Apport De La Télédétection Et De La Géophysique Dans La Cartographie Des Fractures Hydrauliquement Actives En Zone De Socle Au Centre-Ouest Du Benin

The hilly region is famous in Benin for its water supply problems because it is located in an area composed only of crystalline rocks. It is experiencing acute difficulties of access to water resources. The present study aims to map fracture networks from satellite images and to use high resolution electrical resistivity tomography for their ground locations. The methodological approach used consists of the use of remote sensing techniques for the structural mapping of lineaments and the application of electrical resistivity tomography for the validation of hydraulically active lineaments. The use of the Selective Principal Component Analysis techniques and Sobel (7x7) directionalspatial filtering techniques made it possible to map the major lineaments. The validation of lineaments was made during several geophysical prospecting surveys of electrical resistivity tomography. The map generated as a result of satellite image processing has 638 linear structures ranging in size from 7 to 34 km and having two preferred directions N0 ° - N10 ° and N80 ° - N90 °. At the end of the geophysics, 13 lineaments were validated in major fractures with widths of the fissured-altered zone ranging from 15 to 40 m and whose depths are beyond 50m. The range of electrical resistivity characterizing these fractures varies from 100 to 800 (Ω.m).La région des collines est célèbre au Bénin pour ses problèmes de ravitaillement en eau car située dans une zone constituée uniquement de roches cristallines. Elle connait des difficultés aiguës d’accessibilité aux ressources en eau. La présente étude a pour objectifs de cartographier les réseaux de fractures issues des images satellitaires et d’utiliser la tomographie de résistivité électrique pour leurs localisations au sol. L’approche méthodologique utilisée est basée sur l’utilisation des techniques de la télédétection pour la cartographie structurale des linéaments et l’application de la tomographie de résistivité électrique pour la validation des linéaments hydrauliquement actives. L’utilisation des techniques de l’Analyse en Composantes Principales Sélectives et des filtrages spatiaux directionnels de Sobel (7x7) ont permis d’établir la carte des linéaments majeurs. La validation des linéaments a été faite au cours de plusieurs travaux de prospection géophysique de tomographie de résistivité électrique. La carte générée à la suite du traitement des images satellitaires compte 638 structures linéaires dont la taille varie de 7 à 34 km et qui présentent deux directions préférentielles N0°- N10° et N80°- N90°. A l’issu destravaux de géophysique, 13 linéaments ont été validés en fractures majeures avec des largeurs de la zone fissurée-altérée allant de 15 à 40 m et dont leurs profondeurs sont au-delà de 50 m. La gamme de résistivité électrique caractérisant ces fractures varie de 100 à 800 (Ω.m)

Contribution of Aeromagnetic Data to the Structural Discontinuities Identification of Black Volta Catch-ment Aquifer System in Côte d'Ivoire

This study aims to improve the structural knowledge of the fissured aquifers of the Black Volta catchment in Côte d'Ivoire based on the mapping of magnetic lineaments that represent magnetic discontinuities such as magmatic body contacts or tectonic faults. Four filtering methods: pole reduction (equator), gradient (vertical and hori-zontal), upward extension, and angle tilt were applied to the residual magnetic field map to extract magnetic discontinuities using the Oasis Montaj (Geosoft) program. Euler deconvolution coupled with the analytical signal provided, in addition to the horizontal location of the magnetic contacts, an indication of their depths. The re-sulting structural map contains 458 structures with lengths ranging from 9.03 to 66.54 km. Three directions: NW-SE, E-W and NE-SW were detected, with a predomi-nance of the NW-SE direction. Depths estimated by Euler solutions range from 6.8 to 2847 m. This map is consistent with many faults already recognised or assumed by traditional structural studies and tectonic events affecting the Ivorian basement. These results contribute significantly to the improvement of the structural map of the Black Volta Basin in Côte d'Ivoire. In addition to the major known tectonic faults, numerous lineaments, particularly those at depth, have been highlighted by the pre-sent study.  &nbsp

Contribution of Aeromagnetic Data to the Structural Discontinuities Identification of Black Volta Catch-ment Aquifer System in Côte d'Ivoire

This study aims to improve the structural knowledge of the fissured aquifers of the Black Volta catchment in Côte d'Ivoire based on the mapping of magnetic lineaments that represent magnetic discontinuities such as magmatic body contacts or tectonic faults. Four filtering methods: pole reduction (equator), gradient (vertical and hori-zontal), upward extension, and angle tilt were applied to the residual magnetic field map to extract magnetic discontinuities using the Oasis Montaj (Geosoft) program. Euler deconvolution coupled with the analytical signal provided, in addition to the horizontal location of the magnetic contacts, an indication of their depths. The re-sulting structural map contains 458 structures with lengths ranging from 9.03 to 66.54 km. Three directions: NW-SE, E-W and NE-SW were detected, with a predomi-nance of the NW-SE direction. Depths estimated by Euler solutions range from 6.8 to 2847 m. This map is consistent with many faults already recognised or assumed by traditional structural studies and tectonic events affecting the Ivorian basement. These results contribute significantly to the improvement of the structural map of the Black Volta Basin in Côte d'Ivoire. In addition to the major known tectonic faults, numerous lineaments, particularly those at depth, have been highlighted by the pre-sent study.  &nbsp

Cartographie Automatique des Zones Inondées et Evaluation des Dommages dans le District d’Abidjan à l'Aide de l'Imagerie Satellitaire Radar Sentinel-1 Depuis Google Earth Engine

La connaissance de la localisation et de l'étendue des zones inondées dans le District d’Abidjan au sud de la Côte d'Ivoire, fréquemment affectée par les inondations pendant la saison des pluies et avec une réponse différente des précipitations et du ruissellement dans chacun de ses sous-bassins, a des implications importantes pour la gestion du risque. L'objectif de cette étude est de générer automatiquement des cartes de l'étendue des inondations dans le district d'Abidjan et d’évaluer les zones touchées, grâce au potentiel du cloud, aux algorithmes d'apprentissage automatique et à l'utilisation de données provenant de divers capteurs de télédétection optique Sentinel-2, SAR Sentinel-1 et MNT Polsar. L’approche méthodologique a consisté à implémenter dans Google Earth Engine un script qui permet d'abord de cartographier avec précision l'étendue des zones inondées en utilisant une méthode de détection des changements basée sur les données Sentinel-1 (SAR) avant et après une crue spécifique. Ensuite, les différentes classes d'enjeux (telles que les cultures, les zones habitées, les bâtiments, les routes et la densité de la population) ont été extraites à partir de diverses sources de données gratuites et superposées aux zones inondées cartographiées, permettant ainsi d'évaluer la superficie des zones touchées. De plus, une interface web a été conçue à l'aide des packages de Google Earth Engine, offrant à l'utilisateur la possibilité de visualiser l'étendue des zones inondées et les cartes des enjeux de surfaces affectés, avec une estimation statistique, pour une date donnée dans l'intervalle allant de 2013 à la date actuelle. La cartographie des zones inondées à la date du 20 juin 2020 a révélé une superficie totale de 21 763,05 hectares de zones inondées dans le District d'Abidjan. Une estimation des dégâts causés par cette crue du 20 juin 2020 indique que 13 170,17 hectares d'enjeux ont été affectés en moyenne, ce qui représente 60,5 % des zones inondées. Cette répartition se décompose en 7 875,06 hectares (soit 36,2 %) de terres agricoles touchées et 5 295,11 hectares (soit 24,3 %) de zones urbaines touchées, impactant en moyenne 64 877 personnes. Les résultats de cette étude ont permis de constater que la partie centrale de la zone d'étude, au-dessus de la lagune, présente le plus grand potentiel de risque d'inondation en raison de la morphologie du terrain et de la vulnérabilité élevée des zones construites qui occupent la plaine inondable.   Understanding the location and extent of flooded areas in the Abidjan District in southern Côte d'Ivoire, which is frequently affected by floods during the rainy season, and with a unique response to precipitation and runoff in each of its sub-basins, has significant implications for risk management. The objective of this study is to automatically generate maps of flood extent in the Abidjan district and assess the affected areas, leveraging the potential of the cloud, machine learning algorithms, and data from various optical remote sensing sensors, including Sentinel-2, Sentinel-1 SAR, and Polsar DTM. The methodological approach involved implementing a script in Google Earth Engine that first accurately maps the extent of flooded areas using a change detection method based on Sentinel-1 (SAR) data before and after a specific flood event. Then, various asset classes (such as crops, inhabited areas, buildings, roads, and population density) were extracted from various free data sources and overlaid on the mapped flooded areas, allowing for an assessment of the affected area. Additionally, a web interface was designed using Google Earth Engine packages, providing users with the ability to visualize the extent of flooded areas and maps of affected asset classes, along with statistical estimates, for a given date within the range from 2013 to the present. The mapping of flooded areas as of June 20, 2020, revealed a total area of 21,763.05 hectares of flooded zones in the Abidjan District. An estimation of the damages caused by this flood on June 20, 2020, indicates that an average of 13,170.17 hectares of assets were affected, representing 60.5% of the flooded areas. This breakdown includes 7,875.06 hectares (36.2%) of affected agricultural lands and 5,295.11 hectares (24.3%) of affected urban areas, impacting an average of 64,877 peoples. The results of this study have shown that the central part of the study area, above the lagoon, has the highest potential for flood risk due to the terrain morphology and the high vulnerability of built-up areas occupying the floodplain

Cartographie Automatique des Zones Inondées et Evaluation des Dommages dans le District d’Abidjan à l'Aide de l'Imagerie Satellitaire Radar Sentinel-1 Depuis Google Earth Engine

La connaissance de la localisation et de l'étendue des zones inondées dans le District d’Abidjan au sud de la Côte d'Ivoire, fréquemment affectée par les inondations pendant la saison des pluies et avec une réponse différente des précipitations et du ruissellement dans chacun de ses sous-bassins, a des implications importantes pour la gestion du risque. L'objectif de cette étude est de générer automatiquement des cartes de l'étendue des inondations dans le district d'Abidjan et d’évaluer les zones touchées, grâce au potentiel du cloud, aux algorithmes d'apprentissage automatique et à l'utilisation de données provenant de divers capteurs de télédétection optique Sentinel-2, SAR Sentinel-1 et MNT Polsar. L’approche méthodologique a consisté à implémenter dans Google Earth Engine un script qui permet d'abord de cartographier avec précision l'étendue des zones inondées en utilisant une méthode de détection des changements basée sur les données Sentinel-1 (SAR) avant et après une crue spécifique. Ensuite, les différentes classes d'enjeux (telles que les cultures, les zones habitées, les bâtiments, les routes et la densité de la population) ont été extraites à partir de diverses sources de données gratuites et superposées aux zones inondées cartographiées, permettant ainsi d'évaluer la superficie des zones touchées. De plus, une interface web a été conçue à l'aide des packages de Google Earth Engine, offrant à l'utilisateur la possibilité de visualiser l'étendue des zones inondées et les cartes des enjeux de surfaces affectés, avec une estimation statistique, pour une date donnée dans l'intervalle allant de 2013 à la date actuelle. La cartographie des zones inondées à la date du 20 juin 2020 a révélé une superficie totale de 21 763,05 hectares de zones inondées dans le District d'Abidjan. Une estimation des dégâts causés par cette crue du 20 juin 2020 indique que 13 170,17 hectares d'enjeux ont été affectés en moyenne, ce qui représente 60,5 % des zones inondées. Cette répartition se décompose en 7 875,06 hectares (soit 36,2 %) de terres agricoles touchées et 5 295,11 hectares (soit 24,3 %) de zones urbaines touchées, impactant en moyenne 64 877 personnes. Les résultats de cette étude ont permis de constater que la partie centrale de la zone d'étude, au-dessus de la lagune, présente le plus grand potentiel de risque d'inondation en raison de la morphologie du terrain et de la vulnérabilité élevée des zones construites qui occupent la plaine inondable.   Understanding the location and extent of flooded areas in the Abidjan District in southern Côte d'Ivoire, which is frequently affected by floods during the rainy season, and with a unique response to precipitation and runoff in each of its sub-basins, has significant implications for risk management. The objective of this study is to automatically generate maps of flood extent in the Abidjan district and assess the affected areas, leveraging the potential of the cloud, machine learning algorithms, and data from various optical remote sensing sensors, including Sentinel-2, Sentinel-1 SAR, and Polsar DTM. The methodological approach involved implementing a script in Google Earth Engine that first accurately maps the extent of flooded areas using a change detection method based on Sentinel-1 (SAR) data before and after a specific flood event. Then, various asset classes (such as crops, inhabited areas, buildings, roads, and population density) were extracted from various free data sources and overlaid on the mapped flooded areas, allowing for an assessment of the affected area. Additionally, a web interface was designed using Google Earth Engine packages, providing users with the ability to visualize the extent of flooded areas and maps of affected asset classes, along with statistical estimates, for a given date within the range from 2013 to the present. The mapping of flooded areas as of June 20, 2020, revealed a total area of 21,763.05 hectares of flooded zones in the Abidjan District. An estimation of the damages caused by this flood on June 20, 2020, indicates that an average of 13,170.17 hectares of assets were affected, representing 60.5% of the flooded areas. This breakdown includes 7,875.06 hectares (36.2%) of affected agricultural lands and 5,295.11 hectares (24.3%) of affected urban areas, impacting an average of 64,877 peoples. The results of this study have shown that the central part of the study area, above the lagoon, has the highest potential for flood risk due to the terrain morphology and the high vulnerability of built-up areas occupying the floodplain

Dynamique Passée et Récente et Prévision de L’occupation du Sol: cas du Bassin Versant de la Lobo (Centre – ouest de le Côte d’Ivore)

Le développement agricole et la forte croissance démographique ont contribué à la perte du couvert forestier ivoirien. L’objectif de cette étude est d’analyser l’évolution passée (1986 à 2019) et de prédire (2019 à 2050) l’occupation des sols dans le bassin versant de la Lobo. La classification supervisée par maximum de vraisemblance des images landsat de 1986, 2003 et 2019, couplée à la méthode intensity analysis a été appliquée dans cette étude. Les résultats montrent sur la période de 1986 à 2019 une baisse de la végétation de 4708,82 km2 à 4054,72 km2 au détriment de l’augmentation de la mosaïque des cultures et jachères de 6376,56 km2 à 6925,05 km2. Les plans d’eau et sols nus et bâtis ont également augmenté respectivement de 3,73 km2 et 101,88 km2. Par contre, les aires protégées sont restées inchangées avec une superficie de 1516,13 km2. Les changements des classes d’occupation du sol par intensity analysis ont été lent au premier intervalle de temps (1986-2019) mais s’accéléront au deuxième intervalle de temps (2019 à 2050). La mosaïque des cultures et jachères et la végétation ont connu respectivement des pertes de superficies moins intenses et plus intense, tandis que les autres classes ont gagné intensément de superficies durant le premier intervalle de temps. Pendant le deuxième intervalle de temps, seuls les plans d’eau et la mosaïque des cultures et jachères connaîtront des intensités de gains de superficies moins intenses. Il n’y a pas eu de transition des autres classes vers la végétation et la mosaïque des cultures sur toute la période d’étude (1986 à 2050).               Agricultural development and high demographic growth have contributed to the loss of ivorian forest cover. The objective of this study is to analyse the past (1986 to 2019) and  to predict (2019 to 2050) evolution of land use in the Lobo watershed. Supervised maximum likelihood classification of landsat images from 1986, 2003 and 2019, coupled with intensity analysis was applied in this study. The results show a decrease in vegetation from 4708.82 km2 to 4054.72 km2 over the period 1986 to 2019 at the expense of an increase in the mosaic of crops and fallow land from 6376.56 km2 to 6925.05 km2. Water bodies and bare and built-up land also increased by 3.73 km2and 101.88 km2 respectively. Protected areas, on the other hand, remained unchanged at 1516.13 km2. Changes in land use classes by using intensity of analysis were slow in the first time interval (1986-2019) but will accelerate in the second time interval (2019 to 2050). Crop and fallow mosaic and vegetation experienced less intense and more intense area losses respectively, while the other classes gained area intensively during the first time interval. During the second time interval, only water bodies and the crop-fallow mosaic will experience less intense area gains. There was no transition from the other classes to vegetation and crop mosaic over the entire study period (1986 to 2050)

Cartographie De La Vulnérabilité À La Pollution Des Aquifères Du Socle Précambrien : Cas De La Région D’oumé (Centre-Ouest De La Côte D’ivoire)

Development of agricultural and mining activities and the rapid growth of the population (3.8%) in Oumé’s area, constitute a potential risk of groundwater’s pollutions. This study interested Oumé’s area and aims to elaborate a map of vulnerability to pollution for a better management of groundwaters. The database is constituted by various data in particular, geological, hydrogeological, geomorphological, pluviometric data and satellite image. Methods DRASTIC and GOD were used in this study. Results of DRASTIC method indicated that 31.86% of study area have a very low vulnerability, 68.12% a low vulnerability and 0.02% a moderate vulnerability. With GOD method, 12.8% of study area have a very low vulnerability, 86.46% a low vulnerability and 0.74% a moderate vulnerability. The chemical analysis of groundwater showed that weaker rates of nitrates (0-14mg/l) registered inside the surely well protected zones. These observations have led to validate some various maps obtained. Result of the test of Kappa revealed that coefficient K is reliable and gave moderate agreement (K=0.43) between both methods. This agreement is confirmed by the surface analysis (79.01%). In this study, the DRASTIC method supplied better results and is thus better adapted for the evaluation of vulnerability in the pollution of groundwater of this area

Assessment of Spatio-Temporal Changes of Land Use and Land Cover over South-Western African Basins and Their Relations with Variations of Discharges

West African basins play a vital role in the socio-economic development of the region. They are mostly trans-boundary and sources of different land use practices. This work attempts to assess the spatio-temporal land use and land cover changes over three South Western African basins (Volta, Mono and Sassandra basins) and their influence on discharge. The land use and land cover maps of each basin were developed for 1988, 2002 and 2016. The results show that all the studied basins present an increase in water bodies, built-up, agricultural land and a decline in vegetative areas. These increases in water bodies and land use are as a result of an increase in small reservoirs, of dugouts and of dam constructions. However, the decline in some vegetative clusters could be attributed to the demographic and socio-economic growth as expressed by the expansion of agriculture and urbanization. The basic statistical analysis of precipitation and discharge data reveals that the mean annual discharge varies much more than the total annual precipitation at the three basins. For instance, in the entire Volta basin, the annual precipitation coefficient of variation (CV) is 10% while the annual discharge CV of Nawuni, Saboba and Bui are 43.6%, 36.51% and 47.43%, respectively. In Mono basin, the annual precipitation CV is 11.5% while the Nangbeto and Athieme annual discharge CV are 37.15% and 46.60%, respectively. The annual precipitation CV in Sassandra basin is 7.64% while the annual discharge CV of Soubre and Dakpadou are 29.41% and 37%, respectively. The discharge varies at least three times much more than the precipitation in the studied basins. The same conclusion was found for all months except the driest months (December and January). We showed that this great variation in discharge is mainly due to land use and land cover changes. Beside the hydrological modification of the land use and land cover changes, the climate of the region as well as the water quality and availability and the hydropower generation may be impacted by these changes in land surfaces conditions. Therefore, these impacts should be further assessed to implement appropriate climate services and measures for a sustainable land use and water management

Apport Du SIG Et De L’analyse Multicritère Dans La Prospection Hydrogéologique Du Socle Précambrien De Gagnoa (Centre-Ouest De La Côte d’Ivoire)

Côte d’Ivoire is constituted in more than 97.5% of crystalline basement which contains the main part of groundwater resources of the country. These resources are contained in the weathering material and fissured aquifers. Gagnoa area located at the Central West of Côte d'Ivoire, on crystalline and metamorphic basement. In this area, surface water and groundwater are exploited for the water supply of the populations. However surface water are often doubtful quality and can affect the health of these populations. Thus find water resources in sufficient and good quality amount is a necessity in this area. The study aims to identify favourable areas for the archievement of drillings with important discharge using geomorphologic, hydrogeological data and remote sensing. Multi-criteria analisys methods were used in this study. The database is constituted by various data in particular, data sheets of drillings, hydroclimatic statements and satellite image. This database was stored, analyzed, treated and managed using ARCGIS 10.2. software. Various treatments allowed to establish various thematic maps (accessibility, exploitability, availability). The synthesis of all information inside a SIG allowed to establish the maps of favourable areas to achieve drillings with large discharges. The classes of good and excellent sensibility cover 26% of the study area. These favourable areas for the achievement of drillings with important discharge are located at the center and south of the study area. Validation of this map showed that 66% of the water points (drillings) with discharge superior to 5 m3/h overlap on the good and excellent classes of sensibility. The various zones of productivity of Gagnoa area were able to be characterized. These results are very interesting and useful for the hydrogeological prospecting. They contribute to a better knowledge and a groundwater resource management

Contribution of Aeromagnetic Data to the Structural Discontinuities Identification of Black Volta Catchment Aquifer System in Côte d'Ivoire

This paper focuses on improving the structural knowledge of the fissured aquifers of the Black Volta catchment in Côte d'Ivoire based on the mapping of magnetic lineaments, which represent magnetic discontinuities such as magmatic body contacts or tectonic faults. Four filtering methods, pole reduction (equator), gradient (vertical and hori-zontal), upward extension, and angle tilt, were applied to the residual magnetic field map to extract magnetic discontinuities while using the Oasis Montaj (Geosoft) program. Euler deconvolution coupled with the analytical signal provided, in addition to the horizontal location of the magnetic contacts, is an indication of their depths. The resulting structural map contains 458 structures, with lengths ranging from 9.03 to 66.54 km. Three directions, NW-SE, E-W and NE-SW, were detected with a predominance of the NW-SE direction. Depths estimated by Euler solutions range from 6.8 to 2847 m. This map is consistent with many faults already recognised or assumed by traditional structural studies and tectonic events affecting the Ivorian basement. These results contribute significantly to the improvement of the structural map of the Black Volta Basin in Côte d'Ivoire. In addition to the major known tectonic faults, numerous lineaments, particularly those at depth, have been highlighted by the present study