Studia Geophysica et Geodaetica最新文献

The finite difference scheme is now widely used in the reverse time migration and full waveform inversion. Their results are dependent on the accuracy of finite difference operators. In this paper, we combine the cosine function with the original window function to construct a new window function, in order to obtain higher precision finite difference operators. The absolute error curves of the optimized finite difference operators are close to zero for low wavenumbers. In other words, we do not observe an oscillating curve of absolute errors produced by other optimized methods. In order to overcome the limitations of a single graphics processing unit (GPU), we developed the multiple-GPU method for the elastic wave equation. Numerical experimental results show that our new window function can control the numerical dispersion better than the binomial window and scaled binomial window, and the multiple-GPU computation is very stable.

Downward continuation is known as one of the crucial steps in interpreting gravity or magnetic data. As the continuation depth and the influence of noise increases, the results of downward continuation become unstable. Based on the computation of the Chebyshev-Padé approximation function obtained by the Tikhonov regularization, this paper proposes a new regularized method intended for the downward continuation of potential fields. The Chebyshev-Padé approximation function is applied to calculate the continuation factor. In this study, the cross-correlation method is adopted to calculate the cut-off wavenumber, while the regularized low-pass filter is designed to calculate the downward continuation of the potential field. In order to validate this method, numerical simulation is conducted. We calculate the root mean square error of the theoretical data on the target plane and the data of downward continuation, as obtained using the improved regularization operator method, the Chebyshev-Padé approximation function method, the regularized Chebyshev-Padé approximation function method, and the method proposed in this paper, based on which a comparison is conducted. According to the simulation and experimental results, the effects of the continuation depth can be reduced significantly by the proposed method. Besides, the method demonstrates strong resistance to noise.

Desert seismic events are disturbed and contaminated by strong random noise, which complicates the subsequent processing, inversion, and interpretation of the data. Thus, noise suppression is an important task. The complex characteristics of random noise in desert seismic records differ completely from those of Gaussian white noise such that they are non-stationary, non-Gaussian, non-linear and low frequency. In addition, desert seismic signals and strong random noise generally share the same frequency bands. Such factors bring great difficulties in the processing and interpretation of desert seismic data. To obtain high-quality data in desert seismic exploration, we have developed an effective denoising method for desert seismic data, which performs energy spectrum analysis in the empirical curvelet transform (ECT) domain. The empirical curvelet coefficients are divided into two different groups according to their energy spectrum distributions. In the first group, which contains fewer effective signals, a large threshold is selected to remove lots of random noise; the second group, with more effective signals, a coherence-enhancing diffusion filter (CEDF) is used to eliminate the noise. Unlike traditional curvelet transforms, ECT not only has the multi-scale, multi-direction, and anisotropy properties of conventional curvelet transform, but also provides adaptability to separate the effective signals from the random noise. We examine synthetic and field desert seismic data. The denoising results demonstrate that the proposed method can be used for preserving effective signals and removing random noise.

In an anisotropic model, traveltime can be determined approximately by numerical solution of the eikonal equation in terms of an anellipticity parameter η, using perturbation theory. However, its accuracy decreases under the effect of strong anisotropy at larger offsets. It becomes invalid for determining normal moveout velocity and anellipticity parameter in seismic processing. We propose a new approach using Levin T-transformation to transform the expanded traveltime in the transversely isotropic medium with vertical axis of symmetry (VTI) into rational form. The objective of this study is to provide a new traveltime approximation that is more accurate at larger offsets. In this study, we derive Levin algorithm and determine the optimal value of Levin parameters, which is a key step in achieving better accuracy. In a numerical experiment, we compare the accuracy between Levin T-transformation and second sequence of Shanks transformation in a homogeneous VTI medium. We also implement both approximations in a velocity analysis and stacking traces using synthetic common midpoint gathers on a multilayer earth model. The proposed method shows a superiority in accuracy to existing methods over a range of offsets with offset-to-depth ratio up to 6 and anellipticity parameter 0–0.5.

The relative monitoring gravimeter gPhoneX #108 was installed in the building of Faculty of Civil Engineering of the Slovak University of Technology in Bratislava in the beginning of 2016. Main purpose of the paper is to analyze how external environmental phenomena generating non-gravitational signal, such as strong human activity, ambient noise and tilting of observing site due to solar radiation, can influence the quality of relative gravity measurements and their suitability for geodynamic applications. For this purpose, we accomplished spectral analysis of gravity data with different sampling rate, examination of instrumental drift, and tidal analysis of hourly gravity records. Our study showed that the gPhoneX #108 gravity measurements are adversely affected by the tilting of the building, where the gravimeter is located. This effect produces a significant non gravitational signal with characteristic daily and seasonal variation depending on the weather conditions. The instability of the observing site also generates a strong non-gravitational signal and corrupts the estimation of tidal parameters mainly in a diurnal tidal band. The investigation of the instrumental drift proved its irregular character due to the changes of temperature in the operating room. Another limitation factor for detection of small gravity changes is the level of ambient noise with the average daily amplitude of about 100 nm s^?2. Obtained experimental results can be useful for planning and performing the relative gravity measurements in a noisy environment, or in the case of an instable observing site. The most significant non-gravitational signal with variable daily and seasonal influence was caused by the instability of the observing site. In order to minimize this influence we recommend regular calibration of the built-in tiltmeters.

Ionospheric long-term trend studies are of great scientific interest since they contribute to the more general and controversial climatic change issue. In this paper we analyze the effect of the inclusion of solar cycle 24 on estimation of the nighttime trend of the critical frequency of ionospheric F2 layer (foF2) for Kokubunji (35.7°N, 139.5°E) and Wakkanai (45.4°N, 141.7°E), two mid-latitude Japanese stations. Even though during the night recombination and transport processes prevail, ionization still depends directly on solar activity, so it must be filtered out before any long-term variation assessment. As usual, filtering is done considering the residuals of the regressions between foF2 with the solar radio flux at 10.7 cm (F10.7) and a ratio between the core and wing line intensities of the emitted ionized Magnesium doublet (Mg II index). Similar to the case of daytime foF2 values, night trends become less negative when solar cycle 24 is included since foF2 residuals systematically exceed the values predicted by F10.7 or Mg II from 2009 onwards. This effect is weaker in the case of Mg II, which is expected to be a better solar extreme ultraviolet (EUV) proxy than F10.7 in the case of the solar radiation involved in the F2 layer ionization. A plausible cause for cycle 24 incidence in the trend may be due to the use of F10.7 or Mg II as EUV proxy to filter solar activity. It is because they both seem to be inaccurate for filtering purposes since the last deep minimum in about 2008. Otherwise, there could be a real physical cause for this observation.

We present the first high-precision astrogeodetic vertical deflection (VD) observations collected in Istanbul, Turkey, using a novel lightweight total station integrated with a charge-coupled device (CCD) camera, the QDaedalus system. The observed VDs are unique in that, they were measured for the first time in Istanbul, and they form Turkey’s first dense astrogeodetic network. To establish the Istanbul Astrogeodetic Network (IAN), we selected 30 benchmarks (BMs) with known geodetic coordinates. A total of 21 of these BMs are located in the coastal zone allowing us to investigate the quality of global gravity field models (GGFMs) along the coast of Istanbul. The standard deviations for our VDs are approximately ±0.20″which is commensurate with the VD accuracy of early studies assessing the QDaedalus observations. In particular, dedicated comparison measurements were conducted in two geographic regions—Munich and Istanbul—to control the accuracy of the VD measurements. Our new VD data set within the IAN was compared with predicted VDs from the Global Gravity Model plus (GGMplus) and the Earth Gravitational Model 2008 (EGM2008). The VD residuals between the QDaedalus observations, and predicted values from GGMplus and EGM2008 models tend to increase towards the coastlines, where discrepancies of several arcseconds were found. At 15 coastal BMs, the residuals in the N-S components exceed 2″ and reach values as large as 6″ while residuals in the E-W components exceeded 2″ at 3 BMs. We interpret these large differences as an indication of the current weaknesses in the GGF Ms, most likely reflecting errors in the altimetry-derived marine gravity measurements, which have been incorporated in the EGM2008 and GGMplus models, or the lack of coastal terrestrial gravity measurements, or both. We conclude that the astrogeodetic VDs observed by the QDaedalus are invaluable for independently assessing the quality of coastal-zone terrestrial gravity data sets and GGFMs.

A well-known property of the classical least squares (LS) extrapolation is that a fit is best in the middle of the time span of observed data, but worse near the beginning and end of the time span. This phenomenon is called the edge effect in data processing. The goal of this work is to reduce the edge effect to improve predictions of the Earth rotation parameters (ERP), which comprise the Earth’s polar motion and rotation angle (the difference between the smoothed principal form of universal time UT1 and the coordinated universal time UTC) because a best LS fitting near the end of the data used is better for extrapolation. We first use the LS extrapolation for models consisting of one polynomial and two sinusoids in combination with an autoregressive (AR) technique to extend the observed time series forward. We then re-estimate the LS extrapolation model from the extended time series to reduce the edge-effect. ERP predictions are subsequently generated by combining of the edge effect reduced LS extrapolation and AR technique, denoted as ERLS + AR. Through an example, we demonstrate that the edge-effect in the observed data fitting can be reduced by re-estimating the LS extrapolation model with the extended time series. To validate the ERLS + AR method, we calculate the ERP predictions up to 365 days into the future year-by-year for the 4-year period from 2014 to 2017 using the data from the previous 8 years. The results show that the accuracy of the short-term predictions obtained by the ERLS + AR method is comparable with that achieved by the LS + AR approach in terms of the mean absolute error (MAE). However, an accuracy improvement is found mostly for long-term predictions based on the ERLS + AR method. The MAE for the UT1 ? UTC and polar motion predictions can decrease by approximately 15% to 20%, respectively. It is therefore suggested embedding the ERLS extrapolation algorithm into the existing ERP prediction procedure.

The objective of this study is to provide a well-dated point for a future palaeosecular variation (PSV) reference curve for western Russia. For this purpose archaeomagnetic and magnetic property analyses were carried out on a pottery kiln unearthed at the UNESCO World Heritage site of ancient Bolgar, having a rather precise age dating. The archaeological context provided an age between 1340 and 1360 C.E. The characteristic remanence vector was determined through alternating field demagnetisation and Thellier-Thellier palaeointensity experiments. Some innovations were introduced regarding palaeointensity. The check testing the equality of blocking and unblocking temperature was redefined. This allowed waiving the commonly used additional zero-field cooling steps during the Thellier-Thellier experiment. Another innovation concerns the calculation of archaeointensity at structure level. A Bayesian approach was introduced for averaging individual specimen archaeointensities using a prior probability distribution of unknown uncertainties. Next, an additional prior probability distribution was used to correct for cooling rate effects. This resulted in a lower uncertainty compared to common practice and in eluding time consuming cooling rate experiments. The complex magnetic mineralogy consists of maghaemite, multi-domain haematite and Al-substituted haematite. Some samples contained also some non-stoichiometric magnetite. The magnetic mineralogy was determined through hysteresis loops, backfield and remanence decay curves, measurements of the frequency dependence of magnetic susceptibility and through low temperature magnetisation curves. Accompanying high-temperature thermomagnetic analyses revealed an excellent thermo-chemical stability of the studied specimens. Directions obtained from alternating field demagnetisation and those extracted from archaeointensity experiments are congruent and have low uncertainties. The obtained archaeomagnetic results are fairly in agreement with global geomagnetic field models and contemporary PSV data of the wider area. The geomagnetic field vector obtained for ancient Bolgar is of high quality, deserving thus its inclusion in a future PSV reference curve for European Russia.

There is a growing interest in tidal effects on the global wind-driven oceanic circulation. Tidal models used in such investigations have been verified by comparison with satellite and tide gauge data, but synthetic tests have not been published. In this paper we present three numerical tests in spherical geometry, which are suitable for testing the tidal component of global ocean models. The first test is a tsunami-like propagation of an initial Gaussian depression with no external forcing. The other two tests examine the tidal response of an ocean with an undulating bottom with four Gaussian ridges and an ocean with a flat bottom with a realistic land mask. We provide the results from six model configurations, which differ in the time-stepping scheme and computational grid used. Most of them are implemented in present-day global ocean models. Although the proposed numerical tests are simple compared to realistic simulations, their analytic solutions are not available. We thus check the conservation of time invariants to ensure that the solutions are physically meaningful. We also compare the time evolution of certain physical quantities and the differences in sea surface heights at particular time instants with respect to a reference solution. All tested time stepping schemes are suitable for tidal studies except for the Euler implicit time stepping scheme. Model configurations based on the Arakawa grids B/E use smoothing to suppress the grid-scale noise which results in an energy leakage of around 5%. The B/E-grid energy leakage is probably acceptable if we consider that tuned diffusive terms are used in real-world configurations. The C-grid and B/E-grid solutions differ in the vicinity of solid boundaries as a consequence of different boundary conditions. The B-grid and E-grid solutions are similar, unless the shape of the solid boundaries is complex due to the different shapes of the respective grid cells.