Sandy Formations Research Articles

Building responses to soil removal, dewatering, and artificial recharge during deep excavation in water-rich sandy strata

In water-rich sandy strata, deep excavations necessitate dewatering within the excavation site, leading to significant groundwater drawdowns and surface subsidence behind walls, which can result in severe damage to adjacent buildings. Nonetheless, the individual impacts of soil removal and dewatering in such permeable sandy formations on building responses remain ambiguous,with scant reported protective experience in literature. This study introduces a validated three-dimensional fluid-solid coupling finite-element model to simulate the construction of an 18-19.8 m deep subway station in Nantong, China. Artificial recharge was applied to mitigate adverse effects on nearby buildings caused by drainage. The investigation explores the separate effects of soil removal, dewatering, and water recharging on the settlements of buildings with three different foundations (natural, raft, and piled footings). Additionally, the influence of post-recharging groundwater levels on building responses was scrutinized. The present findings indicate that discharging confined aquifers significantly contributes to building settlements, while artificial recharge can substantially reverse the deformations in buildings, especially those with natural and raft foundations. As excavation depth and the bearing capacity of the building foundation increase, the relative impact of soil removal on building settlements grows, whereas the influence of dewatering diminishes. Compared to enhancing foundation-bearing capacity, artificial recharge emerges as a more cost-effective strategy to safeguard pre-existing buildings near deep excavations in water-rich sandy strata, provided the post-recharging groundwater levels achieve a certain threshold.

Prediction of wellbore sand production potential from analysis of petrophysical data coupled with field stress: a case study from the Shah-Deniz gas field (Caspian Sea Basin)

Identifying the optimal azimuth and inclination for wellbore drilling in sandy formations can be considered a valuable aid in reducing sand production risks, lost time, and decreasing drilling costs in the petroleum industry. Therefore, a numerical systematic approach was provided to predict sand production in wellbore SDX-5, drilled in a deep-water sandstone reservoir in the Shah-Deniz gas field (South Caspian Basin), which has never been done previously. Additionally, this systematic approach uses geomechanical and geodynamical criteria, along with petrophysical information (density and sonic log) and tectonic characteristics of the study area, which are influenced by the active tectonic stresses of the Apsheron-Balkhan zone. The subsurface data sources employed are more eco-friendly, available, and continuous than experimental tests. The computations conducted achieved azimuth, inclination, polar, and depth profile plots for the Lower Balakhany Formation. The calculations reveal that the optimum azimuth for the wellbore drilling trajectories is parallel to SHmax and oblique drilling to near horizontal is the result of optimum inclination. Polar plots showed optimum azimuth, inclination, and effect of wellbore trajectory on critical collapse pressure and collapse drawdown pressure with pressure values simultaneously, which identify safer alternatives for achieving higher petroleum production rates without sanding. Depth profile plots provide a simultaneous overview of the values of critical collapse pressure, critical sanding pressure for instantaneous drawdown, and optimum wellbore production pressure during drilling and production operations. Moreover, optimum reservoir fluid production (maximum discharge) rates can be identified and imposed as upper limits to prevent sand production.

Experimental Study on Dynamic Characteristics of Saturated Soft Clay with Sand Interlayer under Unidirectional and Bidirectional Vibration

The marine and alluvial plains along the southeastern coast of China are widely distributed in sandy formations, including smaller sand lenses and interlayers. The interlayers of sand have a significant impact on the mechanical properties of soft clay. In this paper, a large number of undrained unidirectional and bidirectional cyclic loading tests for soft clay with sand interlayers were carried out by a dynamic triaxial test system. Test results show that, under unidirectional and bidirectional cyclic vibration, the area of the hysteresis loop decreases and the slope of the connecting line at both ends of the hysteresis loop increases with the increasing of frequency. For the same vibration frequency, the area of the bidirectional vibration hysteresis loop and the slope of the connecting line at both ends are smaller than that of the unidirectional cyclic vibration. Under the same dynamic stress ratio, cumulative axial deformation caused by unidirectional and bidirectional vibration increases with the increasing frequency. Under unidirectional vibration, dynamic elastic modulus decreases at first, and then increases with the increasing frequency. For the same frequency, dynamic elastic modulus of the sample increases with the increase in cycles. Due to the effect of radial cyclic stress, the curves of dynamic elastic modulus and damping ratio with frequency under bidirectional vibration are opposite to those under unidirectional vibration.

Artificial Neural Networks to Predict Lost Circulation Zones at Southern Iraq Oilfield

Drilling soft and fragile areas such as (high permeable, cavernous, fractured, and sandy formations) have several problems. One of the most critical problems is the loss of drilling fluid into these formations in whole or part of the well. The loss of drilling fluid can lead to more significant and complex problems, such as pipe sticking, well kick, and closing the well. The drilling muds are relatively expensive, especially oil-based mud or those that contain special additives, so it is not economically beneficial to waste and lose these muds. Artificial neural networks (ANN) can predict drilling fluid losses before they occur based on drilling parameters data and drilling fluid properties of wells effected by lost circulation problems located in the same area. This paper developed two artificial neural network models to predict drilling fluid losses in the Dammam formation- Rumaila oil field in southern Iraq. The two models have the same topology and structure. The first model used the early stopping technique to stop the training when we get the global minimum and the second model used specific epochs to complete the training. The models could predict various types of losses with high accuracy. The accuracy of implementing R2 for the first and second models was 0.9302 and 0.9493, respectively. The early stopping technique lead to obtain a model with acceptable accuracy in a short time without relying on a specific number of epochs.

Groundwater Exploration of Federal College of Education (Technical) Akoka Lagos Nigeria Community Using Schlumberger Array

A total of thirteen (13) Vertical Electrical Soundings (VES) using Schlumberger array were carried out within Federal College of Education (Tech) Akoka, Yaba, Lagos State, Southwestern, Nigeria. This area is between Longitude N06º31’10” and Longitude N06º31’30” and Latitude E3º22′ 55″ and Latitude E3º23′ 5.3″. This study was necessary because of the roles water plays in human life, the demand for it and the failure of boreholes in some locations within the study area. Resistivity method of geophysical survey was applied to determine good locations for groundwater prospecting using PASI Terrameter and its accessories. The resistivity values obtained were plotted on a log-log graph scale for curve matching and computer iteration was carried out using WinResist software to generate computer models for the curve types. The Geoelectric sections beneath each VES point revealed about four to five geoelectric layers, comprising topsoil, sandy-clay, clayey-sand, clay, peat, and sandy formation. Analysis of geophysical investigation showed that sand at the third geoelectric layer in VES 1 and VES 11 with a resistivity value of 143.3 Ωm and 240 Ωm constitute aquifer units where groundwater can be tapped. The shallow groundwater could be tapped at depth interval 6.6-20.2 m in VES 1 and 25.2m and above in VES 11. The failure of borehole located within the female hostel was due to overburden of this shallow aquifer due to population explosion. The recommendation was made that groundwater development through borehole drilling of depth not less than 150m is needed for high groundwater yield based on available information from nearby well log since wider spread for the deep investigation is not possible within the study area.

LITHOLOGY AND SEDIMENTOGENESIS OF THE EARLY OLIGOCENE LOWER PLANORBELLIAN DEPOSITS IN THE NORTHWESTERN PART OF THE INDOLO-KUBAN DEPRESSION

On the basis of lithological, lithmological, mineralogical-petrographic, and granulometric studies of the Lower Planorbellian deposits of the northwestern side of the Indolo-Kuban depression, in combination with the data of facies interpretation of logging results (radioactive methods), the environments of Early Planorbellian sedimentation were reconstructed. It has been established that the lithological structure of the Lower Planorbellian sequence of the Maikop of the northwestern side of the Indolo-Kuban depression is characterized by an almost uniform distribution of mudstones (34–71%, average 49%) and siltstones (27–57%, average 41%) with a subordinate development of sandstones (3–27%, average 10%). According to the features of the spatial distribution of these components within the studied region, four lithological complexes of three series were distinguished: psammite-siltstone-argillite (sandstones 0–25%, siltstones 50–75%, mudstones 25–50%) complex of the siltstone series; predominantly mudstonesiltstone (sandstones 0–25%, siltstones 25–50%, mudstones 25–50%) and mixed terrigenous (sandstones 25–50%, siltstones 25–50%, mudstones 25–50%) complexes of mixed psammitic-siltstone-argillite series and argillite with interlayers of sandstones and siltstones (sandstones 0–25%, siltstones 0–25%, mudstones 50–75%) complex of the argillite series. The genetic interpretation of the results of granulometric analyzes of terrigenous rocks showed the dominance within the region of formations of shelf basins with a directed system of watercourses. A cyclic (transgressive-regressive) character of the Early Planorbellian sedimentation was revealed, which manifested itself in the upsection replacement of sandy formations of the initial stages of the development of the basin by siltstoneargillaceous silts in the middle and again by the accumulation of clastogenic deposits at the final stage. On the whole, the sedimentation basin was a sublittoral shoal with local directed watercourses, within which formations of estuarine and alongshore bars, alluvial fans, and shelf plains are distinguished.

Lost Circulation Prediction Using Decision Tree, Random Forest, and Extra Trees Algorithms for an Iraqi Oil Field

Drilling soft and fragile areas such as high permeable, cavernous, fractured, and sandy formations are often accompanied by many problems. One of the most important of these problems is the loss of drilling fluid into these formations in whole or in part. The loss of drilling fluid can lead to bigger and more complex problems, including pipe stucking or kicking and finally closing the well. The drilling muds are relatively expensive, especially the muds that contain oil-based mud or that contain special additives, so it is not economically beneficial to waste and lose these muds. The treatment of drilling fluid losses is also somewhat expensive as a result of the wasted time that it caused, as well as the high cost of materials used in the treatment such as heavy materials, cement, and others. The best way to deal with drilling fluid losses is to prevent them. Artificial intelligence through supervised machine learning provides the possibility of predicting these losses before they occur based on equivalent circulating density, yield point, plastic viscosity, rate of penetration, flow rate, and losses rate of wells suffered from losses problem located in the same area. In this paper, three supervised machine learning models, namely, decision tree, random forest, and extra trees, were built to predict drilling fluid losses in the Rumaila oil field in southern Iraq. The results show that the extra trees model with an R2 of 0.9681was able to predict the new lost circulation events with high accuracy.

Hydrodynamic, hydrochemical and isotopic characterization of the waters of the quaternary aquifer of Bahr-El-Ghazal, north central Chad.

Located in the middle of the Sahel, the study area is an integral part of the Bahr-El-Ghazal region. It is crossed by a fossil valley, which has a geological history similar to that of rivers that disappeared under the influence of climate change where evaporation phenomena were intense. From the hydrogeological point of view, the free water table of this region is housed in sedimentary geological formations of different natures. It passes from the sand tablecloth to that of the fluviolacustrine clays. In some cases, the latter confer particularly bad chemical qualities to the water, even though groundwater is the main source of drinking water for the local population. For this reason, a study aimed at understanding the different mechanisms that govern the chemical functioning of the aquifer of the Bahar-El-Ghazal region has been carried out. It is with this in mind that a piezometric study is being carried out in the first instance. This reveals the presence of some hydrodynamic anomalies. Thus, a shallow depth of the body of water is observed in the Harr and in the sandy formations of the extreme West. However, underground flows oriented from these two zones towards the central part characterize a deepening of the water level. Subsequently, a hydro-chemical study was undertaken. As regards the chemical facies, they also show an evolutionary character. They pass from calcium and magnesian bicarbonate facies to chloride or sodium and potassium sulphate facies in the piezometric depression. The values of the isotopic composition of the studied groundwater vary from -4.86 to 4.04‰ and -35.60 to 14.50‰ and delta 2H respectively.

Role of Particle Shape on Ground Responses to a Circular Tunnel Excavation in Sandy Soil: Consequences from DEM Simulations.

Due to the sensitivity of sandy soil’s mechanical behavior to the particle shape, it is thus of importance for interpreting the effect of particle shape on the ground response induced by tunnel excavation in sandy formations. We conducted a series of 2D DEM (discrete element method) simulations on a common circular tunnel excavation in sandy soil with variable-shaped particles, which are characterized as two descriptors, i.e., aspect ratio (AR) and convexity (C). The macroscopic responses and the microscopic characteristics of the sandy ground are elaborated in detail. The simulation results show obvious asymmetrical features of the excavated ground, which results from the ground heterogeneity caused by the irregular particle shape. In addition, we investigate the roles of AR and C on the ground response and find that reducing AR or increasing C will enlarge the ground settlement, i.e., the sandy ground deformation is more sensitive to the particles with more irregular shapes. However, elongated particles are beneficial for the generation of soil arching with stronger bearing capacity and thus reduce the soil pressure on the tunnel lining. Our findings have important implications for the safety assessment of the tunnel excavation, as well as other underground structure construction in sandy formations.

Characterization of elastic moduli with anisotropic rock physics templates considering mineralogy, fluid, porosity, and pore-structure: A case study in Volve field, North Sea

A practical methodology was designed to characterize mineralogy and the rock microstructure by constructing anisotropic rock physics templates (ARPTs) based on the Effective Medium Method (EMM). The ARPTs consider parameters such as pore aspect ratio (α), porosity, fluid saturation, and elastic properties of rock minerals. A three-stage methodology was applied to an oil well for a sandstone formation in the Volve field, North Sea. The first stage is the petrophysical evaluation, which is validated with core data. The second, the inversion scheme with differential effective medium (DEM) was used to predict aspect ratios in depth. The obtained results characterize the sandy formation setting intergranular porosity with 0.12 < α < 0.3, although there is also the presence of stylolites caused by the dissolution mechanism between quartz, feldspar, mica, calcite, and clay. The validation of the pore-structure is accomplished with thin sections. The last stage comprises the construction of ARPTs, which includes the axial and two transversal templates of Young's modulus versus Poisson's ratio. These templates are used to characterize lithology, porosity, pore-structure, and the mechanical behavior of the sandstone formation from well logs and gave invaluable information about the pay zone. In addition, because the lithotype interpretation proposed is merely borne by rock's elastic properties, it has the potential to be scaled to field scale by using seismic inversion data.

Integrated geophysical characterization of saltwater intrusion in Lagos State, Eastern Dahomey Basin, Southwestern Nigeria

Integrated geophysical methods of Electrical resistivity tomography (ERT), Induced Polarization (IP), and Well Logging were employed in this study to delineate the lateral and depth extent of saline water intrusion in Oniru area of Lagos State, South-western Nigeria due to reported saltwater contamination of some wells at relatively shallow depths. Ergo, improving the scientific understanding of fresh and saline waters occurrence in the coastal areas of Lagos State.Five traverses, each of variant lengths (100 to 200 m) were established for both ERT and IP as space permitted. For each profile level, Wenner electrode configuration was adopted with a minimum electrode spacing of 3 m and maximum electrode spacing of 50 m. Ten levels were achieved for profiles 1, 2, 4, and 5 while eight levels were accomplished for profile 3. The ERT and IP data were processed using RES2DINV 2-D resistivity data processing software. The well log data utilized were acquired from two existing lithological logs within the study area to validate the results of the ERT and IP.The unconsolidated sandy formation was characterized by high resistivity with corresponding low to medium chargeability while the clay units were characterized by low resistivity with equivalent high chargeability. The sandy formation saturated by saltwater was characterized by very low resistivity with corresponding low chargeability while that of freshwater was characterized by medium to high resistivity with corresponding low chargeability. The integrated geophysical methods of ERT, IP and borehole logging in the study area identified three lithological sequences. The first layer corresponded to high resistivity values of unconsolidated sandy topsoil, mostly less than 5 m. However, in profiles 3 and 4, it occurred as saturated marshy topsoil. The top layer was underlain by sand/clayey sand formation, which was observed to have pockets of saline water intrusion as evinced from the results of the ERT and IP data. The saline water-bearing zones were observed to intermingle with the freshwater-bearing sand units. The freshwater-saline water transition tends to be conspicuous especially in profiles 3 and 5. The base of profiles 1 and 2 was occupied by clay units which were evident from the results of the borehole log, the ERT, and the IP sections.The integrated geophysical method was able to successfully delineate the fresh and saline water interface and extent of saltwater intrusion within the study area and thus would provide a reliable database for future groundwater development and management within the study area.

Sustainability of Heavily Loaded Raft Foundations on Granular Soils of Arabian Peninsula with Soft Clay Layer

Sandy soil formations are widespread around the globe, particularly in the Arabian Peninsula. The Sandy formations were created as a result of long-term geological activities including but not limited to alluvial and wind deposition. Since the alluvial deposits contain fine soils, the clay layers are often intercepted in the foundation supporting strata. The geotechnical foundation design based on field and lab investigations usually relies on the average response of the layered strata and can satisfy both shear and settlement criteria. However, the enduring sustainability of the heavily loaded foundations is required to be investigated in detail. In this study FEM based numerical investigations have been carried out on a thin raft foundation resting on granular soil, with a soft clay layer in the influence zone. The COMSOL Multi-physics platform has been used for the analysis. The study cases are planned based on the thickness of the clay layer and building load. The analysis results predict the risk involved in the sustainability of rafts in terms of the settlement. The field engineers and designers are recommended to consider the risk and plan accordingly

КОМПЛЕКСНА ХАРАКТЕРИСТИКА ГЕОЛОГІЧНИХ/ГЕОТУРИСТИЧНИХ ОБ'ЄКТІВ СКОЛІВСЬКИХ БЕСКИДІВ ЯК ПОКАЗНИКІВ ТЕКТОНО-СЕДИМЕНТАЦІЙНИХ ПРОЦЕСІВ ЗОВНІШНІХ КАРПАТ

A description of the geological/geotouristic "weekend" excursion from Lviv to the nearest part of the Carpathians is presented with an overview of the exposed deposits of the Western European platform and the Skole Beskyds on the route Lviv - Mykolaiv - Verkhnye Synyovydne - Kamyanka - Lviv. A comprehensive outline of the geological position of the route area is given. The main geological objects and observation points (stops) along the route are described. The proposed route will allow you to get acquainted with the whole complex of the sediments on both the Tethys Paleocean margin and the ancient Paratethys Sea, to trace some indicators of the beginning of growth of the Carpathian Mountains, as well as to see the main oil-generating strata of the entire Carpathian arc. In observation points 1 and 2 near Mykolayiv, we observe Miocene shallow-water sandy formations of Paratethys. Point 3 is the Uhersk gas field located in the Carpathian Foredeep. In point 4 (Verkhnye Synyovydne) the lithofacies of the transition link between the Tethys and Paratetys deposits are exposed, and the Paratetys deposits here are expressed by the Menilite Formation (Oligocene) - the main oil-generating stratum of the Carpathians. In points 5 and 6 (Kamyanka Stream) we observe the deepest-water sedimentary formations of the Outer Carpathian segment of the Tethys – the products of catastrophic (including turbidite) currents and background (hemi)pelagic sedimentation.

Assessment of the interval heat flow and thermal resistance at the Faltu-1 well, Borno basin, NE Nigeria

The heat flowing through horizons in the Faltu-1 well, Borno Basin, NE Nigeria was calculated from their thermal conductivities and geothermal gradients with the aim of determining whether or not it is uniform, and if not, the depths where it is diverted, and the possible heat diversion process. The interval heat flow was assessed to be non-uniform. While fluid convection is adjudged to be the major heat diversion mechanism within the Chad Formation with minor heat refraction, the reversed is adjudged to be the situation for the underlying Kerri Kerri Formaton within which increasing sand content with depth is also predicted, with the lower interval predicted to be the Gombe Formation. Patterns of disruptions to the vertical heat flow within the Fika Formation were inferred to suggest rhythmic bedding of shale and sand beds that could serve as both source and reservoir rocks. Magmatic intrusions that impacted the maturation of organic matter into oil and gas also provided necessary entrapment structures and possible migration pathways. The Gongila and Bima Formations each has single disruption of the heat flows that are attributed to refraction. In the case of the Gongila Formation, the disruption is between the early-deposited more sandy and laterdeposited more shaley lithologies in the marine transgression of the area, while in the case of the Bima, it is between the more shaley Upper and more sandy Middle Bima Formations. Analysis of the Bullard plots also revealed disruptions to the vertical heat flow that are attributed either to convecting fluids or to heat refraction and diffraction. Two such disrupting heat advections were identified within the Chad Formation with the first being attributed to convection, while the other is attributed to a combination of both. Two similar disruptions for the Kerri Kerri Formation were attributed largely to lithological variations with minor contributions from convection of fluids. While unable to discern the rhythmic bedding, the five disruptions of the Bullard plot for the Fika Formation and one each for the Gongila and Bima Formations were interpreted to indicate similar features inferred from interval heat flow plots.&#x0D; Keywords: Interval heat flow, heat convection, heat diffraction, thermal resistivity, shaliness

&lt;p&gt;&lt;strong&gt;Updating and assessing plant endemism in Egypt&lt;/strong&gt;&lt;/p&gt;

This study aimed to update and assess the list of endemic plants in the Egyptian flora, based on information obtained during field trips, literature reviews, and herbaria consultations. Life forms, habitat, national distribution, rarity forms, local threats, and conservation status according to IUCN categories, were estimated for these plants. Twenty field visits were conducted from summer 2015 to spring 2020 to many locations all over Egypt. The recent list includes 42 species (belonging to 37 genera and 20 families inhabiting 10 main habitats). Rocky surfaces and sandy formations have the highest number of endemic species, most of them are therophytes. Sinaitico-Arabian sector is the richest in endemics (19 taxa), followed by the Marioutico-Arishian (17 taxa). Small geographic range-small habitat-non-abundant plants are the most represented rarity form. All endemics are exposed to at least one type of local threat, the most prominent threats being over-collecting and over-cutting. According to the guidelines of IUCN, 31 of the endemic taxa are threatened, thus requiring in situ and ex situ conservation. Out of them, 11 taxa are believed to be Extinct, five are Critically Endangered, and 15 Endangered. In addition, 11 taxa are evaluated as Data Deficient. Further studies are still needed to verify and fill the information gaps about the Egyptian endemics.

Seepage Characteristics of Slurry with Particle Additives in the High-Permeability Sand Layer

In the process of the slurry shield construction, the formation of the filter cake is the key to maintain the stability of the excavation face. In high-permeability formation, ordinary pure bentonite slurry is easy to pass through the formation void. Therefore, it is necessary to study the slurry permeability characteristics of the high-permeability sand layer. For the purpose of researching the permeability characteristics of slurry in sandy formation, this paper studies the influence of slurry viscosity on the formation of the filter cake in high-permeability formation under different pressure gradients by the slurry permeability test. The effect of additive particle size on the film-forming gain was studied by adding additives with different particle sizes into pure bentonite slurries with different viscosities. This paper presents a method to determine the optimum slurry viscosity and particle size for high-permeability formation so as to guide the slurry shield construction in high-permeability formation.

Geology and geotechnical investigations of part of the Anambra Basin, Southeastern Nigeria: implication for gully erosion hazards

BackgroundGeologic and geotechnical conditions of soils where Nanka and Ajali Formations outcropped in Anambra Basin, Southeastern Nigeria were investigated and accessed. This was done using detailed mapping and mechanical soil laboratory tests to unravel the genesis and continued expansion of gully erosion in the study areas.ResultsField study revealed that gully erosions are more pronounced in the study area with poor vegetation cover and a high degree of slope steepness. Grain size analysis revealed that the soils of the Nanka Formation have an average sand content value of 90.90% (sandy) and silt content value of 3.0% (low fine portions). The plasticity index of the fine portions indicates that the soils are weak plastic, with a mean value of 5.29%. The soils have an average cohesion value of 0.30 kg/cm2 indicating a very weak cohesion. The soils are highly permeable; with an average value of 2.67 × 10–3 cm/s. The compaction test further revealed that the soils are loosely compacted. The soils for the Ajali Formation have an average sand content value of 95.10% (sandy) and silt content value of 1.43% (low fine portions). The plasticity index of the fine portions indicates that the soils are weak plastic, with a mean value of 2.70%. The soils have an average cohesion value of 0.30 kg/cm2 indicating a very weak cohesion. The soils are highly permeable; with an average value of 2.70 × 10–3 cm/s. The compaction test revealed that the soils are loosely compacted.ConclusionAfter field surveys and laboratory analyses, it was found that the gully erosions have been developing respectively on steep slopes and non-vegetated areas, and their genesis facilitated by the cohesionless and very permeable nature of the sandy formations. Following those key findings, it was proposed many practices (agronomic and engineering mainly) that can help mitigate the formations as well as the expansion of this very damaging hazard type. The potential implications of these gully erosion include damaging of buildings, residential houses, bridges, and roads, loss of farmland and vegetation, isolation of villages and towns, increased migration of inhabitants as well as degradation of agricultural fertile land.

Study and Evolution of the Dune Field of La Banya Spit in Ebro Delta (Spain) Using LiDAR Data and GPR

La Banya spit, located at the south of the River Ebro Delta, is a sandy formation, developed by annexation of bars forming successive beach ridges, which are oriented and modeled by the eastern and southern waves. The initial ridges run parallel to the coastline, and above them small dunes developed, the crests of which are oriented by dominant winds, forming foredune ridges and barchans. This study attempted to test a number of techniques in order to understand the dune dynamic on this coastal spit between 2004 and 2012: LiDAR data were used to reconstruct changes to the surface and volume of the barchan dunes and foredunes; ground-penetrating radar was applied to obtain an image of their internal structure, which would help to understand their recent evolution. GPS data taken on the field, together with application of GIS techniques, made possible the combination of results and their comparison. The results showed a different trend between the barchan dunes and the foredunes. While the barchan dunes increased in area and volume between 2004 and 2012, the foredunes lost thickness. This was also reflected in the radargrams: the barchan dunes showed reflectors related to the growth of the foresets while those associated with foredunes presented truncations associated with storm events. However, the global balance of dune occupation for the period 2004–2012 was positive.

Geological and Hydrogeological Structure of the Aquifer of the City of N'djamena, Chad

The study of the geological and hydrogeological structure of the aquifer of the city of N'Djamena aims to improve our knowledge for sustainable and prudent management of the groundwater resource. For this study, a methodology combining the geological and hydrogeological structure was adopted. It consists of two steps, the first of which is based on the realization and analysis of lithological sections of different boreholes capturing Quaternary aquifers and the second consists in the realization of the hydrogeological structure of the Quaternary aquifer. The results of the different sections of the boreholes revealed different lithological facies with varying thicknesses (both vertically and laterally). The different Quaternary sections carried out are made up of alternating layers of clay and sand. The deposits are predominantly sandy. The slick is lodged in the sandy formations, whose thickness is more or less constant, which is of the order of 50 to 63 m. The hydrogeological structure allowed us to determine the roof, the wall, the thickness of the Quaternary aquifer as well as its wetted thickness, which averages 44 m. These new data allowed us to realize the 3D architecture of the Quaternary Aquifer of the city of N'Djamena for a better efficient and sustainable management.

The Application of Geophysical Techniques in Tracking Leachate Plumes Migration in a Typical Cemetery within the Sandy formation in Benin City, Nigeria

The survey aim is to track leachate plumes within the sandy formationwhich is below the surface laterite layer. It investigates the presence andmigration of leachate plumes in the sandy region in a typical cemetery(Third Cemetery), in Benin City, Nigeria. The research engaged VerticalElectrical Sounding (VES), 2-Dimensional computation of migrationin both the vertical and horizontal directions. The electrical resistivitydata collected in parallel equidistant lines was processed to obtain geoelectric models using Res2dinv. The leachate plumes in the cemeterymigrate vertically and horizontally at different rates. The maximum andminimum rates of vertical migration are 4.1 and 0.2 cm/day respectively,while the maximum and minimum rates of horizontal migration are 32.8and 1.7 cm/day respectively. Volumetric analysis of the plume zonesindicates that of the 75,231m3 of the subsurface imaged, 6,322 m3 is thezone contaminated by leachate plume, that is, 8.4 % of the earth volumeinvestigated contained leachate plume. The research also showed thatrepeated ERT surveys can track movement of leachate plume emanatingfrom decomposed dead bodies over time in active cemetery. The averagetravel time for a leachate plume to transverse a vertical distance of 6.6 min coarse sand is 366.7 days at constant migration speed of 1.8 cm/day.