Wenbin Jiang

The mechanisms of complex Moho reflections at fast‐ and intermediate‐spreading ridges remain unclear. A potential key factor is hydrothermal activity, which at mid‐ocean ridges may result in serpentinization of peridotites under the stability field of antigorite (400–600°C) or chrysotile‐lizardite (<400°C). Here, we calibrated the relationship between serpentinization degree, density, and seismic velocities of peridotites at 200 MPa. Based on physical properties of rocks in the oceanic lithosphere, we established seismic velocity and density models of a sharp or layered crust‐mantle boundary with different serpentinization types. Synthetic seismograms demonstrate that the oceanic Moho reflections are controlled by both structure and serpentinization of the crust‐mantle boundary. The impulsive Moho may correspond to a sharp mafic‐ultramafic boundary (petrological Moho) with <10% serpentinization, or the bottom of 60%–80% antigorite peridotites. The Moho reflection becomes invisible when antigorite percentage decreases from 60%–80% (top) to 0% (bottom) in the uppermost mantle for a sharp mafic‐ultramafic boundary. Besides the layered crust‐mantle boundary, the diffusive Moho could also result from the sharp mafic‐ultramafic boundary with 10%–60% or 80%–100% antigorite peridotites, or chrysotile‐lizardite peridotites overlying antigorite peridotites. Using synthetic seismograms, the diffusive Moho at 6–9 km to the west of the East Pacific Rise 9°44′N can be interpreted by 30% antigorite peridotites, suggesting localized hydrothermal circulation close to the fast‐spreading mid‐ocean ridges. Moho reflections and wide‐angle PmP waveforms at mid‐ocean ridges can provide new clues to decipher the interplay between magmatic, tectonic, and hydrothermal processes in the oceanic lithosphere.

Seismic velocity inversion of shallow structures in the deep sea is challenging because thick water columns often limit the applicability of conventional inversion techniques. For example, dispersion curves of guided P waves are commonly used for estimating subsurface P-wave velocities in shallow-water environments, where distinct guided P waves can be isolated in waveguides using towed-streamer or ocean-bottom seismometer data. However, these waves are difficult to use in deepwater environments due to interference from subseafloor reflections. In this work, we report the first observation of guided P waves in deepwater environments using towed-streamer seismic data and successfully extract multimode dispersion curves. Synthetic modeling demonstrates that the dispersive modes of deepwater guided P waves are controlled by layered medium beneath the seafloor. The method is further applied to estimate the shallow velocity structures using towed-streamer seismic data from the South China Sea. We theoretically analyze the sensitivity kernel of the extracted dispersion curves to evaluate the detection depth of deepwater guided P waves. The analysis demonstrates that guided P waves can provide a deeper subseafloor imaging capability compared with ray-based refraction methods, particularly when the cable length is limited. In addition, we discuss the applicable water depth ranges and outline the limitations of our method. Analyzing the dispersive modes of guided P waves provides a tool for subsurface structural inversion in deepwater environments.

In seismic exploration, particularly within the domain of oil and gas reservoirs, the accurate imaging of complex fault blocks and the identification of structural traps are important. Geological risk factors, including the implementation of structural traps, reservoir delineation, and precise target drilling, require immediate attention in practical exploration. Addressing these factors involves two primary challenges: ensuring imaging accuracy and minimizing structural distortions. This study introduces a high‐resolution velocity modelling technique with fault control, specifically developed to mitigate misties between seismic image and well‐log data and improve the accuracy of seismic depth imaging and well depth correlation. The method offers a targeted solution to the challenges of implementing structural traps, delineating reservoirs and executing precise drilling operations. By incorporating fault control, it accounts for the structural complexity of subsurface media, enabling an accurate inversion of velocity variations across fault blocks. This approach ensures that velocity models, constrained by geological and structural models, exhibit a high degree of consistency. Utilizing fault‐controlled travel time inversion, the method resolves mistier between seismic imaging and well‐log data, guaranteeing the precision of velocity models and imaging. The methodology provides reliable seismic data for target evaluation, effectively reducing exploration risks and improving the accuracy of velocity modelling.

Amplitude versus offset attribute inversion primarily utilizes the change in amplitude with offset to extract lithologic information of the reservoir. We performed high resolution imaging using three‐dimensional seismic data and simulated four different models of gas hydrate and free gas to optimize the selection of sensitive attributes for gas hydrate characterization. We optimized the selection of sensitive attributes for gas hydrate characterization. Our research identified the gradient parameter G as a highly sensitive attribute for characterizing gas hydrate reservoirs. By comparing theoretical models with drilling site data, we predicted the saturation variations of gas hydrate based on the amplitude of G and summarized the amplitude versus offset characteristics and sensitive attributes of gas hydrate at different saturation levels. The results offer valuable insights for identification of gas hydrate in seismic data and provide a reference for their characterization.

Seismic velocity inversion of upper crustal structures in deep sea is challenging, as the unique condition of thick water column often limits the applicability of conventional inversion techniques. For example, while dispersion curves of guided P waves are commonly utilized for estimating subsurface P-wave velocity in land and shallow water environments, these waves have remained in obscurity in deep-water environments due to interference from subseafloor reflections, making them difficult to observe and, until now, unobserved. In this work, we report the first observation of guided P waves in deep water from multichannel seismic data and successfully extract multimode dispersion curves. The dispersive modes of deep water guided P waves are controlled by layered medium beneath the seafloor. Analyzing the dispersive modes of deep water guided P waves provides a tool for subsurface structural inversion in deep water environments.

Seismic technique is widely used to image the subsurface geology for oil and gas exploration. The image quality depends on the spatial sampling density. However, it is challenging and expensive to acquire high‐density seismic data, particularly in the marine environment. Distributed acoustic sensing data are increasingly used in data acquisition because of their low cost and dense spatial sampling. Here, we present a novel type of high‐density towed streamer based on distributed acoustic sensing technology and report the results of a sea trial. This sea trial was conducted in a gas hydrate province as the major driver to develop this technique is to better characterize gas hydrate deposits. Throughout the experiment, several high‐quality datasets were obtained, and parameters like source energies and filler materials were examined. The trace interval of distributed acoustic sensing streamer data reaches 1 m, which is a significant improvement over the usual 3.125 or 6.25 m in the conventional towed streamer. A detailed analysis was carried out from three different perspectives: amplitude, noise and frequency. One of the datasets was further processed following a routine workflow to obtain the final image. Though direct comparison with the image obtained by a conventional towed streamer along a coincident line is not available, the comparison with the previous image from a nearby line shows the improvement in resolution. The final image is of good quality and the presence of gas hydrate could be inferred. The sea trial results demonstrate the feasibility of the use of a distributed acoustic sensing optical fibre streamer in acquiring high‐density seismic data in the marine environment.

Seismic imaging of crustal structures becomes difficult in the presence of rough basements or complex seabed topography. Here, a 900 km deep seismic reflection profile collected across the southwest subbasin (SWSB) of the South China Sea is evaluated. By analyzing the types and distinctions of noise and effective signals, deep structure migration techniques are used to improve crustal structure imaging, wide-line processing is used to predict 3D space multiples, and f-k domain time-space variable adaptive deghosting and different offset stacking are used to enhance the deep events. The new imaged continental crustal structure in the Penxi Bank exhibits moderate thinning, down to 15 km, and is intersected by continental-ward low-angle detachment or normal faults. Within the limitations of the ocean bottom seismic P-wave velocity model, we find the subhorizontal lower crustal reflections (LCRs) that may be indicative of a weak lower crust. Two small-scale rollover structures along detachment faults rooted and rafted to the top of these weak lower crusts. Based on the narrow continent-ocean transitions (COTs), the presence of more continental-ward detachment faults, and the calculation of high lithosphere heat flow, we deduce that the mantle lithosphere breakup occurred earlier than the crust in the SWSB. Moreover, the COTs and oceanic crust domains demonstrate rough basements with numerous faults and approximately 20% diffuse or weak Moho reflections. From the southern COT to the initial oceanic domain, the thickness of the crust gradually reduces to only 3–5 km. This suggests a relatively low magmatic budget and protracted tectonic extension from the continental breakup to the onset of seafloor spreading. Within the oceanic crust domain, the crust thickness ranges from approximately 4 km to 6 km, indicating a thinner than normal crust. LCRs with a ridge-ward dipping pattern terminate at the Moho reflections and are partly connected to synspreading faults, hinting at their possible generation through synspreading faulting.

High‐resolution shallow oceanic crust velocity models provide crucial information on the tectonothermal history of the oceanic crust. The ocean bottom seismometers record wide‐angle seismic reflection and refraction data to image deeper structures compared with streamer data set. However, most ocean bottom seismometers experiments produce low‐resolution velocity models with limited shallow crustal structure due to sparse ocean bottom seismometers spacing. Multichannel seismic data recorded by towed streamers provide complementary seismic images of the oceanic crust but yield little information on subseafloor velocity because most subseafloor refractions are masked by seafloor reflections. Therefore, it is difficult to obtain fine‐scale velocity structure of shallow upper oceanic crust with both ocean bottom seismometers and multichannel seismic data. Downward continuation technique redatumed the shots and receivers to the seafloor to collapse the seafloor reflections to the zero offset and extract refractions as first arrivals from nearly zero offset, enabling dense ray coverage at the shallow crust. We applied the downward continuation and traveltime tomography methods to two synthetic models, Marmousi and SEAM Phase I Salt models, to demonstrate the performance of the strategy in the situations of flat seafloor and rough seafloor topography. We conducted the first‐arrival traveltime tomography on downward continued towed‐streamer multichannel seismic data across a slow‐spreading ridge of the South China Sea, providing unprecedented details of shallow velocity structure in the sediments. The low velocity sediments revealed by traveltime tomography match well with the prestack depth migration profile.

Urban seismology has recently emerged as a vibrant scientific field, driven by the growing interest in seismic signals generated by major public events, sports gatherings, and transportation services. However, deploying dense traditional seismometers in economically active, densely populated urban areas with heavy traffic poses significant challenges. In this study, we conducted a field experiment utilizing distributed acoustic sensing (DAS) technology during a fireworks display in Guangzhou on 5 February 2023. About 572 m of optical fiber was turned into 286 seismic sensors and deployed on LingShan Island to monitor various vibration signals generated during the fireworks show. Our analysis revealed substantial correlations between crowd motions during different phases of the event and ambient noise features recorded by DAS. Moreover, the cross-correlation functions of the ambient noise with its dispersion characteristics pointed to near-field pedestrian activity as the primary noise source. Real-time heat maps of human crowd motions were reconstructed from DAS recording, offering significant insights into the variations of activity intensity across different locations. Discerning fireworks events on the DAS array is more effective than on a scattered seismometer array, because it is easier to ensure that the same event is picked for all the sites in the DAS dense linear configuration. The DAS data inspection allowed us to pick up a total of 549 firecracker explosions in comparison to the seismometer data that only allowed us to detect 116 firecracker events. The heights of fireworks were located by the grid-search method and predominantly distributed at 75–300 m, closely aligning with actual fireworks explosion locations. Our findings underscore that the DAS technology can monitor crowd motion and detect vibration signals in the air, bridging the gap between fundamental earth science research and human social activities.

The crustal structures of the continent-ocean transition (COT) zone and oceanic domain are key to revealing the tectono-magmatic evolution from rift to drift and the following seafloor spreading. We develop a comprehensive study of the deep seismic reflection imaging and tomographic inversion of a wide-angle seismic line that runs across the COT and extinct spreading center of the Southwest Subbasin (SWSB) in the South China Sea. We reveal a low-velocity (<3 km/s) region in the shallow upper crust of the Longmen Seamount, which may represent serpentinized mud or volcanoclastic. The mature oceanic crust is approximately 4–6 km thick with high-velocity bodies (7.2–7.5 km/s) overlapping the Moho, reflecting relatively rich magmatic additions during seafloor spreading. The northern and southern COT segments exhibit a prominent long-wavelength high magnetic anomaly and synbreakup volcano, indicating magmatic additions in the lower crust and lava flows with magma ascending along faults to the surfaces. However, the COT in the south is wider than that in the north. In addition, the southern COT is characterized by limited PmP reflections, well-developed rotated fault blocks and ocean-ward synrifting faults, and a low-velocity (7.5–7.8 km/s) upper mantle, suggesting that the southern COT is probably underlain by a local serpentinized mantle during continental lithosphere breakup. The differences between the southern and northern COTs on the origin of high-velocity lower crustal layers and faulting styles imply asymmetric continental breakup processes in the SWSB.