Baojin Zhang

Deepwater basins have become major targets for commercial oil and gas exploration. However, as exploration advances into ultradeepwater basins, technical challenges become increasingly severe, as exemplified by the Liwan Sag in the northern South China Sea (SCS). The rugged seafloor terrain and great water depth degrade seismic imaging quality and increase uncertainties in drilling and geologic interpretation. Conventional towed-streamer reflection seismic data provide dense reflection information, but are limited by relatively narrow observation angles and limited offset coverage. By contrast, ocean-bottom seismometer (OBS) data provide first-arrival traveltimes at larger offsets, thereby complementing the limited offset coverage of streamer data. In this study, joint OBS and towed-streamer observations from the Liwan Sag were collated and preprocessed to obtain seismic data with an improved signal-to-noise ratio for velocity model building over the full depth range. An integrated workflow with three stages was established: seawater velocity model building, subseafloor background velocity model building, and subseafloor velocity refining. The resulting velocity models were evaluated using traveltime fitting, migration image quality, and common image gather (CIG) flattening, and were then used for imaging in the depth domain and geologic interpretation of the survey line. This case study provided a practical workflow that integrated seismic data preprocessing, full-depth velocity model building, and stratigraphic imaging for ultradeepwater seismic exploration.

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.

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.

The Zhongnan Fault Zone (ZFZ) is a large-scale tectonic belt in the South China Sea (SCS) oceanic basin and plays an important role in the formation and evolution of the basin. Despite numerous studies over the past few decades, debates persist regarding its location, orientation, nature, time of activity, and genesis. In this study, we evaluate our findings on the characteristics and origin of the fault zone through an integrated analysis of multibeam bathymetric and 2D multichannel seismic data. Our results reveal the ZFZ as a fault zone approximately 400 km long and 50–90 km wide, situated between the east subbasin (ESB) and southwest subbasin (SWSB) of the SCS. The fault zone is oriented N8°W, roughly perpendicular to and laterally displacing the relict spreading center and related spreading lineaments. Bounded by discontinuous linear seamounts, the fault zone comprises two V-shaped subparallel pull-apart basins and a separating basement high. Steeply dipping (>60°) normal basement-involved faults bound these pull-apart basins, forming typical negative flower structures. Numerous northeast-oriented en-echelon linear bathymetric highs within the fault zone are identified as secondary antithetic shears (P′ shears). These shears are characterized by their orientation relative to the principal displacement zones defined along the pull-apart basins. The ZFZ exhibits differences from the adjacent subbasins in water depth, basement burial, stratal thickness, and seismic stratigraphic characteristics. Five seismic sequences (S1–S5 upward) in the ZFZ and nearby ESB and SWSB are defined, dating to the Early Miocene synspreading (S1) and the Middle Miocene to Recent postspreading (S2–S5) stages, respectively. The difficulty in correlating seismic facies in sequences S1–S3, compared with the comparable seismic facies in sequences S4–S5 between the ZFZ and adjacent subbasins, suggests a horizontal displacement during the synspreading and early postspreading stages. We propose that the ZFZ functioned as a right-lateral transform fault zone during the synspreading period (the Early Miocene, approximately 24–16 Ma) of the SWSB and transitioned into a left-lateral strike-slip fault zone during the successive early postspreading period (the Middle-Late Miocene, approximately 16–5.3 Ma).

The distribution and characteristics of the Moho reflections (M-reflections) in oceanic basins record the seafloor spreading process and provide crucial insights into the formation and evolution of the oceanic basins. Approximately 20,000 km of multichannel seismic reflection sections are used to characterize the M-reflections and the crustal structures in the South China Sea oceanic subbasins. The identified M-reflections are mainly distributed in the northwest subbasin, the east subbasin, and the northern and southern flanks of the southwest subbasin (SWSB). They exhibit a banded distribution pattern in two-way traveltime along the north–south direction, with shallower depths observed in the central region and gradually deepening toward the northern and southern sides. In addition, the M-reflection amplitudes generally weaken, and the reflection events become less continuous as the igneous oceanic crust thins along with the seafloor spreading prior to the second ridge jump event. Ultimately, the M-reflection becomes indistinguishable in proximity to fossil spreading centers. These observations suggest a decreasing magma supply toward the cessation of the spreading. Despite the significance of Moho interface imaging at the crust-mantle boundary, the distinct M-reflections are absent in the SWSB and the vicinity of the Zhenbei-Huangyan Seamounts Chain, which are attributed to several factors, especially the vague nature of Moho interfaces.

Compared to 1C towed-streamer (TS) seismic exploration, 4C ocean-bottom seismic (OBS) node exploration has great advantages in complex structure imaging, lithology, and fluid identification using elastic waves. However, sparse spatial sampling of OBS node surveys highlights the problems of the imaging acquisition footprint, poor phase continuity, and low signal-to-noise ratio (S/N) with conventional elastic reverse time migration (ERTM) methods. Therefore, a solution of joint ERTM (J-ERTM) is proposed by combining sparse OBS node data with dense TS data. In the J-ERTM of the hybrid data, a novel weighted boundary condition combined with acoustic-elastic coupling equations and a vector-based crosscorrelation imaging condition are presented to perform PP and PS imaging by receiver-side tensorial extrapolation of TS and OBS node hybrid data. A synthetic example demonstrates that our method can effectively process TS and OBS node hybrid data and improve elastic imaging problems caused by OBS node sparse acquisition. J-ERTM techniques also are applied to an active-source OBS data set from the South China Sea. To improve the imaging quality with limited data, some preprocessing procedures for field TS and OBS node hybrid data sets are necessary, such as denoising, OBS node relocation, OBS node orientation correction, OBS node calibration, and others. After preprocessing, the pressure component and velocity components have a more physical energy relationship, more consistent frequency range and wavelet, and a higher S/N. Finally, the preprocessed hybrid data can be used for J-ERTM. The imaging results demonstrate that our method works well with field data.