Studia Geophysica et Geodaetica最新文献

The ionospheric response to geomagnetic storms is usually investigated by considering the variability of the critical frequency of the F2-layer (foF2) or the total electron content (TEC) because these two parameters are directly measured by the ionosonde stations and the Global Navigation Satellite Systems (GNSS). In the present paper, however, the reaction is explored by using the vertical profiles of the electron density, N(h), reconstructed by manually scaled ionosonde measurements at the station Sofia (42.4°N, 23.2°E). The mid-latitude ionosheric response to three geomagnetic storms that occurred in January 2005 is presented as this period has been selected because no major sudden stratospheric warming occurred during this month, and the winter 2005 is given in the literature as an example of a “normal” year. Hence the observed ionospheric response to the considered geomagnetic storms can be attributed mainly to the external forcing. Besides the traditional parameters foF2 and TEC, a particular attention is paid to the variability of the peak electron density height (hmF2). This study reveals for the first time that the main contribution to the response of the midlatitude ionosphere to moderate/intense winter geomagnetic storms is associated with significant enhancements of short-period quasi-diurnal oscillations with period of 6–7 hours observed in both foF2 and hmF2. An explanation of the main mechanisms responsible for the distortion of the diurnal ionospheric variability during these storms is offered. This result is especially important for the ground-based HF radio communications.

A general triaxial ellipsoid is suitable to represent the reference surface of the celestial bodies. The transformation from the Cartesian to geodetic coordinates on the triaxial ellipsoid becomes an important issue in geodesy. In the literature, the vector iterative method and the Newton’s iterative method for solving the nonlinear system of equations or an algebraic fraction equation is applied to compute the geodetic coordinates, but may lead to the non-convergence regions. In this work, the universal algorithm including the Newton’s iterative solutions of an algebraic sextic equation for the points outside the equatorial plane and the analytic solutions for the points inside the equatorial plane are used to compute the geodetic coordinates. The numerical experiments show the algorithm is fast, highly accurate and well convergent. The algorithm is valid at any point inside and outside the celestial bodies including the points near the celestial bodies’ center and in the singular elliptical disc.

M_split(q) estimation is a development of M-estimation which is based on the assumption that a functional model of observations can be split into q competitive ones. The main idea behind such an assumption is that the observation set might be a mixture of realizations of different random variables which differ from each other in location parameters that are estimated. The paper is focused on the robustness of M_split(q) estimates against outlying observations. The paper presents derivatives of the general expressions of the respective influence functions and weight functions which are the main basis for theoretical analysis. To recognize the properties of M_split(q) estimates in a better way, we propose considering robustness from two points of view, namely local and global ones. Such an approach is a new one, but it reflects the nature of the estimation method in question very well. Thus, we consider the local breakdown point (LBdP) and the global one (GBdP) that are both based on the maximum sensitivities of the estimates. LBdP describes the mutual relationship between the “neighboring” M_split(q) estimates, whereas GBdP concerns the whole set of the estimates and describes the robustness of the method itself (in more traditional sense). The paper also presents GBdP with an extension, which shows how an outlier might influence M_split(q) estimates. The general theory proposed in the paper is applied to investigate the squared M_split(q) estimation, the variant which is used in some practical problems in geodesy, surveying, remote sensing or geostatistics, and which can also be applied in other geosciences.

The aim of this study is to determine an accurate geoid model for Iran based on the Least Squares Collocation method in the framework of the Remove — Compute — Restore technique. In areas suffering from a lack of homogeneous and accurate gravity anomaly data, as is the case of Iran, the choice of the most compatible global gravity model has a significant impact on the estimated form of the geoid. Different combined and satellite-only global gravity models were therefore analyzed for Iran, and EIGEN6C4 was selected as the best one. The Shuttle Radar Topography Mission height model was used for the residual terrain correction. The covariance modeling, a crucial step in the Least Squares Collocation method, was based on two strategies. In the first, the study area was divided into four sub-areas, and then an individual empirical covariance was computed and a covariance model fitted to each of them. In the second, an empirical covariance was computed using all terrestrial gravity data, and a unique covariance model was fitted to it. Despite some border effects, the former strategy showed slightly better performance according to the resulting statistics, and therefore it was preferred for the estimation of the geoid model called IRG2018. To remove the offset of IRG2018 with respect to GNSS/Leveling-derived geoid heights, two alternative approaches were tested: subtracting a fitting polynomial surface or directly using the GNSS/Leveling data as an input to the IRG2018 computation process. Evaluation of the results, based on an independent control set of approximately half of available GNSS/Leveling points, showed an advantage of the latter approach, with an estimated accuracy of about 20 cm in terms of RMS.

A fault slip within the ?a?a tunnel Lima, Peru has been monitored since 2012. The data are recorded using an optical-mechanical 3D extensometer, capable of providing very precise long-term three-dimensional measurements of relative displacement across discontinuities. The ?a?a tunnel has an extremely stable environment and cannot possibly be affected by gravitationally-induced mass movements. The host rock of the tunnel is an aphanitic hornblende-bearing basaltic trachyandesite. Several fault and fracture zones detected in the tunnel represent the major discontinuities of the broader surrounding of the monitored site. The recorded fault slip on the NNW-SSE and E-W striking fracture and fault, with inclinations of 76° to WSW and 78° to N respectively, appoint to compressional event (discontinuity contraction) with a maximum horizontal compression stress axis oriented approximately WSW to ENE corresponding to the direction of the Nazca and South America plates convergence. This event resulted in an aseismic slow fault slip between July 2012 and May 2013. The anticipated compression orientation matches the previously published in-situ stress measurements and fault plane solutions, as well as GPS measurements of the movements of the corresponding part of the Peruvian coast. Nevertheless, the presented monitoring results reflect only short-term fault slip dynamics and need to be considered with caution, even though they correspond to the overall tectonic activity driven by continental subduction.

The ray-matrix method can extend the applicability of ray methods to 3-D heterogeneous velocity models containing thin stacks of fine isotropic or anisotropic layers. Ray-matrix synthetic seismograms are compared with finite-difference synthetic seismograms in a simple 1-D velocity model, consisting of a low velocity layer covering a homogeneous halfspace. The agreement between the ray-matrix and finite-difference synthetic seismograms is very good. Moreover, the ray-matrix seismograms do not suffer from extremely thin layers, from anisotropy, and from reflections from the non-reflecting boundaries typical for finite differences.

Global geopotential models are widely used in the remove-compute-restore technique for local gravity field modeling. In this paper, a method for regional improvement of global geopotential models using GPS/Leveling data is presented. The part of the spherical harmonic expansion degrees that can be subject to the regional improvement is determined depending on the spatial resolution of the GPS/Leveling data and the size of the study region. In this method, a global geopotential model is required as the original model. Using the GPS/Leveling data corrected for the systematic errors, the geoid surface is obtained at the GPS/Leveling points. By expanding the gravity potential of the geoid surface into the spherical harmonics, a mathematical model is made to estimate the spherical harmonic coefficients of the regionally improved geopotential model. To stabilize the mathematical model, pseudo data of the gravitational potential type produced by the original model on the entire Earth’s surface are added to the GPS/Leveling data. The relative weight of the two types of the data, i.e., the GPS/Leveling data and the pseudo data, is selected based on fitting the original model to the GPS/Leveling data. As numerical tests, the regionally improved geopotential model of the USA from degree 8 to 779 and the regionally improved geopotential model of Iran from degree 12 to 339 are developed. To develop both regionally improved geopotential models, the EGM2008 model up to degree 2160 is selected as the original model. The assessments at the GPS/Leveling checkpoints show that the regionally improved geopotential model of the USA has a 23% improvement and the regionally improved geopotential model of Iran has an 8% improvement with respect to the original model. The numerical tests confirm the efficiency of the proposed method for the regional improvement of global geopotential models using the GPS/Leveling data.

A fundamental tectonic boundary between the Precambrian East European Craton (EEC) and the younger Phanerozoic mountain belts of Europe runs through Poland. Whereas the transition zone between both tectonical units was intensely investigated by means of electromagnetic methods in the past, the image of electric conductivity of the EEC itself has not been detailed so far. In the present study the lithospheric structure beneath the northeast of Poland is investigated by means of three parallel magnetotelluric profiles. As a result the Polish part of the craton does not appear as homogeneously highresistive as repeatedly reported. Beneath a shallow (from 2 to 4 km) sedimentary layer a less resistive structure (several hundred Ωm) under all three profiles in mid-crustal to upper mantle depth becomes apparent in the high-resistive background. It spatially coincides with a magmatic body of the Upper Proterozoic age called ?niardwy lake gabbro intrusion.

Magnetovariation methods, which are applicable to study the mantle conductivity, require long lasting registration of natural magnetic field variations. Such data usually can be recorded in stationary conditions on geomagnetic observatories. The magnetotelluric and magnetovariation measurements have been carried out at seven semi-permanent sites on Ukrainian territory for approximately 10–15 years by the Ukrainian State Geological Research Institute as well as the S.I. Subbotin Institute of Geophysics of the National Academy of Sciences of Ukraine for monitoring the effects and phenomena related to electromagnetic field. Since the quality of records was acceptable, we used these data to study the Earth’s mantle conductivity peculiarities to obtain the information on sites remote from geomagnetic observatories. The Z-H method was applied to collected data to estimate the magnetovariaton transfer functions, which were concatenated with magnetotelluric ones and further inverted by the 1D OCCAM algorithm to obtain the geoelectrical cross sections for the measuring sites. To extend the region under study and to obtain more reliable results, similar data from three Ukrainian geomagnetic observatories in Kyiv, Lviv and Odessa were included in this study. Obtained conductivity models were related to the known geological structure in the upper and middle mantle beneath the Ukrainian territory.

In the literature, there are numerous derivative-based transforms for gravity and magnetic data sets, with which relevant features can be highlighted. However, almost all of them face the problem of instability in derivative calculation. Therefore, before applying derivative-based transforms, noise reduction is often applied to improve the quality of the data. Nevertheless, the application of conventional filters typically blurs horizontal gradients in the data, which can adversely affect subsequent transforms, for example, the sharp boundaries of the causative bodies may be obscured. To handle the above issue, this study is the first to employ the bilateral filter, used in digital image processing, for improving the derivative-based transforms for gravity and magnetic data sets. The filter replaces each data point by a weighted average of its neighbors. The established weights take into account both the geometric and amplitude closeness between the data points used. Synthetic tests indicate that the proposed method can effectively filter potential field data without distorting the structural features greatly. Thus, the performance of subsequent derivative-based transforms can be improved. The new method was applied to the magnetic data collected over the Dapai polymetallic deposit in Fujian Province, South China. This real example shows that the results obtained from the proposed method contain more pronounced features of existing faults and thus contributes to further geological interpretation.