Studia Geophysica et Geodaetica最新文献

One of the great challenges that the IAG Sub-Commission on gravity and geoid in Africa faces is the significantly large gravity data gaps. Simple interpolation of the gravity data does not add new information at the large data gaps. In the present study, new and independent signal information is implemented by using the laterally varying density that influences the gravity signal. Consequently, the global crustal density model UNB TopoDens was introduced as an additional source of information covering the whole area of the continent of Africa. This new independent information, entering the process of generating and updating the African gravity data base, has variable laterally varying density values available as a 30″ × 30″ grid. It has been employed in the framework of the non-ambiguous window remove-restore technique, which produces well smoothed reduced anomalies which minimize the interpolation errors. The lateral crustal density model (CDM) assigns extensive water areas with a density value which is equal to 1000 k gm⁻³, and the oceans with density equal to zero. These values are overruled with valid values in order to correctly compute the impact of topographic masses in the window remove-compute-restore (wRCR) technique. Accordingly, a density model compatible with the computation of the effect of topographic masses within the window remove-restore technique has been developed. A set of Digital Density Models is needed for the computation of the effect of the topographic masses. This has been achieved by the block average operator technique. The impact of the variable density by using a lateral density model compared to the traditional use of a constant density value is discussed in detail. The results proved that the variable crustal density has a significant effect on the interpolated gravity at the large terrestrial gravity data gaps over Africa.

The accurate interpretation of magnetic anomaly data relies heavily on knowledge of the total magnetization direction of subsurface sources. Particularly when strong remanent magnetization is present, assessing the total magnetization direction is crucial for interpreting magnetic anomalies. However, magnetic field observations are prone to various noises. This study aims to enhance the accuracy of magnetization direction determination by leveraging transformed fields that are less sensitive to noises. Our primary goal is to develop a robust method to estimate the magnetization direction, especially when dealing with complex magnetic anomalies. We propose a method for determining magnetization direction by exploiting the correlation between the pseudo-gravity (PG) field (which we demonstrate is less affected by noises) and the Normalized Source Strength (NSS). This involves searching for the NSS field derived from the observed magnetic field data and the PG fields computed from a range of assumed magnetization directions. The proposed method is validated through both synthetic modeling experiments and real data applications. The results demonstrate the robustness of the method in handling diverse anomaly geometries, including cases with mutual interference and random noises. Furthermore, it effectively mitigates the influence of remanent magnetization. The experiment on sphere source with 5%, 10%, and 20% random noises showed that our method yields significantly lower errors in estimating magnetization direction than previous approach. The mean errors for inclination and declination are 0.29°, 1.22°, 2.45° and 0.44°, 0.84°, 1.92°, respectively. Consequently, our approach, utilizing the NSS field and the PG field, offers an effective tool for estimating magnetization directions.

The Chicxulub impact ∼66 Ma ago formed a large basin on the Yucatán platform, filled by sediments that provide a record of carbonate deposition, sea level and climate changes in the Gulf of Mexico. We study the post-impact sequence drilled in the Yaxcopoil-1 borehole in the crater terrace zone. The post-impact section is 792 m thick, overlying the impactites and Cretaceous carbonates. Section analyzed is between 400 and 792 m, formed by twelve units of limestones, dolomites, argillaceous/silicified limestones and calcarenites. Carbonates show cross-lamination, flow-currents, parallel lamination, cyclic graded bedding and styolitic structures. Study is based on core analyses, logging, petrography, digital scanned images, magnetic properties and X-ray diffraction and X-ray fluorescence geochemistry. The basal units U1–U4 represent low-energy deep bathyal environments and fine-grained facies varying from mudstone to wackestone. Sediment deposits, reworked and transported from the platform and crater rim, show textural and grain size changes from grainstone to packstone. Geochemical and magnetic susceptibility logs record effects of hydrothermal alteration, with secondary mineral assemblages. The SiO₂ and CaO contents display wide ranges, negatively correlated. Fe₂O₃, TiO₂, Al₂O₃ and K₂O oxides show similar patterns downhole. The Sr and MgO show a positive correlation, except for the basal sediments. Paleocene units U1–U3 show increasing density, increasing seismic velocity and upward decreasing porosity. Upper units U5–U12 are characterized by laminated black shales and marls with microfacies varying from wackestone to packstone, with planktic and benthic foraminifera and bioclasts. Unit U9 shows low density and seismic velocity and increased porosity. Depositional environments vary from low-energy deep bathyal inside the basin to shallow neritic outside the crater rim. Sediments of the internal carbonate ramp to external neritic environments record sea level changes and platform subsidence/uplift.

Marine gravity gradient data, which provide high-precision, multi-component observations, are crucial for detecting subtle variations in oceanic geological structures. This information has profound implications for geological exploration, seabed resource assessment, and seismological research. Typically, satellite altimetry is mainly employed to obtain marine gravity field information. In our study, we proposed a strategy that integrates the deflection of the vertical (DOV), derived from satellite altimetry data, with Fast Fourier Transform (FFT) technology to compute the full tensor of the ocean’s gravity gradient. Initially, the DOV components were estimated using the least squares method from the geoid gradient. Subsequently, the vertical gravity gradient anomaly on the ocean surface was determined utilizing the DOV components. Finally, the remaining five gravity gradient tensor components were derived from the vertical gravity gradient anomaly using FFT techniques. In our experiments, we applied the proposed strategy to SWOT and CryoSat-2 observations. The Scripps Institution of Oceanography model was employed to validate the vertical gravity gradient component, while the results for the full tensor of gravity gradient were verified using the CUGB2023GRAD model. The experimental results validate the processing strategy proposed in this study, demonstrating its effective applicability within the local planar coordinate system.

In this contribution, we use all three components of the gravity vector observations to compute a regional geoid and demonstrate the advantages of using the horizontal components alongside the vertical component. We apply the one-step integration method within the remove-compute-restore framework; where the long-wavelength part of the geoid is recovered from Earth’s gravitational models while the harmonicity of the computational space is ensured by removing the topographic effects. We create a system of linear equations using a discretized form of the one-step integration method and use the Tikhonov technique to deal with the numerical instability due to its implicit downward continuation and to determine the geoid at higher resolution, e.g., 1 ′ × 1′. We propose a novel method to estimate the Tikhonov regularization parameter using the discrepancy principal and a stable solution of the geoid at lower resolution, e.g., 3′ × 3′. The results reported are based on real airborne gravity vector observations collected over Colorado, USA. The scattered observations at flight level are directly inverted to the disturbing potential at grid points on the reference ellipsoid, where geoid heights are then computed using Bruns formula. We evaluate the external accuracy of the geoid by comparing it with GNSS/levelling data and estimate the location-based internal uncertainties (error) of the geoid heights through formal error propagation. As part of this contribution, the airborne gravity vector data used in this study are also available for research purposes upon request to the corresponding author.

Most methods for processing seismological data require a suitable velocity model characteristic for the given region being defined. This is also the case of the Reykjanes Peninsula in SW Iceland, where the REYKJANET seismic network was built to monitor local seismicity in the rift zone. At present, four previously published 1D velocity models (SIL, BRA, TRY and VOG) can potentially be used, prompting us to determine which one is the best. In order to address this issue, we arranged a contest in which all four 1D models and one additional 3D model (T3D) were entered. Uniform methodology for classifying the models was applied and included an analysis of: (i) post-1ocalization travel-time residuals, (ii) residuals of the P-wave first-motion incidence angle and (iii) model-predicted and measured Rayleigh-wave dispersion. We discovered that no single model was unequivocally the most optimal, as the differences between them proved rather minor. A common shortcoming of all the models is the bias of the P-wave first motion incidence angle residuals, which may be a general problem for methods working with P-wave amplitudes (e.g., moment tensor solutions). The VOG model was selected with a weak preference. Finally, we propose a simple method for modifying any of the 1D models by adding a station-dependent surface layer with a vertical velocity gradient. This way, a pseudo-3D model is generated which is fully competitive with a true 3D model while retaining the simplicity of 1D ray tracing. The efficiency of this correction was demonstrated using the VOG model. The corrected VOG model provides post-1ocalization residuals comparable with the true 3D model T3D, has zero bias in predicting the P-wave first-motion incidence angles, and agrees acceptably in predicting the Rayleigh-wave phase-velocity known from other sources. While calculations with a 3D model can be clumsy, the proposed pseudo-3D model is defined by few parameters and is very easy to use. Its applicability is limited to earthquake sources deeper than the deepest lower limit of the topmost layer below the stations.

Staggered-grid finite-difference (SGFD) approaches are universally applied to discretize different seismic-wave equations during wavefield extrapolation. However, the traditional SGFDs may encounter numerical dispersion error and instability owing to the limited approximation accuracy. To increase the simulated accuracy, we develop an optimized SGFD with high-order accuracy based on the orthogonal-octahedral operator for 3D scalar-wave modeling. Compared with the standard orthogonal-octahedral approach, the modified approach has smaller computing cost because we reduce the SGFD stencil. In addition, the corresponding time-space domain dispersion relation is beneficial to generate the least-square-based optimized high-order SGFD coefficients. Dispersion and stability comparsions show that the developed algorithm has better performance than the classical methods. Several simulated experiments verify that the proposed scheme can significantly suppress numerical dispersion in time and space domain and effectively improve the simulated accuracy and efficiency. In conclusion, the developed scheme can provide a reliable wavefield extrapolation tool for seismic imaging and inversion.

Spectral inversion, based on the odd-even decomposition principle of reflectivity, used the relationship between seismic data and wavelet amplitude spectrum to establish the inversion equation and achieve resolution-enhancement processing. Compared with deconvolution based on the L₂ norm, the odd and even components of reflectivity using spectral inversion can weaken the tuning effect, identify thin layers, and obtain data with higher resolution. However, most post-stack seismic data are non-stationary, i.e., attenuation of amplitude, phase, and frequency with time exists. We derived a resolution-enhancement algorithm of non-stationary seismic data with quality factor Q based on the short-time Fourier transform. Due to the instability of the spectral inversion algorithm, the lateral continuity of the obtained result is poor. Therefore, we proposed a multichannel spectral inversion algorithm with lateral constraints. The algorithm inherits the high-resolution characteristics of spectral inversion and effectively enhances lateral continuity. Applications to model and field data sets show that the proposed L₂ norm-based non-stationary multichannel spectral inversion method can be effectively applied to the resolution-improvement processing of non-stationary seismic data.

The earthquake of 9 November 1880 was one of the most important moments in the seismic history of Zagreb (Croatia). It is the strongest earthquake to have occurred in the greater Zagreb area, and as such it defines the seismic hazard in northwestern Croatia, the most populated part of the country. The main objective of this study was to reanalyze the location and magnitude of the earthquake, the input parameters which are crucial for a better assessment of seismic hazard, as there were macroseismic indications that the previous assessments should be revised. In addition, the strongest aftershock occurred two days after the main event, so it can be assumed that the observed intensities were caused by the cumulative effect of these two events. Therefore, a new isoseismal map was created, synthetic macroseismic modelling was performed and additional geophysical and microtremor measurements were taken. Based on all the information collected, the attempt to separate the effects of the strongest aftershock from the effects of the mainshock (to avoid a cumulative effect), a new assessment of the location of the epicentre of the main 1880 earthquake in Zagreb and its magnitude was made. When it comes to historical earthquakes, from a seismological point of view, even small improvements in the definition of the main seismological parameters of an earthquake significant for a given area are very important - for a better understanding of the geodynamics of the area, the earthquake mechanism and the spatial distribution of damage after the earthquake, as well as for the consistent assessment of seismic hazard and thus risk.

The oblique collision zone of Arabia-Eurasia is a seismically active region with complex crustal deformation patterns. While GPS measurements provide valuable data, their sparse distribution limits our understanding of the full extent of deformation. This study addresses this limitation by using a robust interpolation method for GPS velocity data in the collision zone. We utilized biharmonic splines to interpolate horizontal components of sparse GPS velocity data independently and in a coupled manner by altering Poisson ratio. This method is an effective means of interpolating sparse vector data in cases where deformation mechanics can be explained by elasticity principles. The interpolation process included fitting trends to the input data, calculating residuals, and analyzing them. The prediction process consisted of trend and spline fitting stages. We interpolate horizontal GPS velocities onto a standard geographic grid with a 30-minute interval, excluding data points with significant deviation. The data was partitioned into training and testing subsets, with the training set used for calibration and the testing set for evaluation of the interpolation method. Our analysis revealed an irregular spatial distribution of crustal movement. The northern component of the velocity field consistently points towards Eurasia and is greater than the eastern component. The amplitude of the northern component decreases from south to north and from west to east, indicating variations in deformation intensity. The eastern component exhibits a change in direction, moving westward in the western half of Iran and eastward in the eastern half, with a reversed trend in the north. This change in direction highlights the presence of solid blocks within the collision zone. Undeformed regions, major faults, convergence deformation, and compressing high-elevation regions are also observed in the collision zone. These findings provide a detailed picture of present-day crustal deformation in the Arabia-Eurasia collision zone, enhancing our understanding of the collision process.