Moho fold structures provide critical insights into the tectonic evolution of the East China Sea. However, previous models exhibit substantial uncertainties, primarily resulting from the unaccounted gravitational effects of crustal sources and insufficient constraints on inversion parameters. In this study, we applied wavelet multi-scale analysis and the power spectrum method to remove crustal contributions, combined with an improved Bott’s method to achieve robust hyperparameter estimations. The Moho topographic model obtained through this method exhibits a significantly enhanced accuracy, with a root mean square deviation from seismic control points reduced by approximately 30% compared to other models. The resulting Moho fold structure reveals three key findings: (1) The South China Block has undergone vertical stress that forced the mantle to subduct. (2) In the northeastern and central parts of the Ryukyu Arc, vertical subduction forces are dominant. In the southwestern part of the Ryukyu Arc, vertical subduction forces are in balance with another force associated with mantle upwelling. (3) There is no interplate stress beneath the Okinawa Trough, and its crustal thinning may have been influenced by upwelling in the mantle.

A distributed cavity-enhanced Raman spectroscopy system was developed for the simultaneous detection of H2, O2, N2, and CO2 in confined spaces and complex gas environments. The system adopts an external-host/distributed-detection-cavity configuration, in which laser excitation, cavity-enhanced detection, Raman signal collection, and spectral detection are functionally separated to improve deployment flexibility for remote in situ measurements. Multi-peak fitting was used to extract the spectral band areas of different gas components, and band-area normalization was introduced to reduce the influence of laser power fluctuations, fiber coupling variations, and cavity coupling changes on concentration retrieval. The results show that H2, O2, N2, and CO2 exhibit clearly distinguishable Raman peaks and good linear concentration responses. The fitting correlation coefficients for H2, O2, N2, and CO2 are 0.998, 0.997, 0.996, and 0.998, respectively, with RMSE values of 0.03%, 0.21%, 0.35%, and 0.06%. After normalization, the average relative errors are reduced to 1.6%, 1.5%, 1.4%, and 1.3%, while the maximum relative errors are reduced to 3.2%, 3.1%, 2.9%, and 2.7%, respectively. Continuous measurements yield RSD values of 1.5%, 0.90%, 0.56%, and 1.25%, demonstrating good simultaneous detection capability and quantitative stability. The proposed system provides a feasible approach for online multicomponent gas monitoring in confined and complex environments.

Accurate characterization of seafloor sediment properties is critical for marine engineering design, resource assessment, and environmental management. Sidescan sonar offers efficient wide-area mapping capabilities, yet establishing robust quantitative relationships between acoustic backscatter intensity and sediment texture remains challenging, particularly in heterogeneous coastal environments. This study investigates the correlation between sidescan sonar backscatter intensity and sediment grain size parameters in waters southwest of Hainan Island, China. High-resolution acoustic data (450 kHz) were acquired alongside surface sediment samples from 18 stations spanning diverse sediment types. Backscatter intensity, represented by grayscale values, was systematically compared with grain size distributions and individual size fractions. Results reveal that mean grain size shows no meaningful correlation with backscatter intensity; however, fine sand fraction content (0.075–0.25 mm) exhibits a strong negative linear relationship (R2 = 0.87 under optimal conditions). Distribution-level analysis demonstrates that backscatter variability mirrors sediment textural complexity, with coarse sediments producing broad, elevated intensity distributions and fine sediments yielding narrow, suppressed distributions. Inter-survey variability highlights the sensitivity of absolute intensity values to environmental conditions during acquisition. Spatial distribution analysis reveals that sediment grain size follows a systematic NE-SW gradient controlled by hydrodynamic energy, with notable local anomalies controlled by reef structures (producing coarse bioclastic sediment) and topographic sheltering (maintaining fine-grained deposits in shallow areas). These findings provide a quantitative basis for fraction-specific acoustic classification approaches while emphasizing the importance of multi-scale analysis incorporating both regional hydrodynamic trends and local morphological controls. The established relationship between fine sand abundance and acoustic response enables semi-quantitative sediment prediction from remotely sensed data, supporting improved seafloor mapping protocols for offshore infrastructure siting, aggregate resource evaluation, and coastal zone management in morphologically complex environments.

Taiwan is located at the junction between the Philippine Sea Plate and the Eurasian Plate, with intense tectonic movements in the region. A more accurate Moho topography model is significant for the study of the intrinsic genesis within Taiwan’s orogenic movement and the subduction pattern of the Philippine Sea plate. Previously, there were two key issues in Moho topographic inversion that needed to be solved, i.e., extracting signals originating from Moho topography and estimating more accurate inversion parameters. To solve these two issues, we use wavelet multi-scale analysis to separate the gravity signals at different depths and extract the signals originating from Moho topography from them. Then, more accurate inversion parameters are estimated using simulated annealing with available seismic data as constraints. On this basis, a more refined Moho topography of Taiwan is inverted. The Moho topography shows that most of Taiwan has a Moho topography of about 28 km, with the highest point slightly above 30 km and the lowest point between 20 and 25 km. Based on the Moho topography contours, we estimate that subduction of the Philippine Sea plate ends at the 25 km contour, in the middle of the Coastal Range, near 23.5°N. This is one of the possible reasons for the high frequency of earthquakes in the region. Comparing with the existing seismic control points, the corresponding root-mean-square error of our Moho model is 4.98 km smaller than the CRUST 1.0 model, which indicates that our Moho model is more accurate and realistic.

This study presents the first application of a deep-towed transmitter–receiver marine controlled-source electromagnetic (TTR-MCSEM) system for gas hydrate exploration in the Shenhu area of the South China Sea. High-resolution electromagnetic data were acquired along a 13 km transect using dynamic source–receiver offsets and a 500 A transmitter. The results reveal the following: (1) unprecedented near-seafloor resolution (20~100 m) for the precise delineation of hydrate-bearing caprock, surpassing conventional ocean-bottom electromagnetic systems; (2) laterally continuous high-resistivity anomalies (~10 Ω·m) extending from the base of the gas hydrate stability zone to the seafloor, which correlate with seismic bottom-simulating reflector (BSR) distributions and suggest heterogeneous hydrate saturation; and (3) fault-controlled fluid migration pathways that supply hydrate reservoirs and lead to seabed methane seepage at structural highs. Through 2D inversion, we show that the inverted resistivity values (~10 Ω·m) are slightly higher than those obtained from resistivity logs (~5 Ω·m). Saturation values derived from inverted resistivity exhibit remarkable consistency with well-log-based measurements. The high efficiency of the system confirms its potential for the transformative quantitative assessment of hydrate systems, seafloor massive sulfides, and marine geohazards.

As crucial transportation hubs and economic nodes, the underwater security and infrastructure maintenance of harbors are of paramount importance. Harbors are characterized by high vessel traffic and complex underwater environments, where traditional underwater inspection methods, such as diver operations, face challenges of low efficiency, high risk, and limited operational range. This paper introduces a collaborative survey and disposal system that integrates a deformable unmanned surface vehicle (USV) with a lightweight remotely operated vehicle (ROV). The USV is equipped with a side-scan sonar (SSS) and a multibeam echo sounder (MBES), enabling rapid, large-area searches and seabed topographic mapping. The ROV, equipped with an optical camera system, forward-looking sonar (FLS), and a manipulator, is tasked with conducting close-range, detailed observations to confirm and dispose of abnormal objects identified by the USV. Field trials were conducted at an island harbor in the South China Sea, where simulated underwater objects, including an iron drum, a plastic drum, and a rubber tire, were deployed. The results demonstrate that the USV-ROV collaborative system effectively meets the demands for underwater environmental measurement, object localization, identification, and disposal in complex harbor environments. The USV acquired high-resolution (0.5 m × 0.5 m) three-dimensional topographic data of the harbor, effectively revealing its topographical features. The SSS accurately localized and preliminarily identified all deployed simulated objects, revealing their acoustic characteristics. Repeated surveys revealed a maximum positioning deviation of 2.2 m. The lightweight ROV confirmed the status and location of the simulated objects using an optical camera and an underwater positioning system, with a maximum deviation of 3.2 m when compared to the SSS locations. The study highlights the limitations of using either vehicle alone. The USV survey could not precisely confirm the attributes of the objects, whereas a full-area search of 0.36 km2 by the ROV alone would take approximately 20 h. In contrast, the USV-ROV collaborative model reduced the total time to detect all objects to 9 h, improving efficiency by 55%. This research offers an efficient, reliable, and economical practical solution for applications such as underwater security, topographic mapping, infrastructure inspection, and channel dredging in harbor environments.

A methane (CH4) sensor based on off-axis integrated cavity output spectroscopy (OA-ICOS) was developed, equipped with two measurement schemes: direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). The sensor used an optical resonant cavity composed of two high reflection mirrors (reflectivity > 99%). With a cavity length of 7 cm, an effective optical path length of 10.8 m and a cavity volume of 8.9 mL were achieved. A distributed feedback laser was used to precisely target the CH4 absorption line near 1.6537 µm. Compared with the original system, the cavity mode noise of the CH4 sensor was further reduced by adding white noise perturbations. The white noise perturbations were generated by the broadband random noise from the signal generator. The special customized narrowband RF noise source was not required. The system complexity and cost could be reduced. In DAS mode, the signal-to-noise ratio (SNR) of the OA-ICOS was 16.2 and the minimum detection limit (MDL) was 2.2 ppm at 117 s. In WMS mode, the SNR of the OA-ICOS was 113.9 and the MDL was 1.2 ppm at 106 s. Compared with the results obtained from the WMS mode and DAS mode, the SNR and MDL was improved 7.0 times and 1.8 times, respectively. The proposed sensor system not only enabled high-accuracy trace gas measurement, but also demonstrated strong potential for applications due to its compact design and low cost.

As one of the representative multi-ring basins on the Moon, the internal structure of Mare Orientale is crucial for exploring the formation mechanism of multi-ring basins. Due to the sparsity of moonquake data, at present, only gravity data can be more effectively applied to study the internal structure of Mare Orientale. Therefore, based on the GRGM1200A model from Gravity Recovery and Interior Laboratory, we inverted the internal density structure of Mare Orientale from the perspective of signal separation. In addition, based on the gravity anomalies after deducting the influence of the crust, we calculated the depth of its crust-mantle interface. The results of the study show that there is a wide-ranging, high-density body beneath Mare Orientale. Its morphology is similar to that of a prism, with a depth ranging from 32.4 to 64.0 km. The upper tip is located roughly 98°W longitude and 18°S latitude, and the lower tip roughly 94°W longitude and 22°S latitude. Within the internal depression, the crust-mantle interface in Mare Orientale is clearly uplifted with a shallowest depth of 1.47 km. In the basin centre, there is an uplifted rebound of the crust-mantle interface. Based on our results, we suggested that the multi-ring structure of Mare Orientale was formed primarily by a large, tilted meteorite impact, which left meteorite residues in the upper mantle of Mare Orientale and caused a dramatic uplift of the crust-mantle interface within the basin.

Reservoir stimulation technology is important for improving the gas production efficiency of natural gas hydrate exploitation. Based on the data of the first natural gas hydrate field test in the Shenhu Sea area, the gas production capability of Class 1-type hydrate reservoirs was numerically evaluated by horizontal well depressurization with high-pressure rotating water jets (HPRWJ). The results showed that the HPRWJ can effectively improve the production efficiency. The mechanism is enlarging the drainage area and improving the seepage conditions of the reservoir. With the wellbore deployed at the middle part of the three-phase layer, compared with the unstimulation case, the case with HPRWJ results in the V g increased by 140.65% after 1080 days of production with a length of 300 m and the stimulation parameters: Q (quantity), R (radius), S (spacing), W (width), K (permeability), and P (porosity) were set to 25, 5.5 m, 11.5 m, 0.5 m, 5 D, and 0.7, respectively. With comprehensive parameter analysis, the results show that the stimulation zone quantity and radius are the main control parameters for improving production efficiency, and the permeability and porosity had less effect. It is suggested to use low density with a large radius when adopting HPRWJ, which can save construction time and costs.

Multicluster wells are critical for the commercialization of natural gas hydrate (NGH). Based on the on-site data of the first natural gas hydrate trial production in the South China Sea, a numerical model was constructed. The key parameters, including well spacing and well-net patterns, were investigated. Under the preconditions of a fixed well-net pattern of four wells and continuous production for 1080 days, the results show that as well spacing increases from 20 to 60, 100, 140, and 180 m, respectively, productivity increases to 136.3, 169.7, 190.3, and 198.4% with a pressure difference of 2 MPa and 128.4, 158.6, 185.2, and 198.6% with a pressure difference of 6 MPa. The rate of production rise does not increase linearly, and the free gas tends to be trapped in the middle of the multicluster well, causing the "blind zone" phenomenon with increasing well spacing. Under the preconditions of fixed well spacing (60m) and pressure difference (6 MPa), with the number of wellbores increased from 2 to 9 in the well-net patterns, the productivity increases to 124.2, 131.8, 148.2, 158.1, 177.3, 201.9, 208.3, 244.4, and 250.3%, respectively, and the productivity increase is synchronized but not linear, and the increase ratio becomes smaller. Under the CVW15 well-net patterns, when the well spacing is set to 180m, it is estimated that 18 groups of well net can achieve commercial production, which provides a reference for well-net pattern selection and parameter setting for NGH production.

Enhancing the production capacity of natural gas hydrates (NGHs) is critical for its commercial development. Complex structure wells may efficiently increase drainage areas while enhancing exploitation efficiency. Based on the field data of China’s first offshore NGH test production, the numerical method was used to analyze the production performance of different complex structure well types by continuous depressurization production for 360 days under the preconditions of fixed effective completion length of 300 m and a pressure difference of 6 MPa. Results indicated that the complex structure well types deployed at the three-phase layer demonstrated superior production performance within 240 days of production; the DLW2 and HW2 well types stood out, with an average gas production rate Qg reaching 43,333 m3/d and a specific production index J of 24.1. After 360 days of production, benefiting from multi-layer combined production, the Cluster vertical well deployed at the multi-layer had the best production performance, with an average Qg of 34,444 m3/d and a J-index of 19.1. The research results provided insights into the complex structure well-type selection strategy for NGH depressurization in this sea area.

Stepwise depressurization is an important depressurization strategy in the development of natural gas hydrates. This work numerically analyzes the effects of different depressurization gradients and constant pressure durations on gas and water production during stepwise depressurization extraction with a vertical well in the Shenhu Sea area hydrate reservoir of the South China Sea. The results indicate that stepwise depressurization can reduce water production and raise the gas-to-water ratio in the early stages of production while ensuring cumulative gas output. When the vertical well is deployed at the model’s center with a completion length of 70 m and a constant pressure duration of 10 days, a depressurization gradient of 0.5 MPa, stepwise depressurization by 6 MPa, and continuous production for one year is achieved. Compared with direct depressurization, its cumulative gas production is 2.966 × 106 ST m3, which only decreases by 2.94%. However, it maintains a higher gas-to-water ratio in the early stages of production. Considering factors such as engineering operability, cumulative gas output, and gas-to-water ratio, it is recommended to use a small pressure gradient and a medium constant pressure stabilization time for stepwise depressurization Stepwise depressurization can maintain a high gas-to-water ratio while ensuring gas production and reducing water production can alleviate sand production problems and improve economic efficiency. The understanding gained from this work has reference value for the development of similar hydrate reservoirs worldwide.

Improving the production capacity of natural gas hydrates (NGHs) is crucial for their commercial development. Based on the data of the first on-site testing production of NGHs in the Shenhu Sea area, numerical methods were used to analyze the production behavior of radial lateral well (RLW) and horizontal snake well (HSW) with different completion lengths when they deployed at different layers of the Class-1 type hydrate reservoir (with a fixed pressure difference of 6 MPa and continuous production for 360 days). The results indicate that compared with the single vertical well production, RLW and HSW can effectively increase production capacity by enlarging drainage area and the productivity is directly proportional to the total completion length. The RLW and HSW deployed at the three-phase layer (TPL) have optimal mining performance within a 360-day production period. Different to the previous research findings, during a short-term production period of 360 days, regardless of the deployment layer, the overall production capacity of HSW is better than RLW’s. The total gas production of HSW-2 circles well type is about four times that of a single vertical well, reaching 1.554 × 107 ST m3. Moreover, the HSW-1 lateral well type stands out with an average Qg of 3.63 × 104 ST m3/d and a specific production index J of 16.93; it has the highest J-index among all well types, which means the best mining efficiency. It is recommended to choose the HSW-1 circle well type, if the coiled tubing drilling technique is used for on-site testing production of NGHs in the future. The research results provide insights into the potential applications of RLW and HSW in this sea area.

In situ electric heating is an important method used to increase production capacity during the extraction of natural gas hydrates. This work numerically evaluated the sensitivity of different heating parameters on gas production behavior with a vertical well depressurization in the Shenhu Sea area hydrate reservoir, the production pressure difference of 4 MPa, and continuous depressurization for 1080 days. The results showed that the in situ electric heating method can effectively enhance production capability by promoting hydrate dissociation and eliminating secondary hydrates. Compared with scenarios without heating, implementing whole wellbore heating (100 W/m) increases cumulative gas production (Vg) by 118.56%. When intermittent heating is applied to the local wellbore (15 m) located in the three-phase layer (with an interval of 30 days) and stops heating in advance at 480 days, there is no significant difference in Vg compared to the whole wellbore heating case, and the cumulative heat input is only 4.76%. We recommend considering intermittent heating of the local wellbore and stopping heating in advance during vertical well depressurization as this approach significantly reduces heating energy consumption while simultaneously improving production capacity.

Submarine sinkholes are unique and important geomorphological features with a typical cavity structure that are of great scientific value. Submarine sinkholes were discovered for the first time in the isolated Ganquan carbonate platform on the Xisha Islands, the northwestern South China Sea. Based on high-resolution multibeam bathymetric data and seismic profile data, we identified 37 submarine sinkholes at water depths ranging from 550 to 1267 m. They are subcircular to circular negative-relief features, and most of them are V- or compound V-shaped in the cross-section. Their average diameters range from 57 to 667 m, and the depth of the depression ranges from 2.5 to 241 m. By comparing submarine sinkholes in the Ganquan platform with those in other carbonate platforms worldwide, we can infer that the Ganquan platform submarine sinkholes are the largest sinkholes developed on an isolated carbonate platform. Remotely operated vehicle (ROV) “Haima 2” images revealed that the inner walls of submarine sinkholes are characterized by stalactite-like structures, possible dikes, flow marks, and corroded holes, which are typical karstic landscape features. The temperature within submarine sinkholes is 2 °C higher than that of the open ocean at the same water depth. Based on the results of the shallow formation profile and multichannel seismic profiles, we propose that the submarine sinkholes in the Ganquan platform probably formed via the dissolution of the carbonate platform via acidic hydrothermal fluids that originated from magmatic activity and migrated along faults.

An adaptive backstepping dynamic surface sliding mode controller based on nonlinear disturbance observer (NDOABSMC) is designed to track zigzag motion of underwater glider (UG). Firstly, a nonlinear disturbance observer is devised to observe ocean current. Then, the backstepping sliding mode control is used to devise motion attitude controller of UG to ensure that UG can track the target trajectory quickly. Additionally, the stability of UG's control system is analyzed. In the end, the proposed control strategy is compared with other approaches.

This work presents a case study about applying a homemade boomer source working with a well-designed field geometry in an offshore seismic survey. Some measurements were carried out to calibrate the boomer source acoustic characteristics in an anechoic tank. Then a catamaran, taking account of the boomer source signature, is designed to control the source depth and entirely use ghosts as part of the source wavelet. This way, the vertical resolution is sacrificed to signal-to-noise ratio (SNR), and the source wavelet meets the minimum phase characteristics. Furthermore, the detailed analytical results of the calibrated source wavelets also provide a reliable basis for determining post-processing parameters. Two surveys have been conducted on the same line using this source in combination with two different hydrophone streamers. Some valuable experiences in field data acquisition and post-processing are analyzed and summarized. The most important one is taking the ghost part of the source wavelet by controlling source depth. This work is a reliable reference to other research on improving the quality of seismic imaging of offshore shallow strata.

The Kuiyang-ST2000 deep-towed high-resolution multichannel seismic system was designed by the First Institute of Oceanography, Ministry of Natural Resources (FIO, MNR). The system is mainly composed of a plasma spark source (source level: 216 dB, main frequency: 750 Hz, frequency bandwidth: 150–1 200 Hz) and a towed hydrophone streamer with 48 channels. Because the source and the towed hydrophone streamer are constantly moving according to the towing configuration, the accurate positioning of the towing hydrophone array and the moveout correction of deep-towed multichannel seismic data processing before imaging are challenging. Initially, according to the characteristics of the system and the towing streamer shape in deep water, travel-time positioning method was used to construct the hydrophone streamer shape, and the results were corrected by using the polynomial curve fitting method. Then, a new data-processing workflow for Kuiyang-ST2000 system data was introduced, mainly including float datum setting, residual static correction, phase-based moveout correction, which allows the imaging algorithms of conventional marine seismic data processing to extend to deep-towed seismic data. We successfully applied the Kuiyang-ST2000 system and methodology of data processing to a gas hydrate survey of the Qiongdongnan and Shenhu areas in the South China Sea, and the results show that the profile has very high vertical and lateral resolutions (0.5 m and 8 m, respectively), which can provide full and accurate details of gas hydrate-related and geohazard sedimentary and structural features in the South China Sea.

A compact and high-sensitivity sensing system is required for the exploration of natural gas hydrates (NGHs) to measure dissolved CO2 in seawater. In this study, a mid-infrared (MIR) CO2-sensing system was investigated by wavelength modulation spectroscopy (WMS). The system contained an interband cascade laser (ICL) operating in MIR wavelength, an optical multipass gas cell (MPGC), and an MIR mercury cadmium telluride detector. A combination of three absorption lines was utilized to achieve measurement with a wide CO2-concentration range. Additionally, we proposed and introduced in detail a method of optimizing the operating temperature, pressure, and modulation depth. The optimum minimum detection limit (MDL) of 11 ppbv was achieved when the integration time was 145 s. In the northern South China Sea, a deep-sea test was performed, and the test results validated the satisfactory performance of the sensing system for deep-sea NGH exploration.

The concentration of dissolved carbon dioxide in seawater affects the formation of seabed biogenic minerals, and controls the development and evolution of marine organisms, which is one of the most important indices in the marine environment and geology survey. The concentration of free carbon dioxide in bottom seawater has important academic value for the exploration of seabed minerals, marine environment monitoring, the study of the carbon cycle in seabed sediments and even the global carbon cycle. Titration is used to determine the index, whose range is between 4 mg/L and 400 mg/L. However, the concentration of free carbon dioxide in bottom seawater in some sea areas is lower than this range, thus titration may not be suitable. A method for the determination of free carbon dioxide in artificial seawater, whose name is D.B.S/Henry’s Law, is described in this paper. D.B.S/Henry’s Law indirectly determines the content of free carbon dioxide in seawater instead of directly titrating with reagents. It is simple, fast and accurate, and it is based on the law of conservation of mass/Henry’s law and CO2 dissolution mechanisms. This discovery mainly provides a new scientific perspective for the determination of trace free carbon dioxide concentration in seawater. In addition, Henry’s law constants of carbon dioxide in artificial seawater at 3–20 degrees Celsius are determined with D.B.S/Henry’s Law. These data will be used as an important reference for the determination of free carbon dioxide in seawater in the future. D.B.S/Henry’s Law may be used as a supplement of the methods specified in national and industrial standards, which are applied to the detection of carbon dioxide Henry’s constant and free carbon dioxide content in various temperature and salinity seawater. D.B.S/Henry’s Law provides a new perspective for CO2 determination and has extensive practical application value.