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Internal solitary waves (ISWs) exert a significant influence on the deep-sea floor, yet little research has been conducted on their impact on benthic organisms. The objective of this study was to investigate the propagation characteristics of ISWs on the Shenhu continental slope in the northern South China Sea, as well as their mechanisms of influence on benthic activity. In-situ observation was performed at the head of the Shenhu Canyon (water depth 655m) to determine the physical characteristics of ISWs and the changes in benthic organisms. The study revealed that the abundance and density of benthic organisms were positively correlated with the time and intensity of ISWs. The abundance of benthic organisms affected by ISWs is 3-5 times that without ISWs, and the density of benthic organisms is 3-9 times. The impact induced by ISWs, including resuspension of bottom sediments, formation of marine snow, changes in the bottom boundary layer environment, and vertical transfer of seawater environmental factors, played a crucial role in the impact on benthic activity. To our limited knowledge, this is the first in-situ long-term observation study on the effects of ISWs on deep-sea benthic organisms on the continental slope of the northern South China Sea deeper than 600m, providing new insights for exploring the environmental impact of ISWs on the deep-sea bottom boundary layer.

Ocean mining activities have been ongoing for nearly 70 years, making great contributions to industrialization. Given the increasing demand for energy, along with the restructuring of the energy supply catalyzed by efforts to achieve a low-carbon economy, deep seabed mining will play an important role in addressing energy- and resource-related problems in the future. However, deep seabed mining remains in the exploratory stage, with many challenges presented by the high-pressure, low-temperature, and complex geologic and hydrodynamic environments in deep-sea mining areas, which are inaccessible to human activities. Thus, considerable efforts are required to ensure sustainable, economic, reliable, and safe deep seabed mining. This study reviews the latest advances in marine engineering geology and the environment related to deep-sea mining activities, presents a bibliometric analysis of the development of ocean mineral resources since the 1950s, summarizes the development, theory, and issues related to techniques for the three stages of ocean mining (i.e., exploration, extraction, and closure), and discusses the engineering geology environment, geological disasters, in-situ monitoring techniques, environmental protection requirements, and environmental effects in detail. Finally, this paper gives some key conclusions and future perspectives to provide insights for subsequent studies and commercial mining operations.

A number of moderate earthquakes occurred in the South China continental edge or coastal area. Several large earthquakes are sparsely distributed along the seismic belt with the northeast direction between the South China and the continental shelf of the South China Sea. Coulomb stress changes of the 1969 Yangjiang main shock (Ms 6.4) with the following events of the 1986 ML 5.4, the 1987 ML 5.4 and the 2004 ML 5.2 in the Yangbian Bay area are studied. The complex stress patterns of the main shock caused by the dextral Pinggang strike-slip fault imported on the planes of the normal fault, strike-slip fault and thrust fault of three events indicate that the distribution of aftershocks mainly located in the areas of positive stress changes and part of aftershocks fall into the stress shadow area due to the reactivity of the local faults. The 1969 Yangjiang event occurred between the intraplate and the broad plate boundary zone setting when we apply on the empirical relationship between the aftershock durations and the fault-loading rate. The intersection fault model is proposed and could explain the stress changes of the Yangjiang earthquake sequences in South China coastal area. The future hazard assessment requires to take account of the stress increase area along the tip of the main shock source fault and off-fault stress increase area.

Landslides often occur in land slopes, estuaries, fjords, and intertidal zones in the marine environment, while landslides in tidal channels have not been systematically reported. The Zhoushan sea area is dotted with islands and crisscross tidal channels, which is a typical tidal channel development area. The data from the coastal zone and island survey, offshore engineering site selection, and environmental survey show that landslides are very common in tidal channel areas. The detailed study of this kind of landslide will enrich the research theory of submarine landslide and have important guiding significance for marine planning, development and site selection, and construction of coastal engineering in coastal areas. First, the submarine landslide in southwest of Zhujiajian Island in Zhoushan is investigated in this study. It is found that the sliding zone is parallel to the island coastline, with a length of about 250 m and an extension of 2 km, and the buried depth of the sliding surface is generally 8–10 m; then, through the self-developed seabed deformation observation system, combined with the hydrodynamic observation system, the landslide was observed in situ for 75 days. The observation results showed that the landslide slides at a constant speed along the sliding surface under the action of weight; the slides accelerate under the action of waves, and multiple sliding surfaces are generated in the landslide body at the same time. Finally, the genetic mechanism of submarine landslides in tidal channel areas is put forward.

Gas hydrate dissociation may pose serious geohazards. The response of the seafloor to hydrate dissociation is important for geohazards, marine environments and global climate. We studied how hydrate dissociation affects underground temperature and pressure and how dissociation may lead to seafloor deformation. Here, we designed a laboratory model experiment to visualize fluid migration and seafloor deformation during hydrate dissociation at atmospheric pressure. Our experiments show visible gas pipes, migratory gas and gas pockets due to hydrate dissociation and explain the newly discovered discordance between the slide plane and the base of the gas hydrate stability zone. We found that the underlying free gas and gas conduits not only are favorable factors for forming hydrates but also can be promoted by hydrate dissociation. The temperature, pressure, and seafloor deformation are collected during the experiments. Our results show that the hydrate dissociation process is an endothermic reaction. More importantly, hydrate dissociation-induced overpressure exhibits a sudden decrease after slow growth and this inflation–deflation cycle resembles breathing. In our experiments, seafloor deformation are dominated by underground overpressure and lags up to about ten minutes. This deformation pattern could help forecast seafloor subsidence based on deformation observations. Overpressure monitoring is effective and saves time to cope with imminent geohazards. This pattern is important for geologists and engineers to forecast hydrate-related geohazards.

Suspended particulate matter (SPM) plays an important role in material transport, deposition, resuspension and the function of benthic communities’ processes in deep sea. SPM concentration data is usually indirectly measured by optical/acoustic sensors. However, converting these sensors’ signal to SPM concentration is associated with a number of uncertainties, which will lead to mis-estimation of the results. Some researchers recommend combining several optical/acoustic sensors to determine SPM concentration. However, due to the lack of corresponding video images, the interpretation of significant mismatch signals recorded by different sensors is subjective. Consequently, a better understanding of long-period SPM dynamics, especially in deep sea, is still a challenge. In this study, we seek to monitor the dynamics of SPM in deep sea, by firstly obtaining in-situ digital video images at a water depth of 1450 m on the northern slope of South China Sea in 2020, and secondly developing a digital image processing method to process the in-situ monitoring data. In this method, we defined an image signal which was the ratio between the area of the SPM and that of the total image, to characterize the SPM concentration. A linear regression model of the image signal and SPM concentration was established (R2 = 0.72). K-fold cross-validation showed that the performance of the model was well. We calculated the SPM concentration derived from image signal, and manually classified SPM into three distinct morphological groups. The long-period observation revealed that numerous aggregates existed in deep sea. The change of SPM concentration and morphology under hydrodynamics was synchronous. When current speed equaled to or exceeded 0.15 m/s, there was a significantly increase in SPM concentration and size. However, such increase was episodic. When current speed decreased, they will also decrease. In addition, we compared the image signal with the optical/acoustic backscattering signal, analyzed the mismatch period among these three signals. We found that the optical backscatter signal can’t accurately reflect the SPM concentration during the mismatch period. To our best knowledge, this is the first time that the in-situ digital video images were used to analyze the dynamics of SPM in deep sea.

An estuary delta is an area with active human economic activities and a fragile natural ecological environment. With the intensification of global changes and human activities, coastal and seabed erosion around the world is becoming more and more serious. This study compared the hydrodynamic effect and sediment diffusion path at the Yellow River subaqueous delta (river control type) and the Yangtze River subaqueous delta (tide-river type) in the East Asian monsoon area using a Delft 3D numerical simulation to analyse the causes of the distribution of accumulation and erosion landform in the different types of subaqueous deltas. This study found that the river deltas were dominated by the river when the accumulation landforms concentrated in the direction of the Coriolis forces, which was due to hydrodynamic erosion driven by wind, and the sediment diffusion range was mainly concentrated in shallow water areas (water depth < 6 m), resulting the Yellow River subaqueous delta presenting the pattern of “northern erosion and south accumulation”. The Yangtze River subaqueous delta was controlled by tidal effects, making the southern part of the estuary more susceptible to erosion. The research results can not only further deepen the understanding of the physical process and mechanism of erosion and deposition in the subaqueous delta but also provide stronger scientific support for the deltas’ ecological environment protection, geological environment safety, and disaster geology prevention.

The shear wave characteristics of shallow seafloor sediment are vital to marine acoustic study and ocean engineering exploration. We analyzed the shear wave speed and the physical properties of the...

The study of the physical and mechanical properties of marine soil is of great importance for marine geohazard prediction, submarine energy extraction, and submarine foundation design. In this study, a series of basic geotechnical tests and triaxial compression tests are performed on samples taken from the Shenhu sea area in the South China Sea (SCS). Physical and mechanical properties, particularly normalized stress–strain behavior, are investigated. The microstructural and mineralogical characterization is carried out, through scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results indicated that the sample could be classified as high-plasticity silt (MH) with high water content and high compressibility, that the soil has the highest quartz content in its mineral composition, a loose skeleton composed of flocculent structures under the microscope and is distributed with the remains of marine organisms. Furthermore, a new stress–strain-normalized condition is theoretically derived, based on the hyperbola function. In this condition, the concept of standard normalized factor, which is defined as the ultimate value of principal stress difference, is introduced. Meanwhile, the normalized stress–strain relationship of soft soil from the SCS is established under consolidated undrained conditions and the results of the model are compared with the experimental results, with a good normalization effect. It is believed that the work presented in this paper could contribute to the design and construction of offshore engineering.

Under tidal scouring, residual petroleum in the intertidal sediment after oil spills could release again, causing secondary pollution in the marine ecosystem. The current study aimed to investigate the dynamic process and principles of crude oil release from silty intertidal sediment under different influencing factors and screened for the key factors. In this paper, the fitting equations and correlation between the release amount and various factors were explored through the single-factor and orthogonal experiments. Then, the key influencing factors were selected for multi-factor fitting of the release amount. The results showed that the oil release amount rose with the increase in oil concentration, oscillation frequency, and release time, but decreased with an increase in salinity. As the pH decreased, the oil release amount increased. The relationship between release amount and concentration/oscillation frequency can be equipped by the polynomial equation, and the average R

Calcareous sand is often used for island reef filling. The permeability of calcareous sand is a key parameter used to determine the consolidation time and the storage of fresh water during the construction of calcareous sand foundation. Pore structure characteristics are the key factors affecting permeability. In this study, X-ray computed tomography (CT) combined with pore network modeling were used to study the pore characteristics and permeability properties of calcareous sand with different particle sizes. The results show that the number and volume of pores in calcareous sand samples increase with increasing particle size. The pore volume distribution is uneven, and the pore shape does not change significantly with particle size. The permeability of calcareous sand increases with increasing particle size. The number of pores and the pore distribution are the key factors determining the permeability of calcareous sand with different particle sizes. When the particle size is larger than 1.45 mm, the presence of internal pores has a greater impact on the total number of pores and the permeability.

Porosity and permeability are the two most important characteristics of underground gas storage in sandstone reservoirs. Injection of gas into reservoir rocks will cause rock deformation. The deformation will influence the porosity and permeability properties of the rocks. We investigate the evolution of these two properties of storage sandstone by triaxial compression tests and a uniaxial in situ compression CT test. As the deviatoric stress increases, the sandstone is compressed firstly (porosity reduction) and then dilates (porosity enhancement). With the increase in confining stress, the occurrence of volumetric dilation will be delayed. Trapped porosity of this sandstone at different deviatoric stresses is very small (0.122%-0.115%) which indicates that nearly all pores are connected. During the compression stage, the decrease in permeability is related to compression of pores and microcracks. During the volumetric dilation stage, it is related to increase in tortuosity. This interpretation can be confirmed by observations of in situ compression CT. The permeability evolution estimated by pore network modeling is consistent with macroscopic testing results.

The Huanghe River (Yellow River) is known by its high suspended sediment concentration (SSC) in its river mouth tidal flat. However, the factors controlling the high SSC over there are not well understood. Therefore, we conducted 7-d hydrodynamic observations (water depth, wave height, and current velocity) and SSC measurements on the tidal flat off the Huanghe River Mouth. The data shows that in most of time, under the calm sea condition, the SSC ranges 0.1–3.5 g/L, and sediment discharge from the river is the main source. However, when hydrodynamics are enhanced in a tidal cycle and large-scale erosion occurs on the seafloor, resuspended sediment becomes the main source, and the SSC in the water column reaches 17.3 g/L. We find the suspended sediment flux is mainly controlled by the tidal current and Stokes drift, while the wave-induced shear stress could also affect the variation of suspended sediment flux. During the observation period, when sea under calm-rippled conditions, the current-induced resuspended sediment concentration (RSC) was greater than the wave-induced RSC. In contrast, in smooth-wavelet sea conditions, the wave-induced RSC was greater than the current-induced RSC, for instance, a single wave event was found to cause 11.8 cm seabed erosion within 6 h. This study reveals different controlling factors for the high SSC near a river-influenced tidal flat, and helps us get a better understanding of a delta’s depositional and erosional mechanisms.

Submarine pipelines are likely to be impacted by frequently catastrophic landslides, resulting in broken pipelines and causing irreversible economic losses; however, because pipelines located in de...

The objectives of this study are carried out a series of controlled large wave flume experiments using fine-grained sediment from the Huanghe River Delta, exploring the complete sequence of sediment behavior in the bottom boundary layer (BBL) during wave-induced liquefaction. The results show that: (1) The BBL in silty seabed is exposed to a progressive wave, goes through a number of different stages including compaction before liquefaction, sediment liquefaction, and compaction after liquefaction, which determines the range and thickness of BBL. (2) With the introduction of waves, first, the sediment surface has settled by an amount S (S=1–2 cm) in the course of wave loadings with an insufficient accumulation of pore water pressure. And a thin high concentration layer formed the near-bed bottom. (3) Once the liquefaction sets in, the liquefied sediment with an ‘orbital motion’ and the sub-liquefied sediment form a two-layer-sediment region. The range of BBL extends downwards and stopped at a certain depth, subsequently, develops upwards with the compaction process. Meanwhile, re-suspended sediments diffuse to the upper water column. (4) During the dynamics process of the BBL beneath progressive waves, the re-suspended sediment increment ranked as sediment liquefaction > erosion before liquefaction > compaction after liquefaction.

Submarine sediment failures or landslides triggered by storm waves in river delta areas pose a significant risk to coastal infrastructure. Due to the limitations of in situ monitoring technology, existing investigations are mostly carried out with geophysical techniques to provide some basic characteristics (e.g. location, size, runout distance, volume, and potential triggers) of existing submarine landslides. However, it is of equal importance to identify the starting criterion and the in situ evolutionary process of the initial stage of seafloor instabilities—sediment failure, which naturally relies heavily on long-term field observations. A field monitoring system was developed for observing sediment failure, which successfully recorded the wave-induced seabed deformation in the subaqueous Yellow River Delta for the first time. Once sediment failure is initiated, the shallow soil undergoes periodic and reciprocating oscillations under alternating action of wave crests and troughs. The evolution of the maximum deformation depth interface moves from shallow to deep, and then migrates upward from deep to shallow layers (the shallow-deep-shallow pattern), which indicates a stability recovery process following the wave-induced seabed failure, and these processes were found to occur multiple times within one storm event. Laboratory wave flume experimental results reproduced and verified the field observations, while also providing pore pressure data which explains the initiation of sediment failure and the deformation process. Finally, a development pattern of the seafloor instabilities in the subaqueous Yellow River Delta is proposed. The in situ observation methods proposed and the knowledge acquired by field monitoring and flume testing could benefit the investigation of costal seafloor instabilities.

The occurrence of deep-sea geohazards is accompanied by dynamic changes in the physical properties of seafloor sediments. Therefore, studying the physical properties is helpful for monitoring and early warnings of deep-sea geohazards. Existing physical property inversion methods have problems regarding the poor inversion accuracy and limited application scope. To address these issues, we establish a deep learning model between the resistivity of seafloor sediment and its density, water content, and porosity. Compared with empirical formulas, the deep learning model has the advantages of a more concentrated prediction range and a higher prediction accuracy. This algorithm was applied to invert the spatial distribution characteristics and temporal variation of the seafloor sediment density, water content, and porosity in the South China Sea hydrate test area for 12 days. The study reveals that the dynamic changes in the physical properties of seafloor sediments in the South China Sea hydrate zone exhibit obvious stratification characteristics. The dynamic changes in the physical properties of seafloor sediments are mainly observed at depths of 0–0.9 m below the seafloor, and the sediment properties remain stable at depths of 0.9–1.8 m below the seafloor. This study achieves the monitoring and early warning of dynamic changes in the physical properties of seafloor sediments and provides a guarantee for the safe construction of marine engineering.

Sediment dynamics describe the processes of the formation, distribution, and movement of sediments [...]

Nepheloid layers are widely distributed in the marine environment, and their formation and evolution pose many challenges to the current understanding of ocean dynamics and marine sedimentology. In sediment transport processes, nepheloid layers significantly contribute to the exchange of sediment between the continental shelf and the slope. In this paper, we summarize the global research trends on nepheloid layers. In total, 689 publications from 1990 to 2022 were collected from the Web of Science and analyzed using bibliographic software, including Bibliometrix, VOSviewer, CiteSpace, and CorText. Based on these publications, past and present popular research on nepheloid layers is examined and evaluated. The trends in nepheloid layer research are summarized by analyzing keywords, article references, countries, institutions, and authors. Finally, prospects and several key questions related to nepheloid layers are concluded, which can potentially guide future studies. The bibliographic analysis can provide new insights into the history of nepheloid layers. The results also provide valuable information for other researchers and programs investigating geological, geophysical, and biogeochemical processes.

In situ observations of suspended sediment concentrations (SSCs), bed elevations, and concurrent hydrodynamics were conducted with an instrumented tripod on an abandoned lobe of the Yellow River Delta in the winter of 2014–2015. Four typical winter storms were recorded, but no significant local seabed erosion was observed at the observation site. Nevertheless, the sediment dynamic processes in this area were inferred from the data. The bottom sediment is eroded by the waves induced by strong northerly winds, and the erosion source area is located northwest of the observation point. Therefore, during strong wind and wave conditions, the observation point forms a horizontal SSC gradient, which generally decreases from west of to east of the observation point. The gradient mainly passes the observation site through diurnal tidal horizontal advection and is intermittently influenced by advection by the residual current driven by sea‒land wind and semidiurnal tides. Additionally, local resuspension of a “fluffy layer” on the seabed by the semidiurnal tide occurs. Flood tides transport the SSC gradient southeasterly, while ebb tides push the gradient back periodically. An asymmetry in the suspended sediment transport rate exists and is estimated to be 1.77 kg m−2 s−1 to the southeast. A new parameter, the net loss rate of suspended sediment, is defined and is estimated to be 0.0109 g L−1 h−1. These findings provide a better understanding of the erosion mechanism of the Diaokou lobe and some new ideas/methods for analysing hydrodynamics and sediment dynamics with a single-point measured dataset.

Strong nonlinear internal waves generate a significant pressure force on the seafloor and induce a pore-pressure response penetrated in the seabed and are thus an important driver of sediment resuspension and a potential trigger of seabed failure. The following provides an overview of the seabed responses induced by nonlinear internal waves and the theory, models, and limited observations that have provided our present knowledge. The pressure disturbance is generated by the combined effect of interface displacement and near-bottom acceleration by the nonlinear internal waves. Recent observations in the South China Sea have shown that the pressure magnitudes up to 4 kPa, which is the largest known disturbance. Intense pore-pressure changes in roughly the top 1 m of the weakly conductive seabed are expected during the shoaling and breaking of the nonlinear internal waves and lead to 2 cm sediments of the local seabed appearing in transient liquefaction. Since the fluid seepage reduces the specific weight of the bed, results show that the contribution of vertical seepage on sediment resuspension is estimated at 11% for a seabed saturation of 0.97. Finally, in situ observations are needed to confirm theoretical knowledge and to help improve our ability to model the multiscale interaction process between the seabed and internal waves in the future.

The estuary delta is an area where human economic activities are active and natural ecological environment is fragile. With global change and the intensification of human activities, coastal and seabed erosion around the world is becoming more and more serious. In this paper, we used the Delft 3D numerical simulation to compare the hydrodynamic effects of sediment transport paths in the Yellow River delta (river-controlled type), Yangtze River delta (tidal type) and Mekong River delta (tidal wave type) in the East Asian monsoon area, and analyzed the causes of accumulation erosion landform distribution in three different types of subaqueous deltas. This study finds the Yellow River Delta has experienced varying degrees of erosion at the estuary, but its subaqueous delta is still dominated by deposition; the Yangtze River Delta has ensured the stability of its shoreline under the influence of artificial shoreline reinforcement, but the subaqueous delta (water depth: 0–15 m) is in a state of erosion all year round; and in the Mekong River Delta the erosion occurs in both its shoreline and subaqueous delta. Additionally, only by analyzing the erosion and deposition within the transport range of resuspended sediment, the changes in the properties of the entire subaqueous delta could be recognized. The research results can not only be helpful to analyze whether the change of river sediment will lead to the change of delta type under human influence, but also provide more powerful scientific support for the protection of delta ecological environment, geological environment safety and geological disaster prevention.

In response to the accelerating processes of marine natural gas hydrate exploration and gas production from hydrate-bearing sediments, their potential impacts on the environment have been attracting extensive attention from international academia as well as from industry, especially in recent years. Methane seeps related to gas hydrate degradation on the seafloor are ubiquitous on continental slopes in both active and passive continental margins. A previous series of articles suggest that hydrate dissociation (gas release or ebullition at large scale) mainly occurs as a result of faulting or decreased lithostatic pressure triggered by many external driving forces (mainly including tectonic activities, overpressure zone, earthquake and tides), and temperature change. These frequently affect methane inputs into the atmosphere, fueling seafloor oxygen consumption or ocean acidification, and even posing submarine geohazards. Gas hydrate-related areas are usually estimated from indirect biochemical indications (cold-seep communities, element and isotopic anomalies in pore water) and geophysical indications (surface morphology, gas plumes and pathways). Therefore, appropriate application of monitoring and detection methods is of crucial significance for assessing the temporal and spatial variability of related environmental indicators in gas hydrate reservoirs. Monitoring is also as an essential support for harvesting this potential alternative energy in ways that are safer, more economical, and more environmentally friendly. Following the three structural elements of a hydrocarbon seep pumping system (gas/fluid source, fluid migration pathway, and seeping structures at or near the seafloor), this paper sets forth the significance of indicators for dynamics resulting from the destabilization of reservoirs of natural gas hydrates, reviews the corresponding monitoring or detection methods and integrated monitoring systems; and especially, expatiates the in situ observation networks regarding gas hydrate reservoirs and gas hydrate production tests. In the closing section, we discuss and draw conclusions regarding the challenges for future development of hydrate environmental monitoring and significant environmental issues requiring special attention. We intend this paper to provide references and to set the scene for future research activities about gas hydrates, so as to ignite the interest of more research groups around the world.

The technology for in situ testing of the undrained shear strength of deep-sea clay is underdeveloped. Indoor tests remain necessary, and there is a large temperature difference between in situ and laboratory tests. To analyse the effect of temperature on undrained shear strength, in this study the physical characteristics of marine clay samples from the South China Sea were determined, followed by penetration tests by the mini-ball method under low (4 °C) and room (20 °C) temperatures. The results indicated that the clay strength increased by 14.1–30.0% as the temperature decreased from 20 °C to 4 °C, and the strength of the bound water and the viscosity of the free water in the clay sample increased as the temperature decreased, which was the root cause of the increase in the clay strength. Based on the research, it is possible to correct the undrained shear strength values measured in laboratory tests and provide more reasonable parameters for ocean engineering.

The submarine canyons are an important clue to study the evolution process of seafloor geomorphology and they generally indicate the significant linear grooves on the seafloor related to seafloor geodynamic erosion during the evolution of geomorphology. The submarine canyons or canyon groups are not only the channels for the sediment transport from shallow sediments with land-based sources to the deep sea in the sediment source-sink system, but also a key temporary sediment deposition area to study sediment transport patterns and the evolution of submarine geomorphology. In this paper, we processed and analyzed the multibeam bathymetry data acquired in the South China Sea continental margin by the research vessel “Dongfanghong 3” in 2020. Based on fine submarine geomorphological features identified from multibeam bathymetry data, we construct the formation pattern of the cyclic steps. The six cyclic steps (wavelengths of 1–6 km and wave heights of 19–81 m) are found in the lower section of the Penghu canyon and they appeared at the conjunction part of the Penghu and the Taiwan canyon. Based on location and the wavelength variations of the cyclic steps, we propose that the cyclic steps are formed by turbidity current flow along the Penghu and the Taiwan canyons. The axis of the cyclic step CS4–CS6 is shifted westward by about 5° compared to the axis of the cyclic step CS1–CS3. The inconsistency in the axis direction of the cyclic steps CS1–CS3 and CS4–CS6 suggests that is where the migration of the Penghu canyon occurred.

The silty seabed in the Yellow River Delta (YRD) is exposed to deposition, liquefaction, and reconsolidation repeatedly, during which seepage flows are crucial to the seabed strength. In extreme cases, seepage flows could cause seepage failure (SF) in the seabed, endangering the offshore structures. A critical condition exists for the occurrence of SF, i.e., the critical hydraulic gradient (icr). Compared with cohesionless sands, the icr of cohesive sediments is more complex, and no universal evaluation theory is available yet. The present work first improved a self-designed annular flume to avoid SF along the sidewall, then simulated the SF process of the seabed with different consolidation times in order to explore the icr of newly deposited silty seabed in the YRD. It is found that the theoretical formula for icr of cohesionless soil grossly underestimated the icr of cohesive soil. The icr range of silty seabed in the YRD was 8–16, which was significantly affected by the cohesion and was inversely proportional to the seabed fluidization degree. SF could “pump” the sediments vertically from the interior of the seabed with a contribution to sediment resuspension of up to 93.2–96.8%. The higher the consolidation degree, the smaller the contribution will be.

With the increase in demand for metal resources, research on deep-sea polymetallic nodule mining has been reinvigorated, but the problem of its environmental impact cannot be ignored. No matter what method is used for mining, it will disturb the surface sediments of the seabed, thereby increasing the concentration of suspended solid particles and metal ions in the water body, changing the properties of the near-bottom water body and sediments, and affecting biological activity and the living environment. Focusing on the ecological and environmental impacts of deep-sea polymetallic nodule mining, taking as our main subject of focus the dynamic changes in sediments, we investigated the environmental impacts of nodule mining and their relationships with each other. On this basis, certain understandings are summarized relating to the ecological and environmental impacts of deep-sea polymetallic nodule mining, based on changes in the engineering geological properties of sediment, and solutions for current research problems are proposed.

Hydrate formation is an inhomogeneous process, which results in permeability anisotropy, i.e., permeability differs in different directions. Permeability anisotropy of hydrate-bearing sands is analyzed based on CT (computed tomography) scanning and pore network modeling in this study. A digital sample is divided into eight different parts based on hydrate distribution. Eight equivalent micropore structures are extracted by the maximum ball method. The absolute and relative permeabilities of each part in three different directions (x, y, and z) are calculated by a permeability simulator. Before hydrate formation, the sand pack permeability in the different directions is nearly the same. After hydrate formation, the sample pore structures become more complex. Both the absolute and relative permeabilities vary in the different directions. When hydrate saturation exceeds a threshold, the degree of permeability anisotropy is weakened. Hydrate formation induces a more significant relative gas permeability anisotropy compared to relative water permeability anisotropy. The degree of relative water permeability anisotropy increases at high water saturations.

Permeability and porosity are two crucial properties related to underground gas storage. This research investigates the permeability and porosity evolution of a Vosges sandstone under different str...