In 2016, we carried out a coincidental multichannel reflection seismic and wide‐angle reflection/refraction seismic experiment along a 320‐km‐long profile across the northeastern margin of the South China Sea. Based on these new data, we studied velocity structure of the transitional continental crust using both forward modeling and travel‐time tomography. The velocity model shows that Mesozoic sediments have velocities of 4.3–5.3 km/s and are about 3–6 km thick in the magnetically quiet Chaoshan Depression and the attenuated transitional continental crust. Using forward magnetic modeling, we confirm that the presence of this thick Mesozoic layer with low magnetic susceptibility induces the magnetic quiet zone and show that the lower crust high‐velocity materials have limited effect on magnetic anomalies. This Mesozoic layer pinches out seaward near the continent‐ocean boundary (COB). Two apparently isolated high‐velocity anomalies are revealed in the lower crust of the continental slope. The first has velocities of 7.0–7.5 km/s and a maximum thickness of 8 km, and the second has velocities of 7.0–7.3 km/s and a maximum thickness of 3 km. We find that the discontinuity of the high‐velocity zones is not due to the lack of some seismic phases or lose of a seismometer. We suggest that postspreading magmatism caused these high‐velocity zones. A third high‐velocity zone, showing a distinctly large velocity gradient, is revealed near the COB. This indicates upper mantle upwelling and/or exhumation right at the COB, despite recent unsuccess in directly sampling these deep materials in the South China Sea.

In this study, we process four new multichannel reflection seismic profiles acquired in 2015 and 2016 in the continent–ocean transition zone (COT) of the northern South China Sea (SCS). We apply a multi-domain, progressive, and seabed-controlled denoising technique and obtain a good denoising effect. Combining velocity analysis in the multi-round time domain and forward modeling, we analyze the types and characteristics of multiples in the study area and formulate an effective demultiple technique to attenuate strong seabed multiples, diffracted multiples from rough seafloor, and other multiples from deep reflectors. The processing results show that the sea surface-related multiple elimination technique predicts the sea surface-related multiples accurately by data convolution, and has a good effect in attenuating seabed multiples. Diffracted multiple attenuation method extracts high-frequency and high-energy diffracted multiples, and suppresses multiples by the energy ratios of multiples to primary events. To attenuate deep multiples, we select predictive deconvolution to attenuate periodic deep multiples after many trials and detailed analysis. The combination of these different techniques in sequence proves to be quite effective in attenuating different seismic multiples in the COT. The imaged crustal structures near the COT often show strong magmatism and/or basement uplifting. The faulted and thinned continental crust adjacent to the COT corresponds to the lowest free-air gravity anomalies. Gravity anomalies often increase from the COT to the oceanic crust. An exception is to the northeast of the SCS, where the relatively wide COT shows very high gravity anomalies, likely induced by mantle upwelling and serpentinization.

Seismic while drilling (SWD) technology, taking advantage of the vibration signal generated during the drill bit breaking rocks in drilling operations, has been used in the seismic explorations. Since the energy of signals produced by the seismic origin is weak while the noise jamming in the drilling site is strong, the signal-to-noise ratio (SNR) of collected SWD data is usually fairly low. In this paper, we focused on improving the SNR of SWD data via adjusting the impact energy of drilling tools. Based on the explanation of the working principle of SWD systems, the energy formation modes of three kinds of drilling tools were discussed. It was safe to expect that the hydraulic torque impactor could be applicable to improve the SNR of SWD data in evidence.