Results in Geophysical Sciences最新文献

Seawater intrusion into coastal aquifers is a worldwide problem causing water quality challenges in most communities along the coast, and has prompted many investigations into its occurrence, sources and age. However, there is still a major challenge when it comes to evaluating the spatial distribution of the problem in coastal aquifers towards groundwater management. Therefore, this study has employed electrical resistivity tomography technique to delineate both the lateral and vertical distribution of the salinity within some thirteen coastal communities in the Central Region of Ghana. The survey was conducted using the multiple-gradient array technique with the aid of a multi-electrode ABEM Lund Imaging System along selected traverses from the coastline into the communities. 2-D resistivity pseudo-sections generated from the survey data showed a unique resistivity distribution within each community, influenced by the underlying local geology and seawater intrusion. Low resistivity layers (< 50 Ωm), which may be associated with saline water saturated geologic formations, are observed within almost all the communities investigated at both shallow and deeper depths. The study also revealed that, seawater intrusion has affected freshwater aquifers in the communities by more than 5 km in-land from the sea and need to be tackled.

The introduction of water into the pore space of naturally occurring magnesite (leukolite) induces an intense relaxation mechanism, which is related to the electric double layer (EDL) formed on opposing sides of the solid – mater interface. The relaxation of the EDL is studied by using Broadband Dielectric Spectroscopy at different conditions of combined temperature and hydrostatic pressure. The temperature evolution of the characteristic relaxation frequency, reveals two successive temperature regions: in the low temperature one, protonic conductivity over the network of water molecules in on the solid surface, couples to the ionic transport of charged defects occurring in the solid. At higher temperatures, near – zero activation energies are found: the phenomenon is discussed theoretically and attributed to decoupling and an exchange of protons with lattice site magnesium cations, the formation of a proton enriched sub-surface layer of magnesite and subsequent release of cations to the liquid. The values of the activation volume and their independence on temperature, support the aforementioned interpretation.

Many researchers have suspected saltwater occurrence in Agbabu in the eastern Dahomey Basin. However, there is a need to get an insight to the possible origin of saltwater in the area. The aim of this study is to determine the source of saltwater in the inland aquifers of Agbabu, southwestern Nigeria for proper management of scarcely available freshwater resource in the area. 23 vertical electrical sounding, 2D electrical resistivity tomography (ERT), aeromagnetic and hydrochemical data (11 subsurface water samples) analyses were utilized in this study. The results from the magnetic analysis revealed the inland extension of the Chain Fracture Zone (CFZ). This NE-SW trending fracture zone cuts across the entire eastern Dahomey Basin, including Agbabu and its environs. Four (4) geoelectric layers were delineated across Agbabu namely, the topsoil (4 - 2806 Ωm), clayey sand (3 - 1280 Ωm), clayey (3 - 161 Ωm) and bituminous sand (1 - 10390 Ωm) layers. Anomalously low resistivity (1 - 20 Ωm) zones suspected within the bituminous sand correspond to the saltwater intrusion zones. The bituminous sand unit harbouring freshwater exhibits characteristic resistivity values of 33 – 160 Ωm. The interface between the freshwater and saltwater intruded zones in the area was suspected at 10 m depth with resistivity values ranging between 21 Ωm and 33 Ωm. The results from the hydrochemical water analysis also confirmed the presence of saltwater intrusion in the area. The salinity extent chart which was generated from the VES results indicated the mobility of the saltwater into the groundwater system. The fracture connectivity between the complex aquifers of Agbabu and the Atlantic Ocean (CFZ), that was mapped from the aeromagnetic and electrical resistivity results, is suspected to be primarily controlling the localization of saltwater in the study area.

The reservoir component of the petroleum system of the Niger – Delta has historically been accessed at depths ranging between 3.5 and 4.5 km, even though the Delta's sedimentary thickness is put at a maximum of 12 km as the basin has never been drilled to basement. The conventional assumption of the depth extent of reservoir facies within the Delta have been put to question as recent forays past the known limits yielded hydrocarbon production. Filtered, transformed, and enhanced airborne gravity and magnetic data acquired over the Niger Delta, southern Nigeria, were therefore analyzed to gain insight into the interval that has remained unexplored. A careful consideration of the zone suggests that the unexplored interval may accommodate interesting rock units and structures that could support the generation and preservation of hydrocarbon within some parts of the interval. Combined Spectral Depth Analysis (SDA) and 3D Euler Deconvolution employed to determine depth range to basement within the entire Niger Delta indicate that depth to basement rocks beneath the sedimentary unit ranges from 10–15 km, thus suggesting the presence of a Cretaceous sedimentary interval beneath the current drill depths that might be at least 6 km thick. Structural analyses of the basement rocks as well as the basement topography could reveal interesting structural pattern which when reactivated possibly engendered sedimentary structures within the sediments overlying them. Relevant information, such as generated from this study is expected to motivate interest to focus exploration target beyond the current drilling depth.

Our study has investigated spatio-temporal distribution of rainfall and rain day trends during different seasons for all districts of Haryana, India. Gridded rainfall dataset of 120 years (1901 to 2020) from India Meteorological Department (IMD) was analysed using mean rainfall, rainfall deviation, seasonal rainfall ratio (SRR), coefficient of variation (CV), number of rain days, rainfall intensity, trends of rain days, Empirical Orthogonal Functions (EOF) and Principal Component (PC) analysis. Districts lying in eastern Haryana have experienced more rainfall (less variability) than the ones lying in western Haryana during each season. SRR and CV analysis depicted most consistent rainfall during monsoon and maximum variability during post-monsoon season. Highest number of rain days was observed during monsoon season followed by pre-monsoon, winter and post-monsoon season in Haryana. Innovative trend analysis method (ITAM) shows a declining trend in number of rain days during winter and post-monsoon season while an increasing trend was observed during pre-monsoon season. Overall, monsoon season has shown a falling trend in moderate while rising trend was observed in both light and heavy rainfall intensity categories in most of districts. Dominant EOF explained maximum variability during post-monsoon season followed by winter, pre-monsoon, and monsoon season, respectively. PC analysis captured inter-annual variability in rainfall during each season. Our findings highlight qualitative and quantitative aspects of seasonal rainfall dynamics at districts level. This study is beneficial in understanding impact of climate change and climate variability on rainfall dynamics in Haryana which may guide policymakers and beneficiaries in optimizing use of hydrological resources.

The main aim of this study is to carry out a hydrogeological assessment to reveal suitable sites for the implantation of hydraulic structures like water boreholes. Petrographic studies put into place granitic rocks with numerous biotite in the Northern part of the map and basaltic rocks with phenocryst of olivine in the South. Lineaments extracted from satellite images revealed two major trends of N50–60°E and N70–80°E. A total of sixty-four Vertical Electrical Sounding (VES) data were acquired using Schlumberger configuration array, 2-D sections and hydraulic parameters of aquifers were deduced. The geoelectric sections characterize the subsurface into four geoelectric layers namely: topsoil with (resistivity values lower than 300 Ω m), weathered basement with (resistivity values ranging between 300 and 600 Ω m), fractured basement (resistivity ranging between 600 and 800 Ω m) and a more or less fresh bed rock with resistivity value up to 4000 Ω m. The hydraulic assessment of aquifers revealed a high conductivity value of 34 m/day, porosity of 41% and a high transmissivity of 429 m²/day indicating a highly interconnected fractured basement of the study area. Groundwater potential map revealed Toungang, Lafé and Ndionkou villages as suitable sites for water boreholes implantation with an average aquifers roof depth ranging between 1to 31 m.

Preliminary geological preconstruction assessment of a proposed housing estate site in Jigawa State, northern Nigeria has been done from analysis of aeromagnetic data for the purpose of optimised physical planning, particularly in the area of forestalling any potential risk that may be associated with the existence of linear geological structures or sub-surface conditions that may be inimical to engineering structures. The techniques adopted for the data enhancement include the total horizontal derivative, second vertical derivative and Euler deconvolution solutions. Spectral analysis was also performed on the data to determine depth to the magnetic basement rock, hence overburden thickness and basement topography. Data enhancement analyses delineated considerable number of semi-regional to regional-scaled lineaments, the existence of which presented the study area as a suspected shear zone. Basement topography was found to be highly rugged with evidence of rift-controlled subsidence detected in the eastern section. Having just a few lineaments traversing through it and being located where the basement topography is relatively less undulating, a trapezium-shaped area in the central southern region was identified as the most stable location within the study area, geologically. Two unmapped geologic bodies were delineated from the magnetic data as well. Drilling efforts confirmed an initially unmapped quartzite at the location where one of the two unmapped bodies were delineated and this validated the potency of magnetic data as an aid for geologic mapping. If established that lineaments F25, F26 and F28 traversing through this area are not seismically active, the trapezium-shaped area, said to be least affected by geological factors, could be used for building constructions. However, it is advised that high-rise buildings should not be concentrated in the area but cascaded buildings are recommended due to the fact that a portion of the area is located on a hill-slope and the topography of the basement rock underneath is also coincidentally sloppy. The study area was found to be a shear zone with highly undulating basement topography. Therefore, construction on the site must be approached with caution.

Setúbal is a county located in the Lisbon metropolitan area (Portugal) which has been hit by historical earthquakes that were responsible for high social and economic losses. The 1858 earthquake, with an epicentre located offshore just south of the Setúbal region, is a good example of an earthquake that caused high damage (IX-VIII MM) in this county. The main city of the region is Setúbal itself, an important and populated urban agglomeration with several industrial facilities very close by. The old city centre is mostly settled upon shallow geological units and taking into consideration the historical seismicity, it is important to examine the possible site effects that could occur during future earthquakes affecting the Setúbal region.

The objectives of this study are therefore the characterisation of the seismic behaviour of Setubal's shallow geological formations and the evaluation of potential site effects. Shear wave velocity profiles were computed with the Refraction Microtremor (ReMi) method, spatial distribution of V_S30 values was estimated and a map with the V_S30 classification of soils according to the EC8 was produced. Spatial distributions of different data are presented and discussed: V_S30 values, thickness of the shallow formations, top Pliocene surface, and results from Standard Penetration Tests (N_SPT values). Numerical correlations between these parameters were established. Geotechnical characterisation was based on the analysis of 307 logs and 2950 Standard Penetration Tests. A total of 266 refraction microtremor measurements were performed obtaining V_S profiles and V_S30 values for 43 different sites. It was found that in Setúbal the majority of the Holocene units belong to EC8 class C (180–360 m/s) but, in the areas where the thickness of shallow formations (TSF) is lower, the Holocene units may belong to class B (> 360 m/s). The spatial distribution of V_S30 highlighted that the Plio-Pleistocene, Miocene and Mesozoic areas show the best soil conditions corresponding to EC8 classes B and A. However, it was possible to identify a Plio-Pleistocene area with V_S30 varying between 232 and 355 m/s, therefore belonging to class C. This fact highlights that site effects studies should not be performed solely based on geological criteria, but it is essential to estimate S wave velocities (V_S) in situ.