Simulation des variations de stock d'eau et mesures de gravimétrie absolue à Nalohou, Bénin

Dans le cadre du projet ANR GHYRAF (Gravimétrie et HYdRologie en Afrique), trois années de mesures de gravimétrie absolue ont été réalisées en zone de mousson ouest africaine au Bénin. Ces données sont comparées aux variations de stocks d'eau simulées grâce à un modèle numérique contraint par un suivi hydrologique (Sonde à neutrons, piézométrie) et dont la structure est basée sur des données géophysiques (résistivité EM et OC, RMP). La bonne concordance obtenue permet d'évaluer la zone de sensibilité du gravimètre, et d'estimer les contributions relatives de la zone non saturée et de la nappe phréatique dans la variation totale de stock. On montre que la gravimétrie absolue est une source de données indépendantes permettant de contraindre le modèle hydrologique. (Résumé d'auteur

Intravenous alteplase for stroke with unknown time of onset guided by advanced imaging: systematic review and meta-analysis of individual patient data

Background: Patients who have had a stroke with unknown time of onset have been previously excluded from thrombolysis. We aimed to establish whether intravenous alteplase is safe and effective in such patients when salvageable tissue has been identified with imaging biomarkers. Methods: We did a systematic review and meta-analysis of individual patient data for trials published before Sept 21, 2020. Randomised trials of intravenous alteplase versus standard of care or placebo in adults with stroke with unknown time of onset with perfusion-diffusion MRI, perfusion CT, or MRI with diffusion weighted imaging-fluid attenuated inversion recovery (DWI-FLAIR) mismatch were eligible. The primary outcome was favourable functional outcome (score of 0–1 on the modified Rankin Scale [mRS]) at 90 days indicating no disability using an unconditional mixed-effect logistic-regression model fitted to estimate the treatment effect. Secondary outcomes were mRS shift towards a better functional outcome and independent outcome (mRS 0–2) at 90 days. Safety outcomes included death, severe disability or death (mRS score 4–6), and symptomatic intracranial haemorrhage. This study is registered with PROSPERO, CRD42020166903. Findings: Of 249 identified abstracts, four trials met our eligibility criteria for inclusion: WAKE-UP, EXTEND, THAWS, and ECASS-4. The four trials provided individual patient data for 843 individuals, of whom 429 (51%) were assigned to alteplase and 414 (49%) to placebo or standard care. A favourable outcome occurred in 199 (47%) of 420 patients with alteplase and in 160 (39%) of 409 patients among controls (adjusted odds ratio [OR] 1·49 [95% CI 1·10–2·03]; p=0·011), with low heterogeneity across studies (I2=27%). Alteplase was associated with a significant shift towards better functional outcome (adjusted common OR 1·38 [95% CI 1·05–1·80]; p=0·019), and a higher odds of independent outcome (adjusted OR 1·50 [1·06–2·12]; p=0·022). In the alteplase group, 90 (21%) patients were severely disabled or died (mRS score 4–6), compared with 102 (25%) patients in the control group (adjusted OR 0·76 [0·52–1·11]; p=0·15). 27 (6%) patients died in the alteplase group and 14 (3%) patients died among controls (adjusted OR 2·06 [1·03–4·09]; p=0·040). The prevalence of symptomatic intracranial haemorrhage was higher in the alteplase group than among controls (11 [3%] vs two [<1%], adjusted OR 5·58 [1·22–25·50]; p=0·024). Interpretation: In patients who have had a stroke with unknown time of onset with a DWI-FLAIR or perfusion mismatch, intravenous alteplase resulted in better functional outcome at 90 days than placebo or standard care. A net benefit was observed for all functional outcomes despite an increased risk of symptomatic intracranial haemorrhage. Although there were more deaths with alteplase than placebo, there were fewer cases of severe disability or death. Funding: None

Geology and Petroleum Systems of the Offshore Benin Basin (Benin)

This paper summarizes the tectonosedimentary development and petroleum system of the Offshore Benin Basin (OBB). In accordance with structural development, the stratigraphic succession of this basin was divided into 4 sequences: pre-rift (up to Late Jurassic); rift (Neocomian to Lower Cretaceous); transitional (Cenomanian to Santonian) and post-rift (Maastrichtian-Holocene) sequences. Only one Upper Cretaceous petroleum system is well known within this basin. Source rocks of this system contain Type II-III kerogens with a TOC (Total Organic Carbon) average of 2.9%. Oil is produced from sandstone facies within Abeokuta formation. Currently exploration data and geochemical characteristics of bituminous sands which outcrop in some onshore areas of the Dahomey Embayment point to the existence of another petroleum system of Lower Cretaceous age (Neocomian to Albian) in this basin

Construction d’un modele lithostratigraphique tridimensionnel pour les departements de l’oueme- plateau au Sud-Ouest du Benin

Using the data of drillings distributed on the territory of the Ouémé-Plateau departments, it was built a three-dimensional lithostratigraphic model. This model required the collection, the classification and the insertion of the data in the software of modeling "HydroGeo Analyst 2.0". Some constraints like the nature and the thickness of the layers were imposed on the model numerically built in a georeferential system. The use of the model obtained aims to improve lithological knowledge of the zone of study and to facilitate the extension of the hydrogeologic prospecting in the northern zone of the plate. A l’aide des données de forages répartis sur le territoire des départements de l’Ouémé-Plateau, il a été construit un modèle lithostratigraphique tridimensionnel. Ce modèle a nécessité la collecte, la classification et l’insertion des données dans le logiciel de modélisation “HydroGeo Analyst 2.0”. Certaines contraintes comme la nature et l’épaisseur des couches ont été imposées au modèlenumériquement construit dans un système géoréférencé. L’utilisation du modèle obtenu a pour objectif d’améliorer la connaissance lithologique de la zone d’étude et de faciliter l’extension de la prospection hydrogéologique dans la zone nord du plateau

Geology and Petroleum Systems of the Offshore Benin Basin (Benin)

This paper summarizes the tectonosedimentary development and petroleum system of the Offshore Benin Basin (OBB). In accordance with structural development, the stratigraphic succession of this basin was divided into 4 sequences: pre-rift (up to Late Jurassic); rift (Neocomian to Lower Cretaceous); transitional (Cenomanian to Santonian) and post-rift (Maastrichtian-Holocene) sequences. Only one Upper Cretaceous petroleum system is well known within this basin. Source rocks of this system contain Type II-III kerogens with a TOC (Total Organic Carbon) average of 2.9%. Oil is produced from sandstone facies within Abeokuta formation. Currently exploration data and geochemical characteristics of bituminous sands which outcrop in some onshore areas of the Dahomey Embayment point to the existence of another petroleum system of Lower Cretaceous age (Neocomian to Albian) in this basin

Logiciel visualisation de donnees geologiques pour le champ de seme (lovidoges) au Sud-Ouest du Benin

The insufficiency of numerical supports for the geological data in Benin in general and on its field of Sèmè in particular, required the development of a software of visualization of geological data for this oil-bearing field. Entirely written in the language "BASIC", this software is completely integrated into "Windows". Having amongst other things for source the results of drilling of eight (8) well offshore, this software of visualization of data constitutes a collection of information detailed on the geological characteristics and geophysics of the sedimentary layers of the gulf of Benign which it reconstitutes in the various shapes of cuts and graphs. This software thus worked out facilitates the access and the handling of the data by any user. This article details the structure of the software and explains its operating mode summarily. L'insuffisance de supports numériques pour les données géologiques au Bénin en général et sur son champ pétrolifère de Sèmè en particulier, a nécessité l'élaboration d'un logiciel de visualisation de données géologiques pour ce champ. Entièrement rédigée dans le langage "Basic", ce logiciel est complètement intégré à "Windows". Il s’appuie sur les résultats de forage de huit (8) puits en offshore etconstitue un recueil d'informations détaillées sur les caractéristiques géologiques et géophysiques des couches sédimentaires du golfe du Bénin qu'il reconstitue sous diverses formes de coupes et de graphes ;facilitant ainsi l'accès et la manipulation des données par tout utilisateur. Le présent article détaille la structure du « Lovidoges » et explique sommairement son mode de fonctionnement

Dynamique hydrosédimentaire et migration de l’embouchure du fleuve mono dans la zone margino-littorale au Sud-Ouest du Bénin

En milieu tropical, la dynamique hydrologique au sein du domaine margino-littoral associe souvent des jeux d’estuaires et de deltas dans lesquels se réalisent des débouchés (embouchures) de formes variées.Au Bénin, partant des données d’interprétation de photo aériennes et des levés de terrain, l’embouchure "bocca del Rio" comprise entre Ouidah et Grand-Popo a été étudiée. La présente étude met l’accent sur la dynamique hydrosédimentaire en cause dans les phénomènes de migrations est-ouest de la dite embouchure, le bilan sédimentaire et le fonctionnement hydrodynamique entre l’estuaire du Mono, et l’océan atlantique. Des scénarii d’évolution sont formulés dans l’optique d’aménagement à des fins touristiques respectueux de l’équilibre dynamique de la bocca del Rio.Mots clés : Bénin, bocca del Rio, dynamique hydrologique, bilan sédimentaire, ligne de rivage.In tropical coastal environment, the hydrological dynamic often associates games of estuaries and deltas in which achieve varied shapes of outlets (mouths). In Benin, using aerial photographs data and field surveys, the mouth "bocca del Rio" between Ouidah and Grand Popo was studied.The present study focuses on the dynamics hydrosedimentary involved in the East-West migration phenomena of "bocca del Rio" mouth, the sediment balance and the hydrodynamic behaviour between Mono Estuary, and the Atlantic ocean. Scenarios of evolution are formulated in terms of planning for tourism respectful of the dynamic equilibrium of the bocca del Rio.Keys words: Benin, bocca del Rio, dynamic hydrologic, sedimentary balance, shoreline

Groundwater in hard rocks of Benin : regional storage and buffer capacity in the face of change

Groundwater plays a major role in supplying domestic water to millions of people in Africa. In the future, the ability to increase reliable water supplies for domestic and possibly irrigation purposes will depend on groundwater development. Groundwater storage is a key property because it controls the buffering behavior of the aquifer as it is subjected to time-varying conditions such as increased pumping or land-use change. However, quantitative knowledge of groundwater storage in Africa is very limited. This lack of knowledge is a major concern in hard rocks, which cover about 40% of the surface area of Africa. This paper presents a unique quantitative assessment of groundwater storage in different types of hard rocks and a first estimate of the capacity of hard rock aquifers to buffer changes in climatic and anthropogenic conditions. Our study area in Benin (West Africa) is composed of various grades of metamorphic rocks. We used the latest developments in the application of the magnetic resonance geophysical method to confront the methodological difficulty of quantifying groundwater storage. We successfully conducted 38 magnetic-resonance measurements in eight (8) different geological units; each measurement was quantitatively interpreted in terms of groundwater storage. We determined the groundwater storage of our study area to be 440 mm 70 mm (equivalent water thickness). To assess the buffer capacity of aquifers, we compared groundwater storage to groundwater discharge. Groundwater discharge is the sum of natural discharge plus human abstraction. We estimated natural discharge (i.e. deep drainage plus evapotranspiration) from water table fluctuations monitored in six (6) piezometers. Human abstraction was calculated based on the number of operating boreholes and their average daily abstraction. We found that human abstraction (0.34 mm/year +/- 0.07 mm) is far less than natural discharge (108 mm/ year 58 mm). We conclude that increased abstraction due to population growth will probably have a smaller impact on storage than observed land-use change, which may lead to a change in the evapotranspiration rate. We calculated buffer capacity as the ratio of current storage to total discharge, and obtained a result of 6 years +/- 47 months. This buffer capacity confirms groundwater's ability to buffer changes. Finally, our study is intended to promote a more quantitative approach to assessing groundwater resources in Africa and to support our ability to adapt to current and future changes

The use of magnetic resonance sounding for quantifying specific yield and transmissivity in hard rock aquifers : the example of Benin

Hundreds of thousands of boreholes have been drilled in hard rocks of Africa and Asia for supplying human communities with drinking water. Despite the common use of geophysics for improving the siting of boreholes, a significant number of drilled holes does not deliver enough water to be equipped (e.g.40% on average in Benin). As compared to other non-invasive geophysical methods, magnetic resonance sounding (MRS) is selective to groundwater. However, this distinctive feature has not been fully used in previous published studies for quantifying the drainable groundwater in hard rocks (i.e. the specific yield) and the short-term productivity of aquifer (i.e. the transmissivity). We present in this paper a comparison of MRS results (i.e. the water content and pore-size parameter) with both specific yield and transmissivity calculated from long duration pumping tests. We conducted our experiments in six sites located in different hard rock groups in Benin, thus providing a unique data set to assess the usefulness of MRS in hard rock aquifers. We found that the MRS water content is about twice the specific yield. We also found that the MRS pore-size parameter is well correlated with the specific yield. Thus we proposed two linear equations for calculating the specific yield from the MRS water content (with an uncertainty of about 10%) and from the pore-size parameter (with an uncertainty of about 20%). The later has the advantage of defining a so-named MRS cutoff time value for indentifying non-drainable MRS water content and thus low groundwater reserve. We eventually propose a nonlinear equation for calculating the specific yield using jointly the MRS water content and the pore-size parameters, but this approach has to be confirmed with further investigations. This study also confirmed that aquifer transmissivity can be estimated from MRS results with an uncertainty of about 70%. We conclude that MRS can be usefully applied for estimating aquifer specific yield and transmissivity in weathered hard rock aquifers. Our result will contribute to the improvement of well siting and groundwater management in hard rocks

Time-lapse electrical surveys to locate infiltration zones in weathered hard rock tropical areas

International audienceIn West Africa, infiltration and groundwater recharge processes in hard rock areas are depending on climatic, surface and subsurface conditions, and are poorly documented. Part of the reason is that identification, location and monitoring of these processes is still a challenge. Here, we explore the potential for time-lapse electrical surveys to bring additional information on these processes for two different climate situations: a semi-arid Sahelian site (north of Burkina and a humid Sudanian site (north of Benin), respectively focusing on indirect (localized) and direct (diffuse) recharge processes. The methodology is based on surveys in dry season and rainy season on typical pond or gully using Electrical Resistivity Tomography (ERT) and frequency electromagnetic (FEM) apparent conductivity mapping. The results show that in the Sahelian zone an indirect recharge occurs as expected, but infiltration doesn't takes place at the center of the pond to the aquifer, but occurs laterally in the banks. In Sudanian zone, the ERT survey shows a direct recharge process as expected, but also a complicated behavior of groundwater dilution, as well as the role of hardpans for fast infiltration. These processes are ascertained by groundwater monitoring in adjacent observing wells. At last, FEM time lapse mapping is found to be difficult to quantitatively interpreted due to the non-uniqueness of the model, clearly evidenced comparing FEM result to auger holes monitoring. Finally, we found that time-lapse ERT can be an efficient way to track infiltration processes across ponds and gullies in both climatic conditions, the Sahelian setting providing results easier to interpret, due to significant resistivity contrasts between dry and rain seasons. Both methods can be used for efficient implementation of punctual sensors for complementary studies. However, FEM time-lapse mapping remains difficult to practice without external information that renders this method less attractive for quantitative interpretation purposes