The accelerometer is the core payload of a gravity satellite, and the calibration of accelerometer data is a crucial step in gravitational field solution. Its calibration accuracy directly affects the accuracy of gravitational field inversion. Based on the two-step method, this paper proposes an improved two-step accelerometer calibration strategy. Through a multi-stage processing method of "high-frequency tracking + low-frequency filtering", a multi-resolution error model is constructed to suppress the influence of parameter errors on high-frequency signals. Using actual satellite gravity data, the effects of the two-step method and the improved two-step calibration strategy are compared. The results show that the bias parameters and scale factors estimated by the improved two-step accelerometer calibration strategy have higher stability. The signal-to-noise ratio (SNR) of the inverted time-varying gravitational field model is higher. Compared with the two-step method, the SNR of the 60th, 96th, and 120th degree models increases by 4.4%, 20.3%, and 11.3% respectively; the cumulative degree variance of the time-varying gravitational field decreases by 2.5%, 19.4%, and 16.5% respectively, and the north-south strip noise of the global equivalent water height is also reduced. The research analysis shows that the improved two-step accelerometer calibration strategy has higher calibration accuracy than the two-step calibration strategy.
Active-source seismic survey technology is one of the most important and widely used exploration methods in marine geoscience. However, in marine areas with complex topography, the energy of active-source acoustic waves excited in water is often significantly attenuated due to scattering as they convert into seismic waves on the seabed. Nevertheless, a systematic delineation of key topographic controls on this attenuation remains lacking. Based on 3D wide-angle seismic survey data from around islands and reefs, we identified a systematic absence of signals generated above a submarine hill by analyzing discontinuities in Pg phase travel-time curves. Through comprehensive comparison of recordings from multiple Ocean Bottom Seismometers (OBSs) at different azimuths and offsets, we ruled out potential influences from the receiving environment, sound source, offset and subsurface media. Using a fluid-solid coupled 3D spectral-element method, we verified that seafloor topography exerts a primary control on the energy intensity of acoustic waves penetrating the seabed and converting into P-waves. The key findings are as follows: (1) When the relief height of a convex seafloor topography is comparable to the incident acoustic wavelength, the P-wave energy undergoes strong scattering attenuation, exhibiting an amplitude reduction of over 50% compared to that excited above a gentle slope. (2) After penetrating the seafloor, the converted P-wave energy is distributed unevenly across different azimuths: scattering is stronger toward gentle slopes and weaker toward steep slopes and topographic convex features. (3) Signal acquisition strength can be significantly enhanced by relocating the sound source from above a convex feature to an adjacent gentle slope and deploying receivers along propagation paths that avoid the convex feature. This study informs the design and optimization of sound source deployment for active-source seismic surveys in marine areas with complex topography.
Utilizing observations from the GRACE satellite between 2002 and 2017, this study presents a systematic analysis of the climatological and morphological characteristics of topside plasma irregularity (TPI) and neutral density disturbance (NDD) at low-to-mid magnetic latitudes. TPI events were identified using in-situ electron density measurements, while NDD events were extracted from thermospheric neutral density data. We quantitatively analyzed the statistical relationship and spatiotemporal distribution between the two phenomena. TPI events were further classified into equatorial plasma bubble (EPB) and medium-scale traveling ionospheric disturbance (MSTID). Results show that EPBs are predominantly distributed over the South America-Atlantic-Africa region, whereas MSTIDs tend to occur in the Africa-Asia-Pacific region. Notably, the climatological characteristics of NDDs were discovered to resemble those of TPIs in their respective regions. Furthermore, NDD events predominantly exhibit a characteristic "backward-C" morphological pattern, with their occurrence probability rising alongside background neutral and electron densities. These results collectively provide strong support for the physical mechanism of TPI-driven NDD, thereby offering systematic observational evidence for understanding energy transfer in ionosphere-thermosphere coupling.
Passive-source Ocean Bottom Seismometers (OBSs), with their high-sensitivity recording of broadband signals from natural earthquakes and ambient noise, have become a crucial technique for investigating fine-scale structures and dynamic processes of the oceanic crust and mantle. In recent years, significant advances in imaging the velocity and anisotropy of the crust-mantle structure in the South China Sea have been achieved based on passive-source OBS arrays, revealing critical geodynamic processes such as heterogeneous oceanic crust formation, lithospheric rupture, mantle convection, deep subduction-induced dragging and spatiotemporal variations of ocean ambient noise. These findings have substantially deepened our understanding of the full life-cycle evolution and scientific framework for multi-sphere system science of marginal seas. The National Natural Science Foundation of China Shiptime Sharing Project has played a pivotal role in facilitating multi-institutional joint expeditions, advancing OBS research and application, developing data-sharing platform, and cultivating scientific talent, thereby providing essential support for these advances. Notably, a collaborative mechanism- characterized by unified planning, coordinated deployment, shared risk, and open data access-has been established under the Shiptime Sharing Project. This mechanism has effectively integrated instrument resources across institutions, enabling the construction of high-density and large-scale observational networks, and providing critical support for acquiring high-quality seismic data. In the future, high-density threedimensional observation networks should be constructed, long-term real-time monitoring capabilities developed, multidisciplinary integration deepened, intelligent inversion and data assimilation methods promoted, and scientific data-sharing mechanisms further improved, to provide a solid observational foundation for Earth system science research in the South China Sea.
Based on a simplified tunnel geometry, this study derives a staggered-grid finite-difference discretization scheme with second-order accuracy in time and arbitrary even-order accuracy in space from the first-order velocity-stress equations in polar coordinates. To handle absorbing boundaries, free surfaces, and angular-periodic boundaries in the tunnel environment, the split-field perfectly matched layer, an enhanced vacuum formulation, and periodic virtual grid extension are applied, respectively. By designing homogeneous and annular-interface tunnel models, full-wavefield simulations are performed in both Cartesian and polar coordinates. A comparison of seismic wave propagation patterns and response characteristics in the two coordinate systems confirms that the polar-coordinate approach effectively suppresses spurious diffraction waves caused by staircase approximation in Cartesian grids. Numerical examples demonstrate that polar coordinates can describe the behavior and features of seismic wavefields in tunnels more clearly and accurately, thereby providing more reliable data support and theoretical guidance for tunnel engineering exploration.
