Abstract: Global positioning system (GPS) refers positioning, timing and navigation services for different engineering applications. GPS positioning accuracies vary depending on the several parameters such as surveying method, data processing strategy and software packages. Bernese v5.2 software package is an important tool for processing and analyzing of the GPS measurements especially for the precise applications in scientific community. Although the accuracies of the estimated coordinates are sufficient, Variance–Covariance (VCV) matrices obtained from Bernese v5.2 are very optimistic because the correlations between different observables may be ignored by choosing identical weights for each measurement type in the analysis. This situation causes wrong interpretations for statistical analyses based on these VCV matrices. Therefore, the VCV matrices obtained from software should be scaled. In this study, the VCV matrices obtained from Bernese v5.2 were investigated for GPS measurements to estimate appropriate scale factor (SF) values. Baselines whose lengths ranging from 55 to 268 km and session durations between 2 and 24 h were processed with single baseline strategy for 31 consecutive days. According to the results, SF values do not depend on baseline lengths; but they vary depending on the session durations. A logarithmic function was defined for time-dependent SF values. This function has been tested in deformation analysis at the global test step and the obtained results, when the SF values are taken into account, are more reliable than the results when the unscaled VCV matrices are implemented. PubDate: 2019-03-27

Abstract: This work is concerned with refining the Helmert solutions in two different cases, namely combining different solutions and updating old solutions with newly available measurement data. The parameters to be estimated in the solutions here not only include the transformation parameters, namely the rotation, the translation and the scale, but also the coordinates in the final frame of all relevant stations. The rotations are not limited to small angles, however, the rotation estimation errors are assumed with small angles. After developing in detail the functional model and the stochastic model of either of the two cases, the parameters are estimated directly using the least-squares method, with the full variance–covariance matrix of the parameter estimates readily provided as a by-product. These variances and covariances of/between the parameters fully capture the statistical information of the final solution, which are important components of the resulting frame network besides the parameters themselves. Simulations are conducted to check the method for both the combination and update cases. It is found that in both cases, the accuracies of the transformation parameters and coordinates of some stations are improved after the combination or update. There are also some stations whose coordinate estimation accuracies remain unchanged; and it is emphasized that including these stations is also necessary in the combination and update for completely describing final frame network. PubDate: 2019-03-19

Abstract: The Global Navigation Satellite System (GNSS) precise point positioning (PPP) technology is currently used to process GNSS water vapor observations in real time or near real time. Further developments are required to improve the accuracy and real-time performance of processing tropospheric delays from which the water vapor observations are extracted. In real-time BDS/GPS precise clock correction estimation with square-root information filtering and PPP solution with Kalman filtering, a fixed variance is often assigned for the process noise of the troposphere dynamics model. However, this fixed value may deviate from reality as the weather conditions change, especially for extreme weather. In this paper, a new adaptive Kalman filter is proposed for tropospheric delay processing, in which the variance of the process noise for the zenith tropospheric delay (ZTD) dynamics model is tuned in real time using the least-squares variance component estimation technique. MGEX/IGS data of 15 consecutive days were processed in a stepwise manner. Namely: Real-time BDS/GPS precise clock corrections were estimated firstly, followed by comparison among ZTD solutions of four schemes based on these real-time clocks of first step and GFZ multi-GNSS precise clock (GBM) final clocks: (1) real-time solution of ZTD (G(GPS), GC(GPS and BDS)) without VCE; (2) real-time solution of ZTD (G, GC) with VCE; (3) final solution of ZTD (G, GC, GR (GPS and GLONASS), GE (GPS and GALILEO), GRC (GPS, GLONASS and BDS), GREC (GPS, GLONASS, GALILEO and BDS) without VCE; and (4) final solution of ZTD (G, GC, GR, GE, GRC, GREC) with VCE. The performance of the ZTD and positioning solution was analyzed. Results showed that the accuracy of estimated real-time satellite clock correction was 0.27, 1.31, 0.29, and 0.21 ns for GPS, BDS/GEO, BDS/IGSO, and BDS/MEO, respectively. For the ZTD solutions, the results of schemes 1 and 2 were 12.8 mm (GC) and 10.1 mm (GC) in terms of mean root-mean-square (RMS) values and 2.1 mm (GC) and 1.6 mm (GC) in terms of minimum RMS values, respectively, thereby showing an improvement for scheme 2 of 1.8–81.4% over scheme 1 with average increasing rates of 20.7% (GC) and 20.2% (G). The results for schemes 3 and 4 were 7.6 mm (G) and 6.3 mm (GRC) in terms of mean RMS values and 2.1 mm (G) and 1.9 mm (GRCE) in terms of minimum RMS values, respectively, thereby showing an improvement in scheme 4 over scheme 3 by 1.3–85.6% with average increasing rates of 22.1% (GRCE), 21.9% (GRC), 18.4% (GR), 15.9% (GC), 15.2% (GE), and 12.1% (G). Similar results can be observed for the positioning solution, especially in the height component. These findings clearly show the advantage of the proposed method, which is consistent with the theoretical analysis. Notably, the advantage of the adaptive VCE becomes significant with the inclusion of additional satellite systems. PubDate: 2019-03-01

Abstract: Locating the edges of anomalous bodies provides a fundamental tool in the geologic interpretation of potential field data. This paper compares the effectiveness of the commonly used edge detection methods such as the total horizontal gradient, analytic signal, tilt angle, theta map and their modified versions in terms of their accuracy on the determination of edges of source bodies. This paper also introduces an edge detector method for the enhancement of potential field anomalies, which is based on the logistic function of the total horizontal gradient. The new method is tested on synthetic data calculated using 3 models, and also on real magnetic and gravity data from Vietnam. The effectiveness of the method is evaluated by comparing the results with those of other popular methods. These results demonstrate that the method is a useful tool for the qualitative interpretation of potential field data. PubDate: 2019-03-01

Abstract: We discuss the determination of gravity gradients from the orbital ceiling to the depth of the Mohorovičić discontinuity (Moho) for Central Europe. Components of the Eötvös tensor were derived from “Heterogeneous gravity data combination for Earth interior and geophysical exploration research” project (“GOCE+”) by using the gridded data with a resolution of 0.2° per 0.2°. Gravity gradients to Moho boundary depth were modelled forward to the 255 km orbital height. We calculated gradient sensitivity using a 3D model divided into: sediments and consolidated crust including the precise location of the Moho boundary. To define tesseroids as mathematical model we need to set two parameters of the crust: density and thickness for each spherical layer separately. Altitudes for topography/bathymetry were derived from ETOPO1 model, sediments thickness from EuCRUST-07 model, and Moho boundary from Grad and Tiira (Geophys J Int 176(1):279–292, 2009. https://doi.org/10.1111/j.1365-246x.2008.03919.x) seismic map. For high latitudes, we noted the largest changes for the gradients towards the poles, with particular values of 689.07 mE (milli-eotvos) and 1138.19 mE for VXX and VZZ gradients, respectively. We obtained extreme values for the location of the deep and shallow areas of the crust (Alps, North-Eastern Poland and areas of seas) equal to − 3 E and + 1.5 E, respectively. Most of the gradients showed strong correlation with anomalies in crustal density of − 2.5 E for VZZ and + 1.5 E for VYY in the extreme cases. We showed that changes in crust density and thickness by respectively 50 kg/m3 and 10 km entail changes in gradient values by 15% for density and 10% for depths. Numerical analysis considering Preliminary Reference Earth Model (PREM) showed importance of density modeling for determination of gravity gradients. PubDate: 2019-03-01

Abstract: The Navigation with Indian Constellation (NavIC), also known as Indian regional navigation satellite system, is a regional navigation satellite system recently developed by India. Its service area covers from 30°E to 130°E and from 30°S to 50°N. In this contribution, an assessment of NavIC from aspects of data quality, usability and single point positioning (SPP) performance is carried out using real measured data collected from four sites both within NavIC’s primary and secondary service areas. Data quality of NavIC’s signal is assessed measuring its carrier-to-noise-density ratio and each satellite’s orbital period is calculated using its broadcast ephemeris. Visible satellite number and DOPs values of each site in modes of NavIC-only, GPS-only and GPS/NavIC are counted and calculated respectively. SPP solutions in modes of NavIC-only, GPS-only and GPS/NavIC are also carried out for these four sites. The results show that: the signal strength of NavIC’s L5 frequency generally equals that of GPS in site IISC (Fig. 1); mean orbital periods of IGSO and GEO satellites are 86160.70 s and 86152.03 s respectively; in site IISC (within the primary service area), currently NavIC system can provide an independent positioning service with an accuracy of less than 1 m in the east direction and less than 2 m in north and up directions respectively; the usability and SPP performance can be improved significantly in the mode of GPS/NavIC compared with those in either single mode. PubDate: 2019-03-01

Abstract: The article presents the results from a static part of the load test for the Zglavje viaduct, located on the motorway A1 in Slovenia and the feasible usage of modern geodetic instruments for determination of dynamic response of a structure. Therefore, static analysis was employed for comparison of the implemented methods by which vertical displacements were measured and compared to a calculated displacement. High agreement of the results was established. In addition, experimental part of the geodetic non-contact methods to determine dynamic response of the structure was implemented on the railway bridge across the Mura river on the railway line Ormož–Hodoš in Slovenia. Geodetic non-contact methods are evidently more and more applicable at determination of dynamic response for their technologic development in the area of speed and constant data capturing. Our results were provided by employing the Robotic Total Station (RTS). PubDate: 2019-02-22

Authors:P. Nenovski Pages: 555 - 577 Abstract: Recently several cases of observations of unipolar magnetic field pulses associated with earthquakes at different points (California, Italy, Peru) have been recorded. The paper attempts to model unipolar magnetic field pulses based on one mechanism that should be omnipresent for all measurement points, namely, the magnetic field diffusion through a conductive medium. The structure of magnetic fields supported by electric current sources is thoroughly modelled. The source of electric current is considered as an elongated volume of finite cross-section being immersed in a conductive medium. To model the unipolarity feature of the observed pulses prior to and at the earthquake main shock, the electric current of the source is of impulse form. Special attention is paid to the differences in the pulse structure (as amplitude envelope and the pulse width) that are measured by various magnetometers (fluxgate or search-coil). An analysis and comparison with recorded magnetic field pulse characteristics reveal that the observed unipolar pulses may have a common genesis, an electric current source within a conductive medium such as the earth crust. PubDate: 2018-12-01 DOI: 10.1007/s40328-018-0219-y Issue No:Vol. 53, No. 4 (2018)

Authors:Bapan Paul; Barin Kumar De; Anirban Guha Pages: 579 - 606 Abstract: The latitudinal ionospheric response of the three strongest geomagnetic storms of 2015 of the current solar cycle 24 during 16–19 March 2015, 21–24 June 2015 and 19–22 December 2015 is investigated using the total electron content data derived from a latitudinal chain of Global Positioning System (GPS) receivers extending from 70°N to 70°S. The storm time perturbations of the ionosphere during main and recovery phase is presented by the GPS derived vertical total electron content (VTEC) data which is further supported by ionospheric F region critical frequency (foF2) and F region peak height (hmF2) data. We observed symmetrical hemispheric response of the ionosphere during the strongest 17th March (St. Patrick’s Day) storm whereas asymmetrical hemispheric response of the ionosphere during 22nd June and 20th December storm over the Asian-Australian sector. The observations are explained by the combined transport of background inter-hemispheric seasonal wind and storm time disturbed meridional wind and by the global thermospheric compositional variation [O/N2] data. PubDate: 2018-12-01 DOI: 10.1007/s40328-018-0221-4 Issue No:Vol. 53, No. 4 (2018)

Authors:M. Radulian; A. Bălă; L. Ardeleanu; D. Toma-Dănilă; L. Petrescu; E. Popescu Abstract: The purpose of this paper is to present the most comprehensive catalogue of focal mechanisms for Romanian earthquakes which occurred between 1929 and 2000 in the Carpathian Orogen, the Moesian and Moldavian Platforms, and the Transylvanian Basin. The present catalogue (REFMC) is a first step toward creating a centralized and continuous database of earthquake mechanisms in Romania by revising and updating existing data for the twentieth century, which together with the Romanian earthquake catalogue (ROMPLUS)—continuously updated by the National Institute for Earth Physics, provides the fundamental information for any seismicity or seismic hazard assessment. In order to produce a close-to definitive version compatible with more recent and less uncertain focal mechanisms solutions, we revised multiple sets of data (some of which newly found), recalculated and corrected some of the fault-plane solutions and reached a consensus. The catalogue comprises 250 crustal events and 416 intermediate-depth events recorded in the twentieth century starting from 1929. On the basis of the new catalogue data and seismotectonic investigation, we propose a reconfiguration of the seismogenic zones located along Southern Carpathians toward the western side of Romania. PubDate: 2018-12-05 DOI: 10.1007/s40328-018-0243-y

Authors:Ya Mao; Qian-xin Wang; Chao Hu; Hong-yi Yang; Xu Yang; Wei-xuan Yu Abstract: During the on-orbit operation of BeiDou satellites, the on board atomic clocks of these satellites are easily affected by changes in the space environment. Since the clock offsets of BeiDou satellites derived from multi-satellite clock estimation exhibit correlation among one another. In this study, the correlation among the clock offsets of BeiDou satellites is analyzed and the influence of correlation among satellites on clock offset prediction accuracy is investigated. To obtain accurate analysis results, the Baarda outlier detection method is initially improved. The improved method can effectively eliminate small errors in clock offset data. Then, the correlation coefficient among BeiDou satellites is calculated, and the method based on the correlation among BeiDou satellite clocks is used to predict the clock offsets of the satellites. Experimental results show that clock offset prediction accuracy can be improved by 7.3% compared with that of the traditional method when the method that considers correlation among satellite clocks is used. PubDate: 2018-11-29 DOI: 10.1007/s40328-018-0242-z

Authors:Peng Feng; Fei Li; Jianguo Yan; J.-P. Barriot Abstract: In this study, done in the frame of the MGEX (Multi-GNSS Experiment) project, for 20 selected worldwide stations, and for the whole year of 2015, we compare zenithal total delay (ZTD) estimated values from BeiDou/GPS combined signals, to ZTD estimates from GPS-only signals, and also to ZTD estimates from the IGS analysis center CODE (CODE products), in order to assess the intrinsic accuracies of these ZTD estimates. We used the PANDA software from Wuhan university for all our data processing, with precise orbits (PPP) from the GFZ IGS analysis center. We found that the GPS-only ZTD estimates show a very good agreement with the CODE final ZTD products, but that a systematic negative bias of around 3 mm is showing up between the ZTD estimates from combined GPS/BeiDou data w.r.t. GPS-only data. This indicates that the accuracy of Beidou satellite orbits and clock errors still need to be improved w.r.t. to IGS standards. PubDate: 2018-11-26 DOI: 10.1007/s40328-018-0240-1

Authors:Huaien Zeng; Guobin Chang; Haiqing He; Yi Tu; Shuifa Sun; Yue Wu Abstract: The rigid motion involving both rotation and translation in the 3D space can be simultaneously described by a unit dual quaternion. Considering this excellent property, the paper constructs the Helmert transformation (seven-parameter similarity transformation) model based on a unit dual quaternion and then presents a rigid iterative algorithm of Helmert transformation using a unit dual quaternion. Because of the singularity of the coefficient matrix of the normal equation, the nine parameter (including one scale factor and eight parameters of a dual quaternion) Helmert transformation model is reduced into five parameter (including one scale factor and four parameters of a unit quaternion which can represent the rotation matrix) Helmert transformation one. Besides, a good start estimate of parameter is required for the iterative algorithm, hence another algorithm employed to compute the initial value of parameter is put forward. The numerical experiments involving a case of small rotation angles i.e. geodetic coordinate transformation and a case of big rotation angles i.e. the registration of LIDAR points are studied. The results show the presented algorithms in this paper are correct and valid for the two cases, disregarding the rotation angles are big or small. And the accuracy of computed parameter is comparable to the classic Procrustes algorithm from Grafarend and Awange (J Geod 77:66–76, 2003), the orthonormal matrix algorithm from Zeng (Earth Planets Space 67:105, 2015), and the algorithm from Wang et al. (J Photogramm Remote Sens 94:63–69, 2014). PubDate: 2018-11-20 DOI: 10.1007/s40328-018-0241-0

Authors:Luminiţa Ardeleanu Abstract: An inversion scheme using only few good quality high-frequency local waveforms was previously applied to estimate the source mechanism for a pilot set of crustal earthquakes with local magnitudes less than 4, from the bend of the South-eastern Carpathians. Taking advantage of the improved velocity and attenuation models recently determined for the study region, we reevaluate the focal mechanism of a couple of low-magnitude events (local magnitude ≥ 3), and assess the uncertainty of the fault plane solutions. The results evidence the increased reliability of the retrieved mechanisms and emphasize the capability of the approach to provide satisfactorily constrained fault plane solutions for the weak-to-moderate earthquakes in the study region. PubDate: 2018-11-16 DOI: 10.1007/s40328-018-0239-7

Authors:Guiwu Chen; Lei Song; Ray Ruichong Zhang Abstract: The acoustic response has many important roles in seismic exploration and nondestructive testing. It enables the development of fracture classification and sizing. In this paper, we combined Hudson’s effective medium scheme and finite-difference time-domain modeling method to simulate acoustic wave propagation in fractured media. Fractures are represented by discrete fracture networks, allowing for a state-of-the-art representation of natural fracture networks by a negative Exponential Law length distribution. The propagation of acoustic waves that are emitted by a point source and reflected from a fractured area in a 2D digital rock model are examined numerically with the purpose of developing an acoustic inference of fracture properties. In these fractured models, we vary the number and mean length of fractures to explore the relation between internal structure of rock and acoustic wave field characters. The modeling results indicate that acoustic wave field is more sensitive to the fracture number than to the mean of the fracture length. Moreover, a fracture-dependent attenuation analysis of the reflection records of discrete stochastic fractured models is obtained. The frequency- and time- dependent attenuation profiles feature two parts in frequency, (1) fracture-to-background at lower frequencies and (2) fracture-to-fracture at higher frequencies. Our results indicate that accounting for attenuation effects may not only allow for improving estimation of fracture number, but also provide information about geometrical characteristics of length distribution. Such an approach can be used to estimate nature fracture network properties with given acoustic records. PubDate: 2018-11-12 DOI: 10.1007/s40328-018-0237-9

Authors:Miloud Chermali; Fouzi Bellalem; Mohamed Walid Belgroun; Amar Boudella; Mohand Ouabdallah Bounif Abstract: The Wenner and offset Wenner array is among the quite recent configurations used to assess and reduce the disturbing lateral effect in resistivity soundings. Even if this system could be considered as one of the most efficient configurations for this purpose so far, the restricted number of measuring points makes its vertical resolution limited. To fix this limitation, Barker found out a specific equation by which resistance values at points halfway between the actual measuring ones are computed without being measured. This aim was reached by combining expressions of electric potentials at various electrode spacings. For the same purpose, three other equations were worked out and tested. For a single sounding, resistance values were computed at intermediate points by the mean of these equations. Observational errors were also calculated using Barker’s formulae. The computed resistance values were found to be contaminated by these errors, in varying degrees to the extent that some of them were lacking reliability. Furthermore, at each electrode spacing, the differences between the interpolated and the assumed resistivity values on a straight line joining the actual measuring points were calculated. After normalizing these differences to their corresponding averages, the RMS of these normalized differences were computed for a set of soundings using the four equations. Finally, a comparison of the obtained results with the errors of measurements from the reliability of the computed resistance values was made. This comparison allowed classifying these equations from the reliability of the intermediate resistivity values perspective. In addition, Barker’s choice of one of these equations as the one that gives the most satisfactory results is justified. PubDate: 2018-10-11 DOI: 10.1007/s40328-018-0236-x

Authors:Sumesh Gopinath; P. R. Prince Abstract: In the present study, information theoretic distance-based entropies have been employed for a better understanding of nonlinear features of the magnetosphere using proxies such as AE and Dst indices. Among the various distance-based entropies, approximate and sample entropies are considered as potential quantifiers which could track the nonlinear variations of the magnetospheric system. The generalized nonextensive Tsallis q-entropy and Fisher’s information measure are used to study the nonextensive entropy and complexity respectively of the magnetospheric dynamics. For the analysis, 1-min AE and Dst indices are considered during the period 1985–2007. The results indicate that nonlinearity and nonextensive entropy of Dst index are solar activity dependent. But, the nonlinearity and nonextensive measures of AE index are not having any solar activity dependence. This implies that, other than the modulating solar wind, certain other complex phenomena of internal origin are having influence on the dynamics of geomagnetic activity in the auroral zone. PubDate: 2018-09-19 DOI: 10.1007/s40328-018-0235-y

Authors:Erman Şentürk; Murat Selim Çepni Abstract: In this study, daily mean vertical total electron content (VTEC) and daily mean 2-h VTEC values were obtained from Center for Orbit Determination in Europe–Global Ionosphere Maps (CODE–GIM) data over the region of Turkey between January 1, 2003, and December 31, 2016. The time interval is sufficient to reflect temporal changes of the ionosphere. The daily mean VTEC data was used to analyze the space weather effects on the VTEC variability and daily mean 2-h VTEC data was utilized to see the pattern of the diurnal, monthly, seasonal and yearly variation of VTEC values. The highest correlation was found between VTEC and F10.7 (r = 0.83). Totally, 40 major geomagnetic storms were identified that 45% of the storms are caused a decrease and 55% of the storms are caused an increase in VTEC variation. The maximum VTEC is shown at 13:00 LT and the minimum VTEC is shown at 03:00 LT according to diurnal variation of the 14-year mean 2-h VTEC. The maximum VTEC is shown on April and the minimum VTEC is shown on July according to diurnal variation of monthly mean VTEC. Diurnal variation of seasonal mean VTEC and its standard deviations are higher in equinox than solstices. Diurnal variation of yearly mean VTEC has a significant change from low to high solar activity periods. PubDate: 2018-08-09 DOI: 10.1007/s40328-018-0233-0

Authors:Ilke Deniz; Gokhan Gurbuz; Cetin Mekik Abstract: Studies of estimating precipitable water vapor (PWV) from continuous Global Navigation Satellite System (GNSS) stations with high temporal and spatial resolution continuously have become popular in recent years. In this estimation process, the weighted mean temperature (Tm) and the conversion parameter (Q) are the most important parameters to convert tropospheric zenith delay (ZTD) to PWV. In this study, Tm and Q time series are derived by assessing 4103 profile observations of eight Turkish radiosonde stations (RS) for approximately one year. The Tm − Ts linear regression model is developed. In analogy to Tm − Ts model, the Q values are modelled based on different combinations of surface temperature (Ts), station latitude (θ), station height (H) and day of the year (DOY). To test the validity of these models, the GNSS derived PWV (PWVGNSS) values are computed from the GNSS data of just over a year for the Istanbul and Ankara RS-GNSS stations using the most precise Tm and Q models, and later they are compared with the PWVRS values. The mean of the differences obtained for the Ankara and Istanbul stations are found to be 1.4–1.6 mm with a standard deviation of 1.7–1.8 mm, respectively. Moreover, modelling and interpolating meteorological parameters such as temperature, pressure, as well as PWV and ZTD are tested using the spherical harmonic functions (SHF). The results indicate that SHF can be safely and accurately used for modelling and interpolating meteorological parameters and ZTD. PubDate: 2018-08-07 DOI: 10.1007/s40328-018-0232-1