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.
Deep fractured reservoirs hold enormous exploration and exploitation potential, and seismic attributes serving as a crucial tool for identifying subsurface fault systems. Five-dimensional (5D) seismic data, with rich azimuthal seismic reflection information, provides the data foundation for fault system prediction. However, the fusion mechanism of seismic attributes in different observation azimuths is unclear, making it difficult to analyze the mapping relationship of seismic attributes in different coordinate systems. Consequently, this paper proposes a multi-azimuth seismic attribute tensor fusion method based on 5D seismic data. It establishes a mapping relationship of the azimuth seismic attribute tensor components between the constitutive coordinate system and those in the observation coordinate system, thereby effectively improving the prediction accuracy of fracture systems in deep fractured reservoirs. By performing orthogonal projection decomposition on the spatial derivatives of seismic attributes, the seismic attribute tensor is defined and calculated. Subsequently, a multi-azimuth seismic attribute tensor fusion operator is constructed based on tensor analysis, which eliminates the influence of azimuthal uncertainty between the constitutive and the observation coordinate systems on the fusion of multi-azimuth seismic attributes. Multi-azimuth seismic attribute information is then fused by calculating the seismic attribute tensor parameters. Finally, the feasibility and stability of the method are validated using three-dimensional forward modeling and field data. Compared with post-stack and single-azimuth seismic attributes, the seismic attribute tensor parameters constructed by this method provide a clearer description of fault characteristics and offer richer fault information, thereby reducing ambiguity in the interpretation of 5D seismic data. This paper provides a novel theoretical method and approach for predicting deep fractured reservoir fault systems.
Accurate calculation and reliability assessment of small earthquake focal mechanisms are crucial for earthquake monitoring and forecasting based on the source characteristics of numerous minor events. Currently, common approaches include using P-wave first-motion polarity, amplitude ratios, or a combination of both. Each method has its strengths and limitations, but joint inversion using both first-motion and amplitude ratios generally provides better constraints. This study uses the traditional FOCMEC method for focal mechanism inversion and introduces a new tool, FOCMEC_GUI, for reliability assessment. This tool evaluates solutions using metrics such as the minimum spatial rotation angle, P-wave first-motion clarity distribution calibration, the contradiction ratio of first motions, and station geometry. We applied the method to the November 4, 2023, Enping M-L 4.9 earthquake sequence in Guangdong. A total of 28 reliable focal mechanism solutions were obtained. The results indicate that the background seismicity is dominated by strike-slip and normal faulting. The M-L 4.9 mainshock was of a normal-strike-slip type, and the aftershocks mainly exhibited normal-strike-slip, strike-slip, and normal fault mechanisms. Based on the spatial distribution of relocated epicenters and focal mechanisms at the edge of the asperity, we infer that the Enping earthquake sequence was driven by the interaction between concave and convex tectonic bodies and the NE-trending Hailing-Enping-Cangcheng Fault, under a predominantly horizontal stress regime. This interaction produced local extensional effects and stress concentration, facilitating fault slip. The proposed comprehensive selection and reliability assessment scheme-enhanced by the P-wave clarity distribution calibration-demonstrates improved robustness and rationality over traditional approaches.
Effective suppression of collar waves is crucial for the accurate acquisition of formation information in logging while drilling (LWD). In this work, we propose a novel acoustic isolator for LWD based on phononic crystal theory. The frequency response curves, time domain waveforms and the mechanical properties of the isolator are comprehensively analyzed through the numerical simulations. It is found that the proposed isolator effectively attenuates collar waves within a broad low frequency range without generating converted Stoneley waves. This feature is particularly significant for permeability inversion and related studies that rely on low frequency Stoneley waves. Compared with traditional periodic externally grooved acoustic isolators, the new isolator exhibits excellent mechanical properties. Additionally, a workflow utilizing signal processing techniques for residual collar waves suppression is introduced. The coherence of formation P-wave signals is enhanced significantly and the velocities of P-wave and S-wave of the formation are measured accurately by applying the workflow proposed. This study provides valuable guidance for improving the effectiveness of formation parameters inversion in monopole acoustic LWD.
This study utilizes high-temporal-resolution plasma density data from the Swarm satellite and applies the phase screen propagation theory to calculate the ionospheric amplitude scintillation index. A systematic comparison with ground-based GNSS scintillation observations demonstrates that this method can effectively reproduce signal perturbations observed by ground receivers, validating its applicability for scintillation estimation. Using 16 Hz electron density data from Swarm collected between 2014 and 2022, the Global Ionospheric Scintillation Occurrence Model (GISOM) was developed based on a minimal modeling algorithm. The model incorporates multiple variables, including season, local time, solar activity level, and geographic location. The GISOM outputs successfully reproduce the spatial distribution features of ionospheric scintillation in both low-and high-latitude regions, and show good consistency with ground-based GNSS observations in terms of local time variation and seasonal trends. Furthermore, a comparative analysis with nighttime equatorial plasma bubble (EPB) events observed by Swarm during the same period reveals significant seasonal differences in low-latitude scintillation occurrence rates over the American sector, particularly a temporal offset between the seasonal peaks of scintillation and EPB occurrence. This phenomenon is also confirmed by scintillation observations from two ground-based stations, TUC and HAN. Overall, the proposed model effectively captures the key spatiotemporal characteristics of ionospheric scintillation and provides a reliable tool for impact assessment, demonstrating strong potential for prediction and practical application.
Forward modeling of wave equations is fundamental to seismic imaging and inversion. Recent studies in high-speed railway (HSR) seismology have revealed that the propagation characteristics of macroscopic wavefields are affected by the heterogeneous response induced by the complex spatial microstructure within the medium. Consequently, the generalized wave equations have been developed, offering a novel perspective for investigating wave propagation in complex media. Under the framework of generalized continuum mechanics theory, this study introduces the generalized wave equations that incorporate both couple-stress and strain-gradient effects.We derive their reduced-order expressions and develop a corresponding staggered-grid finite-difference (SGFD) scheme with convolutional perfectly matched layer (CPML) boundary conditions. Forward modeling tests are conducted to validate the effectiveness of the proposed numerical scheme and boundary conditions. Furthermore, we perform the characteristic analysis of wave propagation under the generalized wave equations for the layered model and complex model. The results show that the seismic wave propagation is affected by the heterogeneity caused by the complex spatial microstructure characteristics within the medium, and the seismic wave propagates in a dispersive manner. Meanwhile, the synthetic seismic records can reflect the information related to the spatial microstructural characteristics of the medium, which can provide a new reference for the modeling and interpretation of the physical property parameters of reservoirs.
Seafloor heat flow data is a key indicator for evaluating gas hydrate resources. To accurately obtain the geothermal characteristics of deep gas hydrate accumulation in the gas hydrate exploration area of the Qiongdongnan Basin, northern South China Sea, this paper introduces the typical borehole heat flow measurement method used in this area. Temperature measurements of the sediments at four horizons and thermal conductivity of the drilling cores tested at the recovery horizon were carried out from the borehole, the variations in these two parameters were analyzed, along with their underlying causes. The results show that: (1) the geothermal gradient decreases gradually from the seafloor surface to deeper horizons, and the geothermal gradient is 63.2 degrees C & centerdot;km(-1) from 0 to 203.5 mbsf; (2) the thermal conductivity of the sediments is between 1.0 and 1.6 W & centerdot;(m & centerdot;K)(-1), the variation of thermal conductivity is mainly affected by the properties of the sediment; (3) the calculated heat flow of the borehole is approximately 75.8 mW & centerdot;m(-2), which is basically consistent with previously measured data, the deepwater area of the Qiongdongnan Basin has the attribute of a "hot basin". The thermal conductivity of the sediments in the 124.65 and 126.7 mbsf horizons is significantly higher than that in other horizons, and the sediment type is fine silty sand, which is conducive to the storage of sandy gas hydrates.
Metamorphic core complexes are distinctive geological structures that form in extensional tectonic settings and are widely distributed along passive continental margins. However, the role of pre-existing faults in the genesis and evolution of these complexes remains critical yet insufficiently understood. This study employs finite element numerical simulations to comprehensively investigate the impact of pre-existing fault geometry on the development of metamorphic core complexes in the crust and mantle of passive continental margins. The results demonstrate that the orientation and depth of pre-existing faults exert combined effects on core complex formation: deep faults facilitate stress localization in deeper layers, leading to mantle uplift and exhumation, whereas mantle-rooted faults may inhibit vertical displacement. Shallow-dipping faults enhance localized crustal deformation and favor the formation of larger domal structures. In addition, under conditions of a weak upper mantle, partial melting in the lower crust can further suppress vertical displacement. Furthermore, our results indicate that pre-existing faults govern the formation of metamorphic core complexes in the crust and mantle along the northern continental margin of the South China Sea, exemplified by the Baiyun Rift and Kaiping Rift. These findings provide a new geodynamic framework for elucidating the diversity of metamorphic core complexes in passive continental margins.
The Haiyuan fault zone is a large-scale left-lateral strike-slip fault on the northeastern margin of the Tibetan Plateau, exhibiting intense tectonic deformation and frequent seismic activity. The Laohushan fault, located in the "seismic gap", is a potentially high-risk area for large earthquakes. Recently, geodetic and seismological studies have revealed clear shallow creep behavior on the eastern segment of the Laohushan fault. However, the fault rock components, deformation mechanisms, and transition condition between shallow creep and deep earthquake nucleation along the Laohushan fault are still poorly known. To address this scientific issue, this study focuses on the fault rocks of the Laohushan fault, conducting detailed analyses of their mineral compositions, microstructural deformation features, and mechanical properties of these rocks. The results indicate that the shallow creep segment of the Laohushan fault is made up of a wide damage zone with numerous thin fault gouge layers spread throughout it. These clay-rich gouges exhibit widely well-developed foliation with aligned clay minerals, resulting in weak mineral slip zones. These clay minerals have low friction coefficients and stable sliding behavior, serving as the principal materials for shallow creep. Low-grade metamorphic rocks and crystalline rocks representing varied depths were exposed along the Laohushan fault. These fault rocks are dominated by high-strength minerals such as quartz, feldspar, pyroxene, and olivine, with widespread brittle fractures and fluid-rock alteration reactions, which provide conditions for earthquake nucleation at focal depths. Meanwhile, fluid activity affects fault strength by promoting mineral alteration and fracture healing, which influences fault sliding behavior. Based on these findings, we develop a rheological structure model for the Laohushan fault, which includes rock physics constraints on the transition from shallow creep to earthquake nucleation at depth. This provides a scientific framework for understanding fault deformation behavior, seismogenesis, and assessing future seismic hazards along the Laohushan fault.
Logging While Drilling Nuclear Magnetic Resonance (LWD NMR) integrates a precision-engineered NMR device within the drill string, enabling NMR measurements under conditions of rapid drill bit penetration. This technique holds significant application potential and represents a cutting-edge field in the development of NMR scientific instruments. The radiofrequency (RF) coil of the LWD NMR probe is a key component, generating radiofrequency magnetic fields to excite samples and receive NMR signals as the instrument accompanies the bit during rapid horizontal drilling. The RF coil's performance directly influences the detection characteristics of the NMR instrument, making it essential to design coils with high Signal-to-Noise Ratios (SNR) and low power consumption. An ideal RF coil should exhibit performance characterized by high SNR, high transmission efficiency, and low power consumption. However, optimizing these key parameters is often mutually constrained. Traditional coil design approaches generally rely on empirical methods and structural forward modeling of the coil for parameter optimization, which often restricts the achievement of optimal results. This paper proposes a novel inverse design approach for the RF coil of LWD NMR probes, extending the target field method to RF coil design for downhole tools. It establishes a relationship between the current in the solenoidal coil and the RF magnetic field outside the coil, while setting coil inductance as a constraint. The objective is to maximize the probe's SNR, utilizing regularization techniques to solve the objective function. The coil's winding pattern is determined using the stream function. This paper constructs the target RF field distribution based on the static magnetic field generated by the magnet of the LWD NMR probe developed by the research team. The optimal RF field is inversely solved to determine the coil structure. Experimental validation was conducted using conventional solenoidal coils and the newly designed coil. The research team performed experimental verification on existing equidistant coils and the new coil designed via this method. Results show that the signal amplitude measured with the new coil increased by 12%, and the SNR improved by 9%. The target field method based on the inverse design approach can effectively optimize the performance of LWD NMR coils, address the slow parameter optimization issue in traditional designs, offer a viable method for the efficient design and optimization of LWD NMR coils, and aid in enhancing the probe's detection performance.
Upward continuation of gravity anomalies is one of the most commonly used techniques in the datum transformation of observation data for the Earth's gravitational field. The Poisson integral method has long been the mainstream approach internationally for upward continuation calculations. A major technical challenge when using this method is that, when the computation point is infinitely close to a grid point on the spherical data surface, the singularity in the kernel function of the Poisson integral model with geocentric radius factor causes a singularity spillover effect, leading to invalid calculation results. To address this issue, in this paper, two solutions have been proposed: one is based on the central grid data block separation method and the other is based on the grid data removal-restore technique for the computation point, both of which remove the singularity spillover effect of the integral kernel function. Additionally, a practical modification scheme for converting the global integral model into a local numerical integral model has been proposed for solving the precise calculation problem of upward continuation of ultra-low altitude gravity anomalies in the Earth's external space. Then the impact characteristics and equilibrium relationship of data observation errors on the gravity anomaly continuation calculation results have been analyzed, and the main interference sources affecting the accuracy of the gravity anomaly continuation calculation have been identified. Finally, the Mount Everest region was selected as a test area, where numerical verification of the removal solution for kernel function singularity spillover effect and the practical modification scheme for the computation model was carried out using the ultrahigh-order gravity model EGM2008. The results confirm that the proposed solutions are feasible and effective.
Continuously operating global navigation satellite system (GNSS) observation networks can estimate regional terrestrial water storage (TWS) changes through monitored surface deformation data. Compared to observations from the Gravity Recovery and Climate Experiment (GRACE) and its follow-on mission (GRACE-FO), GNSS technology demonstrates greater potential for capturing finer-scale TWS variation features. However, the accuracy of GNSS-only inversion results is dependent on station distribution and degrades in sparsely instrumented regions. To address this limitation, this study proposes a joint inversion method for regional TWS changes, integrating GNSS and GRACE/GFO data. Building upon the GNSS-only inversion model, we enhance spatial coverage by introducing virtual stations constructed from GRACE/GFO data. Furthermore, considering the differences in the frequency response characteristics of the two geodetic datasets to hydrological loading variations, we developed this integrated approach. Using this method, we inverted TWS changes in the Yellow River Basin from 2011 to 2023. The joint inversion results were systematically compared with GNSS-only inversion results, GRACE/GFO data, Global Land Data Assimilation System (GLDAS) outputs, and precipitation records. Spatially, the joint inversion results align closely with the GNSS-only results in densely instrumented regions, while corresponding more closely to GRACE/GFO and GLDAS patterns in the sparsely monitored northern basin, where the maximum annual amplitude reached 120 mm. Temporally, all four TWS datasets exhibit consistent seasonal variation patterns, peaking in autumn/winter and reaching troughs in spring. The time series of the joint inversion results exhibits correlation coefficients of 0.65 and 0.78 with the independent GNSS and GRACE/GFO results, respectively, confirming the effective integration of both geodetic observations. The study also identifies precipitation as the key driver of TWS changes in this region, with TWS variations lagging precipitation by approximately 2 similar to 4 months. Consequently, influenced by precipitation patterns, the basin's TWS exhibits distinct characteristics: spatiotemporal heterogeneity and significant interannual variability. Experimental results demonstrate that, compared to individual GNSS-only or GRACE/GFO inversion methods, the joint inversion approach provides a more comprehensive characterization of the spatiotemporal characteristics of TWS changes in the Yellow River Basin, offering a valuable reference for regional water resources assessment and management decisions.
We collected and picked up the arrival time data of natural earthquakes with M-L >= 0.5 in the middle-eastern section of the Hexi Corridor and its adjacent areas. These data were recorded by 57 fixed seismic observation stations of the China Earthquake Networks Center and 89 mobile seismic observation stations deployed in the northern section of the North-South Seismic Belt as part of the second phase of the Himalayan Seismic Experiment Array. Using the double-difference seismic tomography method for inversion, we relocated the earthquake source positions and simultaneously obtained high-resolution 3D crustal P-wave velocity structure in the research area. After relocation, the spatial locations of the earthquake sources (especially aftershocks in the source areas of historical major earthquakes) are significantly improved, showing more distinct clustered or linear distribution along the seismogenic faults. Most aftershocks are located on the northern side of the seismogenic faults, which may be attributed to the fact that most faults in the study area are thrust faults. The velocity structure analysis reveals the following characteristics: (1) The shallow crust of the Hexi Corridor Block exhibits high-velocity anomalies, while the middle-lower crust is a transition zone between high and low velocity or presents low-velocity anomalies. This is probably because the shallow part of the corridor is mainly composed of acidic felsic minerals, which have high density, low porosity, and minimal possibility of partial melting-resulting in an overall rigid structure. However, the middle-lower crust is relatively fragmented due to the strong compression from the Northern Qilian and Alxa Blocks on both sides, creating a complex seismic wave propagation environment. This leads to the underdevelopment of surface rupture zones for most strong earthquakes; instead, the rupture of most earthquakes occured along blind faults or blind folds. (2) Except for the Menyuan-Tianzhu area, the Northern Qilian Block has a distinct low-velocity body in the middle-lower crust (15-40 km), indicating that it may be the area with the strongest crustal shortening and deformation at the northeastern margin of the Qinghai-Tibet Plateau. (3) Chaoshui basin and Minqin-Wuwei basin of Alxa Block extend into the Hexi Corridor as tongue-shaped velocity bodies with gradually increasing scale within the depth range of 25 similar to 40 km. This may be because the two basins have transformed into stable structure at this depth range and are gradually subducting beneath the Northern Qilian Block. (4) The focal areas of the five major historical earthquakes with M >= 6.0 in the study area are all located in the high-low velocity transition zones or within low-velocity bodies. The high-low velocity transition zone is prone to stress accumulation, which can trigger major earthquakes, while the low-velocity bodies have a brittle structure that is more susceptible to damage during major earthquakes.
Marine seismic data contain abundant guided P-waves, whose velocities are frequency-dependent and exhibits dispersion characteristics. Continental shelf margin areas often present inclined features, and the dispersion equation of guided P-waves on a dipping seabed can only be calculated with horizontal layer approximation. Based on the acoustic assumption, an analytical recursive algorithm is derived for guided Pwaves in inclined multi-layered seabed. By applying a three-dimensional coordinate transformation method, the boundary equations of guided P-waves are established. Using the acoustic wave equations, recursive formula and dispersion equations for the guided P-waves in arbitrarily inclined multi-layered model are derived. Validation tests indicate that the obtained guided P-wave solutions satisfy the acoustic wave equations within each inclined layer and enforce the acoustic boundary conditions at any inclined interface. Dispersion curves computed from a simplified horizontal layer model using these derived equations agree with those obtained from conventional equations, which further confirms its validity. Based on these derived dispersion equations, the effects of dipping angle, azimuth, water depth, mud layer thickness, and spatial position on the dispersion characteristics of guided P-waves are investigated. This derived algorithm is further applicable for shallow geophysical exploration in coastal sea areas.
As a crucial branch of geophysical electromagnetic methods, the transient electromagnetic method (TEM) is widely employed in mineral resource exploration. Most mineral targets exhibit significant induced polarization (IP) effects. Forward modeling demonstrates that IP effects cause rapid attenuation of mid-to-late TEM signals and may even induce sign reversal. Consequently, data inversion and interpretation that disregard IP effects not only struggle to fit observed data adequately but also risk yielding erroneous conclusions. Building upon 3D TEM forward modeling that incorporating IP effects, this study develops a corresponding 3D inversion scheme. Given that IP properties are described by multiple interdependent parameters whose simultaneous inversion would incur prohibitive computational costs, we consider a simplified scenario: fixing IP parameters and inverting solely for electrical conductivity. Although idealized, this approach remains valuable for geological settings with minor IP parameter variations and serves as a foundation for future multi-parameter IP inversion. The finite-volume method in the time domain is adopted for 3D forward modeling, with IP effects characterized by the Debye model. For inversion, a Gauss-Newton optimization framework is implemented, integrating implicit sensitivity matrix calculation with conjugate gradient solving of the normal equations to reduce computational time and memory consumption. The efficacy of the proposed 3D inversion algorithm is validated through multiple synthetic models. We compare IP-incorporated versus IP-ignored TEM inversions to underscore the necessity of modeling IP effects, investigate the impact of heterogeneous IP parameter distributions, and present inversion results for a funnel-shaped conductive model of practical geological significance.
The seismic cyclic shear stress ratio (CSR) in the soil layers is the most fundamental indicator for characterizing the seismic effect of the soil layer. However, the widely used Seed ' s calculation formula internationally has significant issues and can no longer meet new engineering demands. This paper focuses on the issue of soil seismic shear stress on hard sites, utilizing a large number of strong earthquake observation records and advanced numerical simulation technique to analyze the applicable conditions and limitations of the Seed ' s formula. The influence of the main controlling factors of the site and ground motion on CSR is discussed and new formulas for calculating CSR are established. Compared with the Seed ' s formula, two additional elements, namely the peak ground acceleration PGA and the average shear wave velocity v(s,30) of the site, have been added. The new formula includes the average CSR calculation formula and the CSR calculation formulas for different probability intervals that better reflecting the uncertain effect of ground motion and, all the formulas have passed the verification through numerical simulation experiments. The CSR variation law obtained in this paper can deepen the understanding of the dynamic stress response of soil layers under strong earthquakes, and the new formula proposed can provide a more scientific and reasonable assessment method for the analysis of site seismic response.
