Contributions to Geophysics and Geodesy

Geomagnetic variations induced by the partial solar eclipse of November 25, 2011

2025-06-30T07:11:36+00:00

We examine the geomagnetic variations observed from the SAMBA network during the partial solar eclipse on November 25, 2011, in Antarctica. The eclipse reached nearly 90% obscuration in the western Antarctic Peninsula at the point of greatest eclipse at 06:21 UTC. Six magnetic ground stations distributed across Chile and Antarctica, around 0° longitude and at various latitudes, were used to collect geomagnetic field data in X, Y, and Z coordinates at a 1-second resolution. A baseline from the international Q-days of the month was applied to filter out isolated signals caused by the eclipse. Results indicated a systematic decrease in both the X and Y components, synchronized with the passage of the penumbra.

Integration of radioactive, conventional logs and geochemical analyses to evaluate the hydrocarbon system of Rudeis Formation, October oil field, Gulf of Suez, Egypt

2025-06-30T07:11:37+00:00

The objective of this research is to determine the source rock by using a combination of well logging data, spectral gamma-ray logs, as well as laboratory geochemical analysis. This involves quantifying the total organic carbon content (TOC) as a key parameter to study the source rock of Rudeis Formation rocks in the October oil field, Gulf of Suez, Egypt. After theoretical calculations of well logging, the mean TOC value comes out to 1.2 wt%. The TOC values in the lab ranged from 0.4 wt% to 1.4 wt%, with a mean value of 0.9 wt%. According to Rudeis Formation's petrophysical properties, the hydrocarbon saturation levels in the upper and lower Rudeis reservoirs were recorded 57% to 60%, and the percentages of shale content in each reached 63% and 51%, respectively. The Radiogenic Heat Production (RHP) values ranged from 0.85 μW/m³ (minimum) to 2.38 μW/m³ (maximum) with a mean of 1.37 μW/m³ and a standard deviation of approximately 0.19 μW/m³. Five sections along the Rudeis Formation (Source 1 to Source 5) are considered hydrocarbon-bearing zones. Three of the zones were classified as source rocks (Source 2, Source 3, and Source 5) associated with a high volume of shales (mostly clay), while the other two zones (Source 1 and Source 4) are associated with a moderate volume of shales (shaly sandstones) considered to be source and reservoir rocks in the Upper and Lower Rudeis Formation. The lower Rudeis unit appears to have fair to good hydrocarbon potential, with TOC ranging from 0.46 to 1.28 weight percent and S2 varying from 0.88 to 3.76 mg/g. The upper Rudeis member is classified as a fair oil source rock, based on TOC values ranging from 0.42 to 1.29%. The equations (1), (2) and (3) were found to be the best fit to the TOC data.

Integration of aquifer resistivity and hydrogeochemical data for groundwater quality assessment in a basement complex terrain of Osogbo metropolis, Southwest Nigeria

2025-06-30T07:11:40+00:00

In this paper, one hundred and nine vertical electrical sounding data were acquired and quantitatively interpreted to delineate the subsurface sequence and determine the geoelectric parameters. One hundred and eighteen shallow wells with measured depths to the water table were sampled and thirty-five ground-water samples were analysed for physicochemical parameters, in a view to assessing the groundwater quality in Osogbo metropolis, Southwest Nigeria. The delineated subsurface layers included the topsoil/laterite, weathered layer; partly weathered/fractured basement and the basal fresh basement with respective resistivities and thicknesses: 13 – 4727 Ωm and 0.2 – 8.9 m; 70 – 641 Ωm and 0.8 – 32.0 m; 18 – 826 Ωm and 1.8 – 27.9; and 349 – ∞ Ωm and ∞ m. The main aquifer unit identified was the weathered layer which was classified into ≤ 100 Ωm (low), 101 – 350 Ωm (moderate) and 350 – 750 Ωm (high) resistivity zones. Depths to the water table ranged from 3.1 – 40.7 m. The analysed physicochemical parameters included the pH (5.31 – 7.33), Electrical Conductivity (EC) (90 – 1709 μs/m); Total dissolved Solid (TDS) (89 – 1227 mg/L); Ca²⁺ (1.9 – 93 mg/L); Na⁺ (9.2 – 349 mg/L); K⁺ (0.8 – 60 mg/L); and NO₃⁻ (0.9 – 4.9 mg/L). The central district of the study area with relatively shallow water table < 12 m) and cluster of ancient waste dumpsites fell within the low resistivity zone having physicochemical characteristics of high TDS (623 – 1227 mg/L) and EC (903 – 1709 μs/m) and elevated concentrations of Na⁺ (120 – 349 mg/L), Ca²⁺ (48 – 93 mg/L) and K⁺ (25 – 60 mg/L) relative to WHO (2022) and SON (2015) thresholds for potable water and adjudged a poor groundwater quality zone. The pollutant is suspected to be leachates from waste dumpsites and pit latrines.

Intermodel and method comparison of mean radiant temperature from numerical weather prediction models: Evaluation of enhanced spatial resolution in Europe

2025-06-30T07:11:43+00:00

Thermal stress indices play a vital role in evaluating human health risks related to heat and cold. Mean Radiant Temperature (MRT), derivable from numerical weather prediction (NWP) models, is a critical input for many such indices. This research generates high-resolution (5.5 km × 5.5 km) MRT estimates across Europe using the CERRA NWP system. We evaluate different computational approaches, benchmark the high-resolution MRT against the established ERA5-HEAT dataset, and validate results using BSRN ground observations. The study focuses on diverse European environments. Our high-resolution MRT product demonstrates comparable performance to ERA5-HEAT over flat terrain but offers substantial accuracy gains over complex terrain, including mountainous and coastal regions. These findings highlight the value of enhanced spatial resolution for accurate MRT estimation, crucial for robust thermal stress assessment and contributing to improved environmental health and safety strategies.

An enhancement filter utilizing the modified arctangent function for the structural and tectonic interpretation of causative sources: Application to WGM2012 gravity data from the Rafsanjan Plain, Iran

2025-06-30T07:11:44+00:00

Gravity data edge detection methods play a crucial role in identifying the horizontal positions of buried sources and enhancing the interpretation of subsurface structures. Among these methods, the Total Horizontal Gradient (THG) filter is widely used due to its noise stability and straightforward formulation. However, the THG filter has inherent limitations, prompting the development of refined edge detection techniques. To address these shortcomings, various approaches based on local phase analysis or normalized filters have been introduced, many of which incorporate the arctangent function to combine horizontal and vertical gradients. While these methods improve edge detection, they also exhibit drawbacks, such as the generation of spurious edges and restricted resolution. In this study, we propose the Modified Arctangent Function (MAT), which enhances gravity source edge detection by integrating the total horizontal gradient with a modified arctangent function. The effectiveness of the MAT filter is systematically evaluated against conventional filters that utilize the arctangent function and/or the total horizontal gradient. Its performance is validated through synthetic gravity data tests, both with and without noise contamination. To further assess its applicability, the MAT filter is applied to high-resolution gravity data from the WGM2012 (World Gravity Map) over the Rafsanjan Plain in Iran. Additionally, an approximated derivative calculation method is incorporated to mitigate noise amplification during the derivative computation process. The results from both synthetic and real data confirm that the MAT filter effectively detects and delineates gravity anomalies, demonstrating its potential as a valuable tool for geophysical interpretation.

Estimation of the effective elastic thickness using global gravitational, lithospheric structure, and rheology models

2025-06-30T07:11:46+00:00

Geodetic satellite missions become essential tools to predict the ocean-floor relief and study the oceanic lithosphere. Satellite-altimetry measurements of the sea surface topography are converted to marine gravity values that are used to predict bathymetric depths. This procedure requires information on marine sediment deposits as well as the lithospheric elastic thickness. Moreover, the elastic thickness provides information on the lithospheric strength in the context of interpreting tectonism and geological processes. In this study, we estimated the lithospheric elastic thickness beneath the Indian Ocean and surrounding continental regions by applying two methods that determine this parameter individually for the oceanic and continental lithosphere. For the former, we used global lithospheric age and upper-mantle temperature models. For the latter, we derived this parameter using global gravitational, lithospheric structure, and rheology models. Since our estimates are based on global models, the resulting map of the elastic thickness lacks more detailed features of lithospheric strength. Nevertheless, the principal pattern in elastic thickness variations relatively closely resemble tectonic configuration and lithospheric thermal state beneath the Indian Ocean. Active divergent tectonic margins along mid-oceanic rifts are characterized by a weak lithospheric strength. The strength increases due to cooling of the oceanic lithosphere with its age, while reaching maxima ∼50 km. A relatively weak lithosphere is found beneath Madagascar (15–30 km) and Sri Lanka (24–35 km). Within the domain of the Indian Ocean, the maximum elastic thickness (∼130 km) is detected beneath continental crustal fragments of the South Kerguelen Plateau.