theoretically show that the viscoelastic effect of a medium may be considered as the intrinsic body sources accumulated in wavefield history and computed by a recursive convolution formula accurately and efficiently. We also demonstrated that such a generalized viscoelastic wave equation may be solved with the curvilinear MacCormack finite difference method and validated the accuracy and feasibility of the proposed method. The modeling results in homogeneous and heterogeneous media match well with the analytical solutions and the references yielded by the spectral element solutions. [ABSTRACT FROM AUTHOR]

Steel catenary risers (SCRs) provide a cost-effective solution for deepwater oil and gas production. However, SCRs are susceptible to potential fatigue failure due to the cyclic motions of floating platforms. Previous studies on the physical modelling of cyclic SCR–seabed interactions have primarily focused on either the continuous cyclic motion of an SCR or a single rest period between two SCR motion packets. However, our understanding of the development of seabed trenches and excess pore pressure and their effects on SCR fatigue during multiple episodes of SCR motion and soil reconsolidation remains limited. This study presents a newly developed model container capable of modelling three-dimensional SCR motions including heave, surge, sway, and vortex-induced vibration in a geotechnical centrifuge. A centrifuge test is conducted to investigate the vertical cyclic SCR–seabed interaction, considering five vertical cyclic motion packets with intervening periods of reconsolidation. The results indicate that ignoring the effects of reconsolidation leads to an overestimation of the fatigue life of an SCR. In this test, the SCR fatigue life is reduced by 18%–23% after five episodic SCR motion packets and intervening reconsolidation. [ABSTRACT FROM AUTHOR]

Mixed grain‐size bedforms comprise alternating sand‐rich and poorly sorted mud‐rich laminae and bands. These bedforms have been identified in distal submarine settings formed underneath unidirectional flows. This study documents mixed grain‐size bedforms in a proximal submarine slope setting formed beneath both unidirectional and combined flows. Core and outcrop data with well‐constrained palaeogeographical context are used to describe two types of mixed grain‐size bedform. Type A bedforms comprise mud‐rich current ripples and low‐amplitude bed‐waves with alternating concave and planar sandstone–mudstone foresets that pass into mud‐rich troughs, and aggradational sinusoidal laminasets. Type B bedforms consist of sandstone–mudstone laminasets that comprise rounded, biconvex ripples with sigmoidal‐shaped foresets and swale and hummock‐like laminasets and banded sets. These bedforms occur in channel‐margin, internal‐levée and external‐levée, intraslope and disconnected lobe environments, and represent 27 to 63% by stratigraphic thickness of the studied successions. They are interpreted as deposits of clay‐rich transitional flows, whose depositional style is governed by the balance of cohesive and turbulent forces, and the rate of flow deceleration. Type B bedforms are further interpreted as combined transitional flow deposits, resulting from flow deflection and ponding processes by seabed topography. Upward and lateral transitions between different bedforms create distinct bedform sequences, demonstrating progressive spatio‐temporal transformations in flow properties and their topographic interactions. By using a well‐constrained palaeogeographical setting, mixed grain‐size bedforms are shown to be situated close to sites of erosion into muddy substrates, abrupt losses in confinement, and/or changes in slope gradient. These bedforms demonstrate that flow transformation and transitional flow behaviour are not restricted to distal submarine settings. Furthermore, mixed grain‐size bedforms are not a diagnostic criterion for bottom currents, because such flows cannot account for the high mud content in laminasets, or the interlamination of sand and mud. [ABSTRACT FROM AUTHOR]

The evolution of submarine canyons is primarily controlled by turbidity currents, which erode and fill them over time; however, many other hydrodynamic currents operate within canyons. Bottom currents from these other hydrodynamic processes, including internal tides, can be dominant processes, but their deposits are seldom recognized in sediment cores or the rock record. This study combines autonomous underwater vehicle swath bathymetry imagery and sub‐bottom profiles, high‐resolution sediment core analyses (X‐ray imagery and thin sections), and previously collected seabed video and flow measurements within Logan Canyon head (eastern Canada) to provide a detailed, modern record of facies associated with hydrodynamic processes in a canyon head. These results suggest that bottom currents are responsible for maintaining gullies on canyon sidewalls and an axial channel on the canyon floor. Thin sections of sediment cores reveal that muddy sand in the canyon head consists of mud aggregates and silt and fine‐grained sand, both behaving similarly in terms of flow dynamics. Three facies are present at macro‐scale and micro‐scale: laminated, partially laminated and bioturbated sandy mud. Sedimentary structures include rhythmic sand and mud aggregate couplets, planar to wavy laminations, current ripple cross‐laminations and fining‐upward successions, which are attributed to bottom currents induced by internal tides. Bioturbated facies, characterized by discrete biogenic structures and cross‐cutting relationships, predominate and overprint a mottled background. A mottled bioturbation fabric also alternates with or locally disrupts layering within the partially laminated facies. Internal tide currents, capable of bedload transport and forming ripples, were measured during a monitoring period in the canyon head, followed by rapid re‐establishment of benthos and associated biogenic structures, supporting the core interpretations. Preservation of sedimentary facies associated with these internal tides occurs when the sedimentation rate outpaces the rate of bioturbation, likely during stormier conditions on the shelf. These results represent observations of sedimentary facies associated with modern bottom currents and internal tides, and can be used to interpret similar occurrences within the rock record. [ABSTRACT FROM AUTHOR]

The shape of the ocean floor (bathymetry) and the overlaying sediments provide the largest carbon sink throughout Earth's history, supporting ~one to two orders of magnitude more carbon storage than the oceans and atmosphere combined. While accumulation and erosion of these sediments are bathymetry dependent (e.g., due to pressure, temperature, salinity, ion concentration, and available productivity), no systemic study has quantified how global and basin scale bathymetry, controlled by the evolution of tectonics and mantle convection, affects the long-term carbon cycle. We reconstruct bathymetry spanning the last 80 Myr to describe steady-state changes in ocean chemistry within the Earth system model LOSCAR. We find that both bathymetry reconstructions and representative synthetic tests show that ocean alkalinity, calcite saturation state, and the carbonate compensation depth (CCD) are strongly dependent on changes in shallow bathymetry (ocean floor =600 m) and on the distribution of the deep marine regions (>1,000 m). Limiting Cenozoic evolution to bathymetry alone leads to predicted CCD variations spanning 500 m, 33 to 50% of the total observed variations in the paleoproxy records. Our results suggest that neglecting bathymetric changes leads to significant misattribution to uncertain carbon cycle parameters (e.g., atmospheric CO2 and water column temperature) and processes (e.g., biological pump efficiency and silicate-carbonate riverine flux). To illustrate this point, we use our updated bathymetry for an Early Paleogene C cycle case study. We obtain carbonate riverine flux estimates that suggest a reversal of the weathering trend with respect to present-day, contrasting with previous studies, but consistent with proxy records and tectonic reconstructions. [ABSTRACT FROM AUTHOR]

Global scallop fisheries are economically important but are associated with environmental impacts to seabed communities resulting from the direct physical contact of the fishing gear with the seabed. Gear modifications attempting to reduce this contact must be economically feasible such that the catch numbers for the target species is maintained or increased. This study investigated the outcome of reducing seabed contact on retained catch of scallops and bycatch by the addition of skids to the bottom of the collecting bag of scallop dredges. We used a paired control experimental design to investigate the impact of the gear modification in different habitat types. The modified skid dredge generally caught more marketable scallops per unit area fished compared with the standard dredge (+5%). However, the skid dredge also retained more bycatch (+11%) and more undersize scallops (+16%). The performance of the two dredges was habitat specific which indicates the importance of adjusting management measures in relation to habitat type. To realize the potential environmental benefits associated with the improvement in catchability of this gear modification, further gear modification is required to reduce the catch of undersize scallops and bycatch. Furthermore we advocate that technical gear innovations in scallop dredging need to be part of a comprehensive and effective fisheries management system. [ABSTRACT FROM AUTHOR]

Acoustic methods are often used for fisheries resource surveys to investigate fish stocks in a wide area. Commercial fisheries echo sounders, which are installed on most small fishing vessels, are used to record a large amount of data during fishing trips. Therefore, it can be used to collect the basic information necessary for stock assessment for a wide area and frequently. To carry out the quantification for the fisheries echo sounder, we devised a simple method using the backscattering strength of the seabed to perform calibration periodically and easily. In this study, seabed secondary reflections were used instead of primary reflection because the fisheries echo sounders were not equipped with a time-varied gain (TVG) function, and the primary backscattering strength of the seabed was saturated. It was also necessary to use standard values of seabed backscattering strength averaged over a certain area for calibration to eliminate some of the effects of differences in seabed sediment and vessel motions. By using standard values of the seabed secondary reflections, the fisheries echo sounder was calibrated accurately. Our study can provide a reliable framework to calibrate commercial fisheries echo sounders, to improve the estimation and management of fishery resources. [ABSTRACT FROM AUTHOR]

The fish anchor is an economically viable offshore foundation engineering technology that has been proposed in recent years. By fish anchor, we mean a fish-shaped dynamically installed anchor that can penetrate into the seabed through water by its own weight. However, the existing research studies seldom consider the hydrodynamic characteristics of its free fall installation, resulting in uncertainties regarding its underwater installation characteristics. Therefore, based on OpenFOAM, this study first investigated the directional stability of fish anchors during its underwater falling process through the static model analysis method and dynamic model analysis method. The findings indicated that the directional stability of fish anchors was insufficient, rendering them suitable only for release and installation at low heights (i.e., less than 4.2 times the anchor length) without marine environmental loads. Furthermore, the anchor inclination angle during installation is significant, thereby impacting its penetration depth and pull-out capacity in the seabed. To solve this problem, the concept of an innovative booster-assisted fish anchor is proposed, which can significantly enhance the directional stability of conventional fish anchors. Results show that the maximum installation height of the new booster-assisted fish anchor is increased by 2.4 times compared to the original design, thereby enhancing both the penetration speed and uplift capacity. Based on the findings, recommended installation and construction conditions are provided. Finally, a predictive formula for the installation height and verticality of the new booster-assisted fish anchor under current action is presented to offer guidance for engineering applications. [ABSTRACT FROM AUTHOR]

In this study, the sediment transport two-phase flow model named SedFOAM is expanded to include soil cohesion, creating a new model named SedCohFOAM within OpenFOAM. The local scouring flume experiment involving a pile on silty seabed and sandy seabed is conducted in a curved flume. Due to the influence of cohesion, the scouring depth at different locations on sandy seabed is 15%–18% greater than that on silty seabed. Observations from this experiment informed the analysis of force balance, wherein the agglomerated silt particles are modeled as large singular entities and the cohesive force is treated as a downward influence that keeps the aggregated particles stationary. Meanwhile, the experimental outcomes are utilized to validate the accuracy of the SedCohFOAM model. The numerical findings demonstrated that SedCohFOAM can simulate the flow field distribution around the pile, variations in seabed shear stress, and alterations in seabed surface morphology. Compared with the SedFOAM model, the SedCohFOAM model has a significantly reduced simulation error in simulating scour on silty seabed. When comparing the cross-sectional profiles of the scour holes derived from the flume experiments with those simulated by SedCohFOAM, it was observed that the ultimate-equilibrium scour depth predicted by the model is consistently lower, but the scour radius in the numerical simulations is larger. The deviation from the experimental results is nearly within 8%, while when the flow velocity is high, the simulation error of the simulated scouring depth behind the pile and the scouring radius in front of pile is amplified. [ABSTRACT FROM AUTHOR]

The principle of the common heritage of mankind (CHM principle), as enshrined in Part XI of the United Nations Convention on the Law of the Sea, is the fundamental principle of sharing the resources of the international seabed area. The International Seabed Authority (the Authority) is the international institution that implements Part XI of the Convention and has the dual task of managing the exploration and exploitation of resources in the international seabed area and protecting the marine environment. This article dives into a deep analysis of the problems that have arisen in the course of the development of the sharing mechanism for resources in the international seabed area by the Authority, and further engages in the discussion on China's domestic legislative strategies for the mechanism for sharing of resources in the international seabed area. It is argued in this article that China's domestic deep-sea legal regime is an important part of global deep-sea governance, and has the potential to apply a 'corrective' function to the 'institutional misalignment' that occurs in the operation of the international seabed regime with the aim of promoting the sharing of resources in the international seabed area. [ABSTRACT FROM AUTHOR]

Seabed mining is a potential solution for diversifying the supply chains of various raw materials required for zero-emission industrial products. The design of a nodule-lifting apparatus in a seabed mining electric vehicle is a crucial step because it significantly affects the mining efficiency of the entire system. Although the physical properties of polymetallic nodules are vital external parameters, little attention has been paid to their effect on the design of nodule-lifting apparatuses. This study aims to analyze and model the physical properties of the polymetallic nodules, expand the conventional numerical model to deal with more diverse physical properties of the nodules, apply probabilistic analysis to evaluate the harvesting rate, and establish a design methodology to efficiently determine the best feasible design. The advantages of the proposed framework are that it enables: (1) quantification of the physical properties of the seabed polymetallic nodules, (2) consideration of these physical properties in the computational simulation of the nodule-lifting apparatus, (3) efficient evaluation of the harvesting performance of the nodule-lifting apparatus through an information criterion-based probabilistic analysis, and (4) effective design of the nodule-lifting apparatus with an orthogonal array and rule-based decision-making strategy. [ABSTRACT FROM AUTHOR]

The mat foundation is a new type of jack-up wind power installation platform suitable for clay seabed. Because of the large size of the mat foundation, a large suction force must be overcome during the uplift process. In this study, the eccentric uplift of a mat foundation was experimentally and numerically investigated. The effects of the eccentric distance, eccentric position, and foundation shape on the suction force of the mat foundation were discussed. The results revealed that the breakout force of the mat foundation mainly comprised the foundation weight and suction force. Both the uplift position and eccentric distance affected the suction force; however, the effect of the uplift position was considerably smaller than that of the eccentric distance. The dimensionless suction force for the eccentric uplift tended to decrease linearly as the eccentric distance increased. When the dimensionless eccentric distance was 0.5, the suction force was 0.35-0.5 times that in the case of central uplift. [ABSTRACT FROM AUTHOR]

An efficient and precise time-frequency analysis method for real-time ocean bottom seismometer (RTOBS) data in the South China Sea (SCS) is presented. Overcoming the limitations of conventional methods, the method involves temporal segmentation, unique frequency octaves, and Fourier transforms to generate power spectral density (PSD) and probability density function profiles. The method demonstrates superior precision, computational efficiency, and full-bandwidth (0 to Nyquist) capability compared to traditional techniques, as validated through theoretical and empirical evaluations. Applied to SCS RTOBS data, it unveils temporal PSD variations, shedding light on underwater noise sources like earthquakes, offshore blasting, ship-induced disturbances, and tidal effects. Establishing background noise levels in the SCS supports noise source categorization and ocean environment monitoring. Furthermore, comparing onshore and offshore seismic stations advances interdisciplinary research, fostering a comprehensive understanding of acoustics and seismology in the region. [ABSTRACT FROM AUTHOR]

The conversion from seismic to ocean-acoustic waves occurs in different places on the bottom of the ocean, often hundreds to thousands of kilometers away from the epicenter. Here, we investigate this conversion process by studying 15 large-magnitude earthquakes that occurred between 2014 and 2022 along the Kermadec Arc in the southwestern Pacific Ocean. To pinpoint the location where seismic-to-acoustic conversion takes places, we analyze hydroacoustic signals recorded by a hydrophone triplet station of the International Monitoring System in the Juan Fernández archipelago. Results from direction-of-arrival and travel-time calculations indicate that the location of the conversion zone largely matches segments of the Louisville Seamount Chain, its lateral extent ranging from approximately 300 to 1800 km, and its location depending on the geometry between earthquake epicenter and the seamounts. [ABSTRACT FROM AUTHOR]

An ocean acoustics experiment in 2017 near a shipping lane on the New England continental shelf in about 75 m of water provided an opportunity to evaluate a methodology to extract source signatures of merchant ships in a bottom-limited environment. The data of interest are the received acoustic levels during approximately 20 min time intervals centered at the closest position of approach (CPA) time for each channel on two 16-element vertical line arrays. At the CPA ranges, the received levels exhibit a frequency-dependent peak and null structure, which possesses information about the geophysical properties of the seabed, such as the porosity and sediment thickness, and the characterization of the source, such as an effective source depth. The modeled seabed is represented by two sediment layers, parameterized with the viscous grain shearing (VGS) model, which satisfies causality, over a fixed deep layered structure. Inferred estimates of the implicit source levels require averaging an error function over the full 20 min time intervals. Within the 200–700 Hz band, the Wales–Heitmeyer model captures the inferred frequency dependence of the source levels. [ABSTRACT FROM AUTHOR]

Oceanic mesoscale eddy mixing plays a crucial role in Earth's climate system by redistributing heat, salt, and carbon. For many ocean and climate models, mesoscale eddies still need to be parameterized. This is often done via an eddy diffusivity K , which sets the strength of turbulent downgradient tracer fluxes. A well-known effect is the modulation of K in the presence of background potential vorticity (PV) gradients, which suppresses cross-PV gradient mixing. Topographic slopes can induce such suppression through topographic PV gradients. However, this effect has received little attention, and topographic effects are often not included in parameterizations for K. In this study, we show that it is possible to describe the effect of topography on K analytically in a barotropic framework, using a simple stochastic representation of eddy–eddy interactions. We obtain an analytical expression for the depth-averaged K as a function of the bottom slope, which we validate against diagnosed eddy diffusivities from a numerical model. The obtained analytical expression can be generalized to any constant barotropic PV gradient. Moreover, the expression is consistent with empirical parameterizations for eddy diffusivity over topography from previous studies and provides a physical rationalization for these parameterizations. The new expression helps to understand how eddy diffusivities vary across the ocean, and thus how mesoscale eddies impact ocean mixing processes. Significance Statement: Large oceanic "whirls," called eddies, can mix and transport ocean properties such as heat, salt, carbon, and nutrients. Mixing plays an important role for oceanic ecosystems and the climate system. In numerical simulations of Earth's climate, eddy mixing is typically represented using a simplified expression. However, an effect that is often not included is that eddy mixing is weaker over a sloping seafloor. In most areas of the ocean the bottom slope is steep enough for this effect to be significant. In this study we derive an expression for eddy mixing that accounts for oceanic bottom slopes. The present effort provides a physical basis for eddy mixing over oceanic bottom slopes, justifying their use in climate models. [ABSTRACT FROM AUTHOR]

This article explores the advancements and challenges in seabed prospecting technology, emphasizing the importance of seabed geophysical data in understanding the planet and driving marine energy resource exploration and deep earth science research. It calls for further technical refinement in ocean bottom geophysical prospecting techniques and the integration of seabed exploration technology with big data and artificial intelligence. The article also introduces a special issue on seabed prospecting technology, which includes research papers covering a range of prospecting methods and geophysical applications. The document provides summaries of various research studies related to seismic data analysis and exploration, offering key findings and methodologies used in each study. [Extracted from the article]

Similar to conventional ocean bottom seismometers and ocean bottom cables, the attitude and position of vertical cables deployed on the seafloor should be calibrated. However, due to the effect of ocean currents, the cable attitude is not stationary. Therefore, a new method for the dynamic repositioning of vertical cables is required. The dynamic repositioning of the hydrophones on vertical cables plays an important role in accurately describing the structure of the stratum and establishing an accurate velocity model. To realize the dynamic positioning of marine vertical cables, the shot lines are divided according to time, the first break time objective equation is established and the receiver position in each time period is obtained using the inversion methods outlined in this work. By analysing the trace spacing, first break time, shot point coordinate accuracy, temporal resolution, shot line spacing and other factors after repositioning, it is found that the first break time pickup accuracy and shot line spacing are the main factors that affect the repositioning accuracy. Furthermore, the resolution is limited by the timing of a single shot line. Additionally, the trace spacing after repositioning is consistent with the actual situation, which verifies the correctness and feasibility of this method. [ABSTRACT FROM AUTHOR]

Clock drift is common for the autonomous ocean bottom seismometer, and a technique of linear drift correction is usually used to fix this kind of timing error assuming that the internal clock has constant drift rate during deployment. However, non‐linear component of clock drift has also been found recently. In order to test the behaviours of the clock drift for 19 Chinese ocean bottom seismometers from a three‐dimensional controlled‐source seismic survey, we have analysed two types of differences in direct wave travel‐times, including one from all shots to the ocean bottom seismometer onboard and another one from the nearest two shots on different shooting lines to the ocean bottom seismometer on the seafloor. Comparison of the differences in direct wave travel‐times from seismic data with and without clock drift correction shows that linear drift correction can commendably eliminate them for most of the ocean bottom seismometers, but some large residuals remain for part of the ocean bottom seismometers. These residuals are inferred to be possibly related to the non‐linear component of clock drift caused by the temperature shock during deployment. The non‐linear clock drift usually varies from different ocean bottom seismometers and thus difficult to correct completely and resultantly will influence the timing accuracy somewhat. Hence, higher uncertainties are suggested to assign to the picked travel‐times for the ocean bottom seismometer with possibly larger non‐linear clock drift in the further tomographic modelling and the internal clock for this kind of ocean bottom seismometer should be recalibrated prior to the next experiment if which requires higher timing accuracy. [ABSTRACT FROM AUTHOR]

Ocean‐bottom node exploration has developed rapidly in marine seismic exploration. However, due to no illumination for the seafloor, and the observation system of the 'less traces but more shots' mode, the conventional surface‐related multiples elimination meets challenges for the application in the ocean‐bottom node exploration. This paper develops a new three‐dimensional surface‐related multiples elimination technique for ocean‐bottom node data. First, a three‐dimensional Kirchhoff pre‐stack time mirror migration algorithm is proposed to realize the accurate imaging for the seabed. Second, the time domain Kirchhoff demigration method is used to construct the contribution traces for multiples prediction. Finally, the surface multiples can be predicted based on the equation derived in the paper. Model tests and field data processing prove that our method can accurately predict the surface‐related multiples for ocean‐bottom node data and be helpful to the multiple elimination, which shows that the proposed technique has potential in actual ocean‐bottom node data processing. [ABSTRACT FROM AUTHOR]

The source vessel noise is a very common noise type in offshore seismic surveys. The state‐of‐art deep learning‐based methods provide an end‐to‐end framework for seismic data denoising. The denoising performance of a pretrained network is, however, highly dependent on the completeness of the training set. When training a denoising network with only field data, especially for attenuating erratic noise, it is hard to obtain a noise‐free data as the training target for the network. Transfer learning, by combining the synthetic and field data, is an alternative solution for improving the generalization capabilities of the network, although being able to model such erratic noise represents also a challenge. Although the denoising results by traditional methods are not accurate enough for creating a complete training set, the features in residual noise by subtracting the denoised data from noisy data are enough for the network to learn. Considering the aforementioned factors, we develop a deep learning‐based workflow for the attenuation of the erratic source vessel noise from ocean bottom node 4C data. Instead of using denoising results directly, we use the conventional methods to extract noise and add them to the high signal‐to‐ratio region of the field data. The created noisy dataset is different from the original noisy data in noise regions; thus, the pretrained network can also be used for predicting the same original data. The denoising results of synthetic and field data all show that even the network is trained on a noisy labelled dataset, we still can obtain high signal‐to‐noise ratio denoising result. Besides, when compare with the results by filtering‐based methods, our proposed method can attenuate the vessel noise more effectively and preserve the near offsets reflections. [ABSTRACT FROM AUTHOR]

High‐precision P‐wave and S‐wave velocity models are very important for seabed exploration. Elastic full‐waveform inversion is an effective way to obtain the velocity models. Full‐waveform inversion is strongly dependent on source wavelet, and inaccurate wavelet estimation will severely affect the inversion results. Furthermore, elastic full‐waveform inversion is a highly non‐linear problem and has larger calculation and memory cost. Based on the deconvolution method, we develop a multi‐scale source‐independent elastic full‐waveform inversion method in a hybrid domain to alleviate the non‐linearity, to improve computation efficiency and decrease memory cost and to reduce the influence of source wavelet. We reconstruct the deconvoluted objective function and further derive the adjoint source and gradient formulas for elastic full‐waveform inversion with the adjoint‐state method. We can obtain good inversion results by using this objective function, adjoint source and gradient even if the source wavelet is unknown. In order to illustrate the influence of different reference traces on the objective function, we provide the theoretical formula of the objective function changing with the variation of reference traces. Experiments show that the deconvolution objective function cannot completely eliminate the influence of inaccurate source wavelet on the inversion, and the inversion effect depends on the selection of reference trace. Different inversion results of the Marmousi2 model indicate that the minimum offset trace is the best reference trace. Comparison of the inverted Marmousi2 model by the proposed method and the conventional elastic full‐waveform inversion method using correct wavelet shows that the two inversion results are very close, which proves the effectiveness of the proposed method and indicates a potential application of the method in seabed exploration. [ABSTRACT FROM AUTHOR]

Ocean bottom seismometer data usually contain a large amount of random noise, which seriously reduces the signal‐to‐noise ratio of the data and affects subsequent imaging. Hence, random noise attenuation is one of the most essential steps in ocean bottom seismometer data processing. In this paper, a novel approach is proposed to attenuate the marine seismic random noise of ocean bottom seismometers based on a six‐dense‐layer denoising autoencoder. We input the t−x$t-x$ domain data into the denoising autoencoder, the encoder compresses the signal and noise and extracts the main features and the decoder finally reconstructs the denoised data with the same dimension as the input. In this approach, because few raw labelled examples are available, we first constructed the pretraining, training and test data sets by patch processing. Then, we pretrained the encoder based on clean synthetic seismic data through unsupervised learning and pretrained the decoder based on noisy synthetic seismic data through supervised learning. Next, the pretrained model was fine‐tuned with the encoder–decoder on a raw seismic data set in an unsupervised manner. Finally, we used the model to attenuate the random noise in raw ocean bottom seismometer data for testing. Synthetic and raw examples are used to compare the f−x$f-x$ deconvolution, multichannel singular spectrum analysis, deep denoising autoencoder and substep deep denoising autoencoder approaches. Experimental tests demonstrate that the proposed method has higher processing efficiency and precision. [ABSTRACT FROM AUTHOR]

With the increasing maturity of ocean bottom seismometer technology in gas hydrate exploration, more and more researchers apply ocean bottom seismometer exploration technology for offshore oil exploration. However, due to the special observation mode of ocean bottom seismometer, it is impracticable to process ocean bottom seismometer data using traditional data processing methods, such as velocity analysis and pre‐stack time migration. This manuscript proposed a new velocity analysis method for ocean bottom seismometer data, which obtains more accurate root‐mean‐square velocity than the existing method. Then we deduced the Kirchhoff pre‐stack time migration formula for ocean bottom seismometer data. Two models demonstrate the correctness of the velocity analysis and migration methods. Finally, the two methods were applied to the actual ocean bottom seismometer data, and the obtained migration profile is consistent well with the profile of towed streamer data in the nearby area. [ABSTRACT FROM AUTHOR]

Due to the limitations of the actual acquisition environment in the field, especially in ocean bottom seismometer (OBS) acquisition, the acquired seismic data are often irregular and incomplete, which affects the subsequent data imaging, interpretation and hydrocarbon‐bearing reservoir prediction. The interpolation reconstruction algorithm based on the compressive sensing theory can reconstruct the data without the limitation of Nyquist sampling interval. However, the reconstruction accuracy and effect are different for different sparse representations of the data. On the basis of compressed sensing theory, we propose a Bregman iterative seismic data reconstruction method based on the sparse decomposition of discrete orthonormal Coiflets and Symlets wavelet transforms. First, the discrete orthonormal matrix is constructed by using the above two wavelet functions to make the original seismic data sparse, then the Bregman iterative algorithm is used to reconstruct the sparse coefficients in the discrete wavelet domain, and finally the recovery matrix is used to reconstruct the seismic data. The discrete orthonormal Coiflets and Symlets wavelet transforms have good sparse representation ability and can compensate for the problem that discrete Fourier transform cannot well sparse representation of the data. After numerical experiments on horizontal‐layered model, Marmousi2 model and actual data, it is verified that the proposed method can reconstruct the missing seismic data under the regular observation system and under the irregular observation system with non‐uniform OBS distribution. Furthermore, this method can hardly bring the interference of random noise, and the reconstruction result is of high accuracy. [ABSTRACT FROM AUTHOR]

Large seabed trenches have recently been found adjacent to suction caissons with taut and semi-taut moorings. Seabed trenching leads to loss of soil support and additional caisson rotation, and can therefore significantly decrease geotechnical capacity. In this paper, upper-bound solutions are presented that enable calculation of the reduction in ultimate load-carrying capacity of a suction caisson in a trenched seabed compared to optimal capacity in an intact seabed. Failure mechanisms for caissons embedded in an intact seabed are augmented to incorporate a kinematically admissible mechanism for a trench extending from the mudline. The rate of work dissipation is calculated to determine the optimised solution of the inclined capacity of suction caissons. Comparisons of the results of the upper-bound solutions with finite-element analyses show that the failure mechanisms and the calculated load-carrying capacities derived from the two methods agree well. Therefore, the analytical upper-bound method presented in this paper can be employed for efficient routine calculations of suction caisson capacity under inclined load considering seabed trenches, which are increasingly encountered in engineering practice. [ABSTRACT FROM AUTHOR]

Quantification and mapping of surficial seabed sediment organic carbon have wide-scale relevance for marine ecology, geology and environmental resource management, with carbon densities and accumulation rates being a major indicator of geological history, ecological function and ecosystem service provisioning, including the potential to contribute to nature-based climate change mitigation. While global analyses can appear to provide a definitive understanding of the spatial distribution of sediment carbon, regional maps may be constructed at finer resolutions and can utilise targeted data syntheses and refined spatial data products and therefore have the potential to improve these estimates. Here, we report a national systematic review of data on organic carbon content in seabed sediments across Canada and combine this with a synthesis and unification of the best available data on sediment composition, seafloor morphology, hydrology, chemistry and geographic settings within a machine learning mapping framework. Predictive quantitative maps of mud content, dry bulk density, organic carbon content and organic carbon density were produced along with cell-specific estimates of their uncertainty at 200 m resolution across 4 489 235 km 2 of the Canadian continental margin (92.6 % of the seafloor area above 2500 m) (10.5683/SP3/ICHVVA, Epstein et al., 2024). Fine-scale variation in carbon stocks was identified across the Canadian continental margin, particularly in the Pacific Ocean and Atlantic Ocean regions. Overall, we estimate the standing stock of organic carbon in the top 30 cm of surficial seabed sediments across the Canadian shelf and slope to be 10.9 Gt (7.0–16.0 Gt). Increased empirical sediment data collection and higher precision in spatial environmental data layers could significantly reduce uncertainty and increase accuracy in these products over time. [ABSTRACT FROM AUTHOR]

A new underwater rammed stone column (U-RSC) is proposed to construct the composite foundation in the seabed. A series of laboratory model tests were conducted to investigate the feasibility and installation behavior of the proposed technology. The test results showed that the U-RSCs installed in clay are influenced by the tamping energy, single filling mass, and soil strength. The profile shapes are wedge shapes tapering toward the base with their maximum expansion position located approximately 1.5 d from the ground surface. The soil heave was observed during the installation of U-RSC into the clay with its maximum influence distances of approximately 4 times hole diameter in the model tests. The tamping-induced excess pore pressure did not fully dissipate to zero but continuously accumulated after the continuous filling and tamping process. It is observed through the lateral earth pressure contour maps that the tamping energy compacted vertically the gravel and laterally the surrounding soil. Based on their influences to the surrounding soil, the soil around the column could be divided into the penetration zone, the high-densified zone, and the low-influenced zone. The proposed U-RSC method provides a new, effective and practical method to construct composite foundation for underwater soil improvement. [ABSTRACT FROM AUTHOR]

The available potential energy of the diurnal internal tide in the Northwest Pacific ocean is studied by using short-term high-frequency mooring observations. The results show that in the upper layer of the ocean, its distribution is relatively chaotic, and it is greater between 1500-1600 m and 2500-3000 m in the middle layer of the ocean. At other depths, it is generally smaller, especially, at the bottom layer of the ocean, it is relatively small. This paper concludes that the marvel of measurement is the best science picks. [ABSTRACT FROM AUTHOR]

The marine animal phylum Placozoa is characterized by a poorly explored cryptic biodiversity combined with very limited knowledge of their ecology. While placozoans are typically found as part of the epibenthos of coastal waters, known placozoan predators, namely small, shell‐less sea slugs belonging to the family Rhodopidae (Mollusca: Gastropoda: Heterobranchia), inhabit the interstitium of seafloor sediment. In order to gain further insights into this predator–prey relationship and to expand our understanding of placozoan ecological niches, we screened publicly available whole‐body metagenomic data from two rhodopid specimens collected from coastal sediments. Our analysis not only revealed the signatures of three previously unknown placozoan lineages in these sea slug samples but also enabled the assembly of three complete and two partial mitochondrial chromosomes belonging to four previously described placozoan genera, substantially extending the picture of placozoan biodiversity. Our findings further refine the molecular phylogeny of the Placozoa, corroborate the recently established taxonomic ranks in this phylum, and provide molecular support that known placozoan clades should be referred to as genera. We finally discuss the main finding of our study – the presence of placozoans in the sea floor sediment interstitium – in the context of their ecological, biological, and natural history implications. [ABSTRACT FROM AUTHOR]

Oxygen crystallising was obtained from olivine, chromite separates from host rock samples, and gabbro from ultramafic cumulates, harzburgite, and dunite samples collected from tectonites of the Guleman ophiolites. The study includes the oxygen isotope compositions from the Guleman ophiolite and the evolutionary scenarios of geochemical and isotopic signatures. The olivine and chromitite δ18Oolivine/chromite and δ18Omelt values point out these minerals and melt isotopic compositions similar to mantle values and rich, heavy stable isotopes. The highest δ18Ochromite isotopic composition values indicate that as the heavy isotope in the melt increases, the concentration in the mineral increases. Excluding a dunite sample, the values of the δ18Owhole rock of the dunite and pyroxenite samples are similar to mantle values. The gabbro samples have higher δ18Owhole rock values than the two harzburgites. Hereby, the chromite formations are the later fractional crystallisation products than olivine and dunites due to the lowering of δ18O. The lower oxygen isotopic composition in studied samples than the normal mantle values, and these values point out subducted hydrothermally alteration. The higher δ18O isotopic compositions could be explained by serpentinization on the ocean floor at low temperatures. [ABSTRACT FROM AUTHOR]