Contributions to Geophysics and Geodesy

Advancing potential field data analysis: the Modified Horizontal Gradient Amplitude method (MHGA)

Hanbing AI — 2024-06-27

Enhancing the detection accuracy of the edges of the geological features within the subsurface remains a significant objective in geophysical data interpretation. Despite numerous advancements, approaches stemming from the directional gradients of gravity and magnetic fields still grapple with challenges such as low-resolution outcomes and susceptibility to noise contamination. In this study, we introduce a novel filtering framework based on the total horizontal gradient and its derivatives, designed to yield more precise and coherent edges free from false boundaries or disruptive artifacts. Validation using synthetic Bishop complex magnetic and gravity datasets, alongside Tuangiao aeromagnetic data from Vietnam, substantiates the robustness and applicability of our modified approach. Furthermore, recognizing the inherent susceptibility of edge detection filters to noise contamination resulting from directional derivatives, we employ the recently developed modified non-local means (MNLM) algorithm to alleviate noise effects prior to the analysis of noisy synthetic and real datasets. Our findings confirm the efficacy of the proposed method in reducing false artifacts and identifying edges with heightened precision, positioning MHGA as a valuable alternative for processing potential field data.

Abu Roash F reservoir hydrocarbon potential, Western Desert, Egypt

Mohamed Said RADWAN — 2024-06-27

The Western Desert of Egypt is recognized for its significant potential in oil and gas reserves, making it a prominent hydrocarbon-producing area with conventional reservoirs. Recent findings from various wells suggest that unconventional reservoirs in the Western Desert may hold considerable promise. The primary objective of this study was to explore this potential. Accordingly, a comprehensive and multidisciplinary approach has been employed to assess the Abu Roash/F (AR/F) reservoir. The methodology involved meticulously examining formation characteristics and integrating borehole images with existing core data and thin sections. This integration aimed to analyse and interpret the lithofacies of the reservoir, with a specific focus on identifying fractures and decoding potential diagenesis effects. As a result, we successfully identified wells exhibiting optimal reservoir quality, unveiling the presence of fractured dolomite or vuggy dolomite in wells containing promising oil and gas pay intervals. Notably, some wells displayed low porosity (3%), yet seismic attribute maps in their vicinity indicated a high density of fractures. This underscores the importance of combining borehole images with seismic attributes to evaluate carbonate formations effectively. The fractures, predominantly oriented NW–SE, were found to be associated with deformation zones. Our findings emphasize the critical role of this integrated approach in unlocking the full potential of hydrocarbon reservoirs in the Western Desert.

Magnetic and resistivity imaging of a probable fault within the Precambrian crystalline rocks in Ogbomoso, Southwestern Nigeria

Oyelowo Gabriel BAYOWA — 2024-06-27

A linear structure which is interpreted to be a fault was identified within the Precambrian basement complex rocks of the Ogbomoso Southwest Sheet 222 geological map, southwest Nigeria. The ground magnetic and electrical resistivity methods were used to investigate the structure and determine its physical characteristics. Five (5) traverses with lengths varying from 700–800 m and trending approximately NW–SE were established perpendicularly to the suspected fault. Total magnetic field data were acquired at 10 m interval, corrected for diurnal variation and inverted into 2D magnetic models using the Geosoft Oasis montaj software package. Dipole-Dipole resistivity data were acquired along the traverses using a dipole length of 20 m with expansion factor n varying from 1 to 5 and modelled to generate 2D resistivity structures with the DIPPRO^TM software package. Thirty-seven (37) depth sounding datasets were acquired, quantitatively interpreted and the results used to generate geoelectric sections across the suspected geologic feature. Three major magnetic dyke lineaments D₁, D₂ and D₃ (60 – 164 m wide and 70 – ∞ depth extent) characterised by peak negative (−44 to −81 nT) magnetic anomalies and equivalent resistivity lineaments R₁, R₂ and R₃ (50 – 140 m wide and 40 to >60 m depth extent) typified by low resistivity (10 – 750 Ωm) vertical discontinuities were delineated. The northern edges of lineaments D₂ and R₂ coincide significantly with the spatial location of the suspected fault F¹ – F¹ while D₁ and R₁ and D₃ and R₃ correlated and are newly identified parallel faults F² – F² and F³ – F³ to F¹ – F¹ and probably syngenetic with it.

Integration of geoelectric and electromagnetic methods to delineate subsurface mineralization in bostonite rocks, South El-Atshan area, Central Eastern Desert, Egypt

Hussein F. ABDELSALAM — 2024-06-27

Bostonite rocks are widely spread in the Central Eastern Desert (CED) of Egypt, they are found in the form of sills or dykes injected in the Precambrian rocks. Postgeosynclinal (Hammamat) sediments were intruded by Postorogenic bostonites. The study of this area, using ground geophysical methods, aimed at tracking the radioactive mineralization under the surface, based on the possible association of this radioactive mineralization with some other mineralizations such as sulphides. The application of the self-potential survey shows that the highest negative anomalies are associated with the South El-Atshan bostonite sill and its contact zones. The depths to the centres of some selected anomalies range from 5.7 m to 24 m, half-widths oscillate from 6 to 32 m, with shallow dips towards the west and east directions. The horizontal-loop electromagnetic (HLEM) survey, which was conducted using four frequencies, revealed the presence of two conductive bodies. These conductive bodies possess narrow widths and depths, ranging from 25 m to 27 m, dipping towards the west direction with shallow angles of 25° and 40°, respectively. In addition, the induced polarization (IP) profile shows high chargeability values reaching more than 7.9 mV/V at depths starting from about 10 m to about 35 m. This zone corresponds to a high-resistance zone that may reflect a subsurface disseminated mineralization. The integration of the results obtained using the three techniques makes it possible to determine the most appropriate zones for the development of exploration in the area of investigation.

Indoor radon monitoring in Zázrivá

Iveta SMETANOVÁ — 2024-06-27

Indoor radon survey in Zázrivá was conducted within the framework of the project “Multidisciplinary research of geophysical and structural parameters, and environmental impacts of faults of the Western Carpathians”. Monitoring was carried out in selected houses and kindergarten using RamaRn detectors. Annual average of indoor radon varied between 45 and 260 Bq/m³, with median of 90 Bq/m³. Seasonal variation with minimum in summer was observed in the majority of monitored rooms. Radon concentration higher than 200 Bq/m³ was measured only in rooms with direct contact with the subsoil. A possible relation between the position of a monitored building close to an assumed fault line and elevated indoor radon levels was not proven.