Acta Geodaetica et Geophysica最新文献

In order to satisfy the Stokes’ solution to the Robin’s problem, the masses above the geoid are removed using an appropriate reduction scheme. This requires heights of the gravity station and at the integration or running points to be known with reasonable accuracy. The height data is used to compute the topographic, atmospheric, and downward continuation effects—essential elements of any geoid model—as well as to estimate free-air gravity anomalies. The height of the gravity station is usually tied to the local vertical network using standard heighting methods, ideally spirit levelling, while a Digital Elevation Model (DEM) is used to derive the heights at the integration points. In most developing countries, the gravity station heights are either not available or are unreliable for geoid modelling applications, since they were mostly observed to cater for the needs of geophysical exploration. By the end of the day, the geodesist has to accommodate the height information in the integral equations for computing topographical effects using the data available in the country. In this study, the different options for height information are investigated using the data available in Auvergne, central France. Geoid models are computed using different height data combinations to represent different scenarios which exist in different countries. Spherical one-dimensional (1D) Fourier transform is used to evaluate the Stokes’ integral in the framework of the Remove-Compute-Restore (RCR) technique. Results show that orthometric heights derived from a high-resolution Global Geopotential Model (GGM) with ellipsoidal heights may be as good as, if not better than, spirit-levelled heights, when used at the gravity station.

Nowadays, dual quaternion algorithms are used in 3D coordinate transformation problems due to their advantages. The 3D coordinate transformation problem is one of the important problems in geodesy. This transformation problem is encountered in many application areas other than geodesy. Although there are many coordinate transformation methods (similarity, affine, projective, etc.), similarity transformation is used because of its simplicity. Asymmetric transformation is preferred over symmetric coordinate transformation because of its ease of use. In terms of error theory, the symmetric transformation should be preferred. This study discusses the topic of symmetric similarity 3D coordinate transformation based on the dual quaternion algorithm, as well as the bottlenecks encountered in solving the problem and using the solution method. A new iterative algorithm based on the dual quaternion is presented. The solution is implemented in two models: with and without constraint equations. The advantages and disadvantages of the two models compared to each other are also evaluated. Not only the transformation parameters but also the errors of the transformation parameters are determined. The detailed derivation of the formulas for estimating the symmetric similarity of 3D transformation parameters is presented step by step. Since symmetric transformation is the general form of asymmetric transformation, we can also obtain asymmetric transformation results with a simple modification of the model we developed for symmetric transformation. The proposed algorithm can perform both 2D and 3D symmetric and asymmetric similarity transformations. For the 2D transformation, replacing the z and Z coordinates in both systems with zero is sufficient.

This study investigates the occurrence of Pc5 pulsations in the 2 to 7 mHz range at mid and low latitudes during the severe geomagnetic storm of June 21–24, 2015. This analysis employs two ground-based magnetic stations from the SAMBA network at magnetic latitudes − 16.55 and − 49.74, located in Chile and Antarctica, respectively, to explore the latitudinal distribution and drivers of Pc5 pulsations throughout the storm. Our results reveal that the intense pulsations recorded at low latitudes are synchronized with those observed at mid-latitudes and coincide with the arrival of the four interplanetary shocks associated with this event. Notably, the amplitudes of Pc5 pulsations exhibit significant latitudinal dependence, decreasing as they propagate from mid to low latitudes. Cross-correlation analyses highlight strong relationships between Pc5 amplitudes and solar wind parameters, indicating that solar wind dynamics played a critical role in the generation and propagation of these pulsations during this storm.

This study assesses the effect of the UNB Topographical Density Model on the accuracy of geoid determination in Sarajevo, Bosnia & Herzegovina. Using the KTH method, 1020 gravimetric geoid models were developed, incorporating both constant and variable density values, simple and complete Bouguer anomalies. The study found that the model computed by the UNB Topographical Density Model and complete Bouguer anomalies achieved the highest precision, with an RMSE of 1.33 cm. The final geoid model was adjusted to the old vertical datum (Trieste height), resulting in an RMSE of 3.44 cm when tested with static GNSS points. These findings underscore the importance of incorporating variable density models for improving geoid accuracy and suggest further refinement using local geological data could enhance precision.

This study explores the application of a rapid 3D cross-correlation imaging technique for analyzing potential field gravity and magnetic data. The approach involves calculating cross-correlation values between observed potential field anomalies and theoretical data, followed by the assessment of correlation coefficients. The method's efficacy was demonstrated through preliminary tests on various synthetic models with differing characteristics, highlighting its ability to detect and accurately locate subsurface sources, even in complex scenarios with inclined and multiple sources. The 3D imaging outcomes provided comprehensive insights into the location and center of anomaly sources. The methodology was then applied to ground magnetic and gravity datasets to evaluate the spatial distribution of underground iron deposits in the Shavaz region. The results closely matched drilling data and prior studies in the area, reinforcing the technique's effectiveness in interpreting real geophysical data. This research emphasizes the advantages of the cross-correlation imaging method in enhancing the understanding of subsurface structures, revealing hidden anomalies, and estimating the depth and arrangement of buried features.

Vertical electrical sounding (VES) technique is applied in this paper to assess and follow the trends of layered gradient resistivity (GR) variations and aquifer vulnerability to contamination in the Khanasser Valley region, Northern Syria. GR parameter, termed also as longitudinal resistance (LR), having the Ohm unit (Ω) is therefore developed in this paper. GR is by definition the inverse of longitudinal conductance (LC) (one of the second order Dar-Zarrouk geo-electric indices). A model composed of three hydrological layers is used to test and characterize the GR parameter, which are the Quaternary aquifer and its overlaying and underlying layers. Thirty four VES points already carried out in the study area with Schlumberger array, and a maximum half current spacing AB/2 of 500 m are used in this research. The one dimensional (1D) quantitative interpretations of those VES show that the first overlaying layer resistivity and thickness ranges are of 0.6–212 Ω.m with an average value of 56 Ω.m, and 1.1–28 m with an average value of 4.64 m respectively. The second Quaternary aquifer layer resistivity and thickness ranges are of 4.3–43 Ω.m with an average value of 15 Ω.m, and 4.5–59.2 m with an average value of 27.5 m respectively. The third underlying layer resistivity and thickness ranges are of 1.7–79.5 Ω.m with an average value of 11.77 Ω.m, and 14.3–283 m with an average value of 82.1 m respectively. The (GR) parameter shows also that the three modeled hydrological units are averagely vulnerable to contaminations.

Two resistivity reflection coefficients K1 and K2 are also developed and used to follow the resistivity distribution trends of the three mentioned modeled hydrological units, where an alternating sequence of clay, alluvial and sandy components of ramel aswad, and marly clay is decided. GR parameter can be therefore employed worldwide to follow the electrical resistivity variations with depth, for several useful applications, such as assessing aquifer vulnerability to contamination, mapping the geologic sequence with depth for evaluating the subsurface for mineral explorations and for locating the engineering sites.

In this paper, we calculate Local Disturbance indices (LDi) using data from two equatorial observatories (Ascension ASC and Fúquene FUQ) to use them as Disturbance Storm-time (Dst) index proxies. We find that the LDi response to geomagnetic storms is different depending on the observatory’s local time at the storm onset. In order to explore this local time influence on the measurements on the ground at low latitudes, we build new proxies using two observatories located at approximately the same longitude, in order to balance measurements in the north and south averaging meridional and measuring only zonal variations. The average of the longitude pairs and Dst-index proxies from single observatories exhibit strong correlation to the Dst index ((ge ) 0.88) during active periods and a moderate correlation ((le ) 0.5) during quiet periods. We find that the storm intensity is associated with local time. We confirm that the fastest variation in the geomagnetic field during the storm is recorded between dusk and midnight, while the region between dawn and noon records more moderate variations, sometimes missing the storm effects altogether. Our results show an azimuthal asymmetry of the magnetospheric ring current, becoming most intense on the night side of the dusk terminator during active periods. We propose a new configuration for local time Dst proxies including the use of equatorial observatories. This will get insights of the evolution of storms in an area where there are limited geomagnetic observatories.

Graphical abstract

Due to rapid increase in demand of building stones the geophysical prospecting of potential dimension stone (Dolerite Dykes, commonly known as “Black Granite”) deposits is of great significant. Dimension stone is widely distributed in the Hazara Valley of KPK, Pakistan. The primary aim is to characterize subsurface structural features and resistivity values crucial for dimension stone deposits, guiding the selection of viable mining zones. Employing electrical resistivity tomography (ERT), an electric current was introduced through electrodes embedded in the ground, demonstrating efficiency, cost-effectiveness, and data coverage. Conducted with the SuperSting R8/IP/SP instrument from AGI USA, the survey utilized a dipole-dipole array with 56 electrodes spaced at 20-meter intervals along eleven profiles. The 2D-ERT technique, or resistivity imaging, employed EarthImager software from AGI USA for data inversion, revealing distinctive low and high resistivity signatures along all profiles. Shallow high resistivity values indicated boulder material, while shallow low resistivity values pointed to subsurface dolerite dyke material. Profiles exhibited deep high resistivity values, suggesting subsurface granitic soft bedrock. EarthImager™ 3D software enabled inferred resource estimations, identifying a volume of 4,349,744 cubic meters and a total tonnage of 13.04 million tons for dolerite (dimension stone) material. The 3D-ERT inversion results further enhanced the understanding of the spatial distribution of the dolerite deposits, providing high-resolution images of subsurface resistivity, which facilitated the identification of economically significant zones. Based on geophysical study it is recommended to drill six confirmatory boreholes of a minimum 150 m depth to assess and confirm dolerite dyke zones/anomalies for future mine planning in the study area.

The lower ionosphere of Earth is greatly impacted by solar flares. During a solar flare event, a sudden enhancement in X-ray flux leads to additional ionization, which increases electron density in the lower ionosphere. In this paper, we first investigated time the behaviour of lower ionosphere during solar flare events (SFE) over the complete solar cycle 24 during the years 2011 to 2018 by using amplitude measurement of very low frequency (VLF) waves and GOES 0.1–0.8 nm X-ray flux. The fixed frequency (19.8 kHz) VLF wave transmitted from the NWC-transmitter, Australia is observed at our low-latitude station Varanasi (geom. lat. 14^o 55^/ N, geom. long. 154^o E), India. The amplitude enhancements associated with solar flares are characterised by the two traditional Wait parameters, virtual reflection height (H^/ in km) and the sharpness factor (β in km^− 1) i.e. electron density gradient. These empirically determined values of H^/ and β were employed in Long Wave Propagation Capability (LWPC) to predict VLF amplitude perturbations induced by the solar flare throughout a wider frequency range than was observable. It is found that the sharpness factor increases with the increasing strength of solar flares, but the virtual reflection height decreases. These observations show a decrease in H^/ from 78 km to 62 km and an increase in β from 0.34 km^− 1 up to a ‘saturation’ level of 0.51 km^− 1. A comparative study of these parameters during different phases of the solar cycle shows that during the rising phase of the solar cycle, β is found to be lower. In contrast, during the declining phase, its value is higher. Also, H^/ decreases more during the decreasing phase of the cycle than during the rising phase. During the peak of the solar cycle, H^/ and β values are found to lie between rising and decreasing phase values, although more dispersed. This indicates that the lower ionosphere behaves differently during different phases of the solar cycle.

This study focuses on analysing the impact of deterministic modifications of the Stokes kernel and terrain correction methods for precise geoid determination using the Stokes-Helmert method over a sophisticated topography. Three deterministic modification methods of Stokes’s kernel (Wong-Gore, Vaníček-Kleusberg, and Featherstone-Evans-Olliver) are tried to minimize the truncation error emanating from the non-availability of gravity data all over the Earth by utilizing two independent satellite only global geopotential models. In parallel to the modified Stokes kernel functions, two terrain correction techniques, i.e., spatial-spectral combined method with mass-prisms and spatial method with mass-cylinders, have also been examined to assess their combined effects on geoid heights over the Konya Closed Basin in Türkiye. The developed geoid models are validated with GNSS-levelling data and inter-compared pixel-wise. The numerical results show that although the overall statistical values depict consistent precision for various combinations of TCs, Stokes kernel modifiers, and GGMs, a holistic validation-comparison analysis reveals significant variations in view of the cm-precise geoid.