Surveys in Geophysics最新文献

Gravitational fields are often modelled by the mathematical apparatus of integral transformations. A basic assumption of these integrals is the knowledge of relatively accurate data available globally. Practically, however, global data coverage is rarely achieved and data are always contaminated by measurement errors. Therefore, integral transformations are properly modified and practical integral estimators are formulated and further employed in numerical experiments. In addition, corresponding statistical characteristics are often desired to indicate the quality of calculated gravitational fields. In this article, we systematically formulate practical integral estimators and their respective errors. We present the practical integral estimators in the combined form (i.e. combining the restricted integrals for the near-zone effects and the truncated spherical harmonic series for the far-zone effects) and in the form of spherical harmonic series. The practical integral estimators form a theoretical basis for an accurate gravitational field modelling, e.g. when solving upward or downward continuation. By employing a unified notation, the mathematical formulas are derived to an unprecedented extent for a broad class of quantities. Namely, the theoretical formulations connect four types of boundary conditions with twenty computed quantities. The practical integral estimators are complemented by point-wise errors and global mean square counterparts. The point errors can be calculated from the errors of the near-zone and far-zone boundary values, the position of the computational point, the size of the integration radius, and the maximum spherical harmonic degree of the far-zone effects. The number of variables is reduced for the global mean square errors, as they are invariant from the horizontal position of computational points. Both statistical characteristics may also be employed in optimisation problems and experimental designs. The basic principles and formulations presented here may be employed in related problems of other potential fields, such as in electrostatics or magnetism.

Using geophysical inversion to determine the geometry of subsurface targets has a long-established history, tracing back to the early days of geophysical interpretation. These methods continue to gain considerable attention because of the growing demand for more precise and interpretable visual representations of subsurface bodies. A recent surge in interest has led to the development of numerous new strategies and algorithms that share the common primary goal of defining target geometries. A comprehensive review of these approaches is currently lacking, which has led to some overlap in research, where studies fail to acknowledge prior work or clarify how their methods compare with, or differ from, work by other researchers. In this paper, we present a thorough review of recent developments in the field, aiming: (1) to provide readers with a broader understanding of the various geometry-based inversion techniques currently being researched, (2) to help practitioners assess the practical applications of these inversion methods, and (3) to provide insight into promising directions for future research. We propose a standardized classification system and terminology to support the research community in formulating, discussing, and disseminating their findings within this rapidly expanding domain. We classify the approaches into five distinct categories. For each, we provide a broad literature search to discuss: the applications and types of geophysical data used, the parameters estimated by the inversion algorithms, the prior information and constraints applied, computational and numerical details, visualization and model evaluation approaches, and interpretation strategies. We finish with a forward looking discussion to summarize.

Previous studies evaluated several characteristics of ionospheric fading events and amplitude scintillation. However, a detailed analysis on how the fading profiles and scintillation probabilities vary according to the dip latitude is still required. In this work, a statistical analysis of data from four ground-based scintillation monitors was performed to evaluate how the α coefficient (first parameter of the “α–μ” probability distribution model); the deepest fading occurrence; the number of fading events per minute; and the duration of fading events change according to the dip latitudes of the ionospheric pierce points (IPPs) of transionospheric propagation paths. The results reveal a nuanced spatial variation in amplitude scintillation, emphasizing an enhanced severity within the equatorial ionization anomaly (EIA) southern crest, resulting in a clear increase in the probability of severe fading events. An increasing trend in the α fading coefficient at more poleward dip latitudes was found, in comparison with results from equatorward locations, suggesting an asymmetry favoring more severe fading events within the former region. The average fading occurrences are significantly larger over the EIA peak region, especially for increasing scintillation levels. Complementary Cumulative Distribution Function (CCDF) curves demonstrate peak probabilities between dip latitudes from − 14.5° to − 10.5° for higher scintillation levels, also displaying an asymmetrical pattern around the EIA boundaries. This study provides important insights on the spatial dynamics of scintillation and fading profiles, enhancing the understanding of low-latitude ionospheric effects on global network satellite system (GNSS) signals.

The gravitational potential and its first-order derivatives induced by finite mass distributions are evaluated numerically and analytically. Three asteroid shape models have been used for the implementation, namely Eros which is the most irregular one, Didymos which is nearly spherical and Dimorphos which is a perfect ellipsoid. For the numerical approach, the spherical harmonic series up to maximum expansion degree 100 were computed. For the analytical approach on the other hand, the line integral algorithm of general polyhedra was applied. The two methods are compared in terms of numerical convergence between them with respect to maximum expansion degree of the corresponding harmonic series and relative position between computation point and modeled body. Additionally, emphasis is given on the different geometric characteristics of the applied shapes and their influence on the induced gravity signal evaluation. The results are separated for points located inside and outside the Brillouin sphere. Inside Brillouin sphere, better agreement between methods is provided for the case of Didymos due to its spherical-like shape. Outside Brillouin sphere, Dimorphos secured the highest convergence between analytical and numerical methods, due to its smooth exterior boundary, with the maximum difference being (6text{E}{-10}text{ m}^2/text{s}^2) for the gravitational potential and (7text{E}{-10}text{ m}/text{s}^2) for its first-order derivatives. For gravitational potential the highest differences are observed for Eros ((2text{E}{-}6text{ m}^2/text{s}^2)). For the first-order derivatives, both Eros and Didymos provided differences of the same magnitude, (1text{E}{-}8text{ m}^2/text{s}^2) and (2text{E}{-}8text{ m}^2/text{s}^2). Finally, regarding the maximum expansion degree, the convergence between the two methods at degree 100 for Eros and Didymos are provided by Dimorphos at degrees 27 and 35, respectively.

In Newtonian theory of gravitation, used in Earth’s and planetary sciences, gravitational acceleration is standardly regarded as the most fundamental parameter that describes any vectorial gravitational field. Considering only conservative gravitational field, the vectorial field can be described by a scalar function of 3D position called gravitational potential from which other parameters (particularly gravitational acceleration and gravitational gradient) are derived by applying gradient operators. Gradients of the Earth’s gravity potential are nowadays measured with high accuracy and applied in various geodetic and geophysical applications. In geodesy, the gravity and gravity gradient measurements are used to determine the Earth’s gravity potential (i.e., the geopotential) that is related to geometry of equipotential surfaces, most notably the geoid approximating globally the mean sea surface. Reversely to the application of gradient operator, the application of radial integral to gravity yields the gravity potential differences and the same application to gravity gradient yields the gravity differences. This procedure was implemented in definitions of rigorous orthometric heights and differences between normal and orthometric heights (i.e., the geoid-to-quasigeoid separation). Following this concept, we introduce the radially integrated geopotential, and provide its mathematical definitions in spatial and spectral domains. We also define its relationship with other parameters of the Earth’s gravity field via Poisson, Hotine, and Stokes integrals. In numerical studies, we investigate a spatial pattern and spectrum of the radial integral of the disturbing potential (i.e., difference between actual and normal gravity potentials) and compare them with other parameters of gravity field. We demonstrate that the application of radial integral operator smooths a spatial pattern of the disturbing potential. This finding is explained by the fact that more detailed features in the disturbing potential (mainly attributed to a gravitational signature of lithospheric density structure and geometry) are filtered out proportionally with increasing degree of spherical harmonics in this functional. In the global geoidal geometry (and the disturbing potential), on the other hand, the gravitational signature of lithosphere is still clearly manifested—most notably across large orogens—even after applying either spectral decompensation or filtering.

The installation of permanent seismic stations in remote areas of the Russian Arctic faces challenges caused by logistical and technical difficulties and Arctic climate features. Considering that organizing permanent seismic observations within scientific academic projects is economically reasonable near accessible and well-developed infrastructural objects (e.g., meteorological stations), among all the steps of organizing seismic observation points, an important one is the selection of a place for seismometer installation that will allow for recording seismic data of the highest possible quality. It is well known that such geological medium features as paleotectonic faults, sand lenses or fractured rocks in the vicinity of the seismic station can decrease the seismic observation quality because of signal scattering. In that case, to minimize the impact of all possible aspects, information about the structural features of the local geological environment under possible installation places is important along with knowledge about the ambient noise level in the vicinity of possible seismological observation points. Since detailed geological information for hard-to-reach areas is scarce or absent, an express way to obtain useful information “on the field” is needed. In this study, we describe a way to select the most optimal place for seismometer installation by analyzing spectral characteristics of ambient noise and qualitative information about structural features of the local geological environment obtained by the set of passive seismic methods as well as some aspects and challenges we faced in the permanent seismic observation organization on the Novaya Zemlya archipelago.

Lightning is a highly energetic electric discharge process in our atmosphere, evolving in several complex stages. Lightning is recognized as an essential climate variable, as it affects the concentration of greenhouse gases. It also threatens electrical and electronic devices, in particular, on elevated structures like wind turbines, and it endangers aircraft built with modern composite materials with inherently low electric conductivity. During the past decades, our fundamental understanding of atmospheric electricity has continued to evolve. For example, during the past 30 years, discharge processes were discovered in the atmosphere above thunderstorms, the so-called transient luminous events (TLEs) in the stratosphere and mesosphere, and terrestrial gamma-ray flashes (TGFs), accompanied with beams of photons, electrons and positrons, were observed from low orbiting satellites passing over thunderstorms. Lightning-like discharges also appear in plasma and high-voltage technology. The SAINT network was formed to bring the different research fields together. SAINT was the “Science And INnovation of Thunderstorms” Marie Skłodowska-Curie Innovative Training Network of the European Union Horizon 2020 program. From 2017 to 2021, 15 PhD students observed lightning processes from satellites and ground, developed models and conducted laboratory experiments. The project bridged between geophysical research, plasma technology and relevant industries. The paper presents a summary of the findings of the SAINT network collaboration.