Détermination de la structure tectonique de l'aquifère crayeux du littoral Nord Pas-de-Calais par prospection géophysique couplée à des observations par forage. Conséquence sur la répartition d'eau salée

Sur le littoral du Nord - Pas-de-Calais (France), la Craie d'âge Crétacé supérieur constitue l'aquifère le plus exploité régionalement pour l'alimentation en eau tant potable qu'industrielle. Sur la frange la plus littorale, l'aquifère crayeux, semi-captif est affecté par de nombreuses failles. Dans certains secteurs, la profondeur de la craie et la localisation des failles, restent aléatoires suite au manque de forages et d'affleurements. Afin de palier à ces lacunes, trois méthodes géophysiques (sondages électriques et sismiques, profilage électromagnétique) ont été appliquées.La synthèse des données obtenues a permis de démontrer que la craie est découpée en une série de compartiments par des accidents tectoniques présentant un affaissement vers le Sud et vers l'Ouest. L'interprétation des données géophysiques a également permis d'obtenir des informations sur la répartition de l'interface eau douce - eau salée au sein de l'aquifère crayeux.Dans la région, le concept classique de biseau salé ne permet pas d'expliquer l'irrégularité spatiale des intrusions. L'étude semble démontrer que la répartition et l'extension des intrusions d'eau salée peuvent être corrélées avec la localisation des accidents tectoniques, les intrusions les plus éloignées du rivage se faisant au droit des zones faillées.Ceci amène à proposer une nouvelle approche concernant l'extension des intrusions d'eau salée : elles seraient directement tributaires de la géométrie et des caractéristiques hydrodynamiques de l'aquifère crayeux, ces dernières étant fortement influencées par la présence des failles.Along the North Pas-de-Calais coast between a point south of Boutonnière du Boulonnais and the Authie River, Cretaceous rocks are limited to the east by a paleocliff that overlooks the Quaternary coastal and dune deposits. The chalk aquifer is connected to the Quaternary aquifers and further west to the sea water. Close to the seashore the top of the chalk plateau falls rapidly from 20 m above sea level to 20 m below sea level, causing the aquifer to grade from being unconfined in the east to confined in the west. This chalky aquifer is cut by several faults. A structural survey (LOUCHE et al., 1997) based on borehole data was carried out to evaluate whether the rate and direction of groundwater flow are controlled by the different faults. Two major fault groups striking N030° and N110° have been revealed by the structural map, with a third minor group (N 160- 170°) to the north of the Canche. The two major groups, created during tectonic activity from the Palaeozoic to the present, are composed respectively of structures with general faulting directions of N 100-110° and N 020-040°, previously described by COLBEAUX et al. (1993). Chalk depth and fault location are unknown in certain areas due to the lack of bore holes and outcrops. With the aim of overcoming these deficiencies, three traditional geophysical techniques (electrical sounding, seismic logging and electromagnetic profiling) were carried out between the Canche and Authie rivers in the coastal area, west of the chalky cliffs.The interpretation of the geophysical results allows us to confirm the presence of previously located faults and identify new ones in the area where the chalk is overlain by Quaternary cover. The chalky aquifer is hence divided by the faults into a series of compartments that are inclined to the south or to the west. In addition, anomalies have been recorded in the fresh water - salt water interface on the Picardy continental shelf (MERCIER & BACROT, 1990). These anomalous zones are aligned along the northern extension of the sub N-S faults proposed by MENNESSIER & BEUN (1980), located a few kilometres to the south. Based on indirect evidence, these latter authors interpreted these faults as being responsible for considerable displacement of a major part of the Quaternary deposits. Given the lack of direct observations on salt-water distribution, geophysical results, and more specifically the electrical soundings, are required to provide supplementary information in order to better understand this phenomenon. Furthermore, the study of the salt water location, using electrical soundings, points out the juxtaposition of deep saline intrusions, demonstrating that the concept of a homogeneous wedge-shaped body parallel to the seashore can no longer be used to describe this chalky aquifer.In the Bas-Champs area, electromagnetic profiling and three electrical soundings provided additional information on the relation between tectonic events and saline water intrusion (Fig. 3). The electromagnetic profiling shows the presence of two faults (Fig. 6). The vertical faults have an apparent low electrical resistivity (2.5 Ω.m) indicating the presence of brackish or salt water. Electrical sounding carried out to the south of the electromagnetic profile (SE9) shows a decrease in the electrical resistivity from 12.5 m depth, also confirming the presence of salt water. The results of an electrical sounding located to the north of the electromagnetic profile (SE10) show that the ground is saturated with fresh water up to at least 27.2 m (resistivity of 90 Ω.m). At 1 km distance, there is a juxtaposition of three sectors separated by faults filled up with salt water. However, an electrical sounding (SE2, Fig. 2) located near the sea showed that the ground is saturated with fresh water up to at least 52.7 m deep.Using geological, hydrogeological and geophysical data correlations, between structural context and saltwater intrusion could be established and a new hypothesis for saltwater intrusion for this study area could be proposed. Intrusion is directly dependent on the geometry and the hydrodynamic characteristics of the chalky aquifer, as reflected by its heterogeneous nature.Salt water seems to follow preferential intrusion directions corresponding to faults rather than the classic model of a salt wedge. These results suggest the hypothesis that the seawater intrusion distribution relates to the tectonic configuration of the aquifer. The chalky series is cut into hydrogeological compartments individualised by tectonic accidents, which are preferential axes for sea intrusion. The presence of a saltwater front located far from the coast, as indicated by electrical soundings, could be explained by the geological history of the maritime plain, where the Pleistocene and Holocene seas covered the maritime plain up to the fossil cliff (LOUCHE et al., 1998)

The empirical basis for modelling glacial erosion rates

Glaciers are highly effective agents of erosion that have profoundly shaped Earth’s surface, but there is uncertainty about how glacial erosion should be parameterised in landscape evolution models. Glacial erosion rate is usually modelled as a function of glacier sliding velocity, but the empirical basis for this relationship is weak. In turn, climate is assumed to control sliding velocity and hence erosion, but this too lacks empirical scrutiny. Here, we present statistically robust relationships between erosion rates, sliding velocities, and climate from a global compilation of 38 glaciers. We show that sliding is positively and significantly correlated with erosion, and derive a relationship for use in erosion models. Our dataset further demonstrates that the most rapid erosion is achieved at temperate glaciers with high mean annual precipitation, which serve to promote rapid sliding. Precipitation has received little attention in glacial erosion studies, but our data illustrate its importance

Imaging groundwater infiltration dynamics in the karst vadose zone with long-term ERT monitoring

Water infiltration and recharge processes in karst systems are complex and difficult to measure with conventional hydrological methods. In particular, temporarily saturated groundwater reservoirs hosted in the vadose zone can play a buffering role in water infiltration. This results from the pronounced porosity and permeability contrasts created by local karstification processes of carbonate rocks. Analyses of time-lapse 2-D geoelectrical imaging over a period of 3 years at the Rochefort Cave Laboratory (RCL) site in south Belgium highlight variable hydrodynamics in a karst vadose zone. This represents the first long-term and permanently installed electrical resistivity tomography (ERT) monitoring in a karst landscape. The collected data were compared to conventional hydrological measurements (drip discharge monitoring, soil moisture and water conductivity data sets) and a detailed structural analysis of the local geological structures providing a thorough understanding of the groundwater infiltration. Seasonal changes affect all the imaged areas leading to increases in resistivity in spring and summer attributed to enhanced evapotranspiration, whereas winter is characterised by a general decrease in resistivity associated with a groundwater recharge of the vadose zone. Three types of hydrological dynamics, corresponding to areas with distinct lithological and structural features, could be identified via changes in resistivity: (D1) upper conductive layers, associated with clay-rich soil and epikarst, showing the highest variability related to weather conditions; (D2) deeper and more resistive limestone areas, characterised by variable degrees of porosity and clay contents, hence showing more diffuse seasonal variations; and (D3) a conductive fractured zone associated with damped seasonal dynamics, while showing a great variability similar to that of the upper layers in response to rainfall events. This study provides detailed images of the sources of drip discharge spots traditionally monitored in caves and aims to support modelling approaches of karst hydrological processes

Modelling of as manufactured geometry for prediction of impact and compression after impact behaviour of variable angle tow laminates

The development of variable angle tow technology has attracted growing attention in recent years due to its strong potential for structural tailoring. However, the full details of the failure mechanisms of variable angle tow laminates have been as yet unknown, and the design complexity also requires use of numerical analysis and novel techniques for variable angle tow composites. This paper addresses the two main problems for use of variable angle tow laminates in design. Firstly, a mathematical model is presented to build a three-dimensional variable angle tow model which exactly captures the features of as manufactured variable angle tow laminates. Secondly, impact and compression after impact models using three-dimensional detailed finite element analyses are presented to predict the failure behaviour of variable angle tow laminates including delamination evolution and crack propagation. Results obtained from the impact and compression after impact models are validated against experimental data. </jats:p

Immediate and long-term impact of the COVID-19 pandemic on delivery of surgical services

Background The ongoing pandemic is having a collateral health effect on delivery of surgical care to millions of patients. Very little is known about pandemic management and effects on other services, including delivery of surgery. Methods This was a scoping review of all available literature pertaining to COVID‐19 and surgery, using electronic databases, society websites, webinars and preprint repositories. Results Several perioperative guidelines have been issued within a short time. Many suggestions are contradictory and based on anecdotal data at best. As regions with the highest volume of operations per capita are being hit, an unprecedented number of operations are being cancelled or deferred. No major stakeholder seems to have considered how a pandemic deprives patients with a surgical condition of resources, with patients disproportionally affected owing to the nature of treatment (use of anaesthesia, operating rooms, protective equipment, physical invasion and need for perioperative care). No recommendations exist regarding how to reopen surgical delivery. The postpandemic evaluation and future planning should involve surgical services as an essential part to maintain appropriate surgical care for the population during an outbreak. Surgical delivery, owing to its cross‐cutting nature and synergistic effects on health systems at large, needs to be built into the WHO agenda for national health planning. Conclusion Patients are being deprived of surgical access, with uncertain loss of function and risk of adverse prognosis as a collateral effect of the pandemic. Surgical services need a contingency plan for maintaining surgical care in an ongoing or postpandemic phase.publishedVersio

Desiccation cracks provide evidence of lake drying on Mars, Sutton Island member, Murray formation, Gale Crater

Mars Science Laboratory (MSL) Curiosity rover data are used to describe the morphology of desiccation cracks observed in ancient lacustrine strata at Gale crater, Mars, and to interpret their paleoenvironmental setting. The desiccation cracks indicate subaerial exposure of lacustrine facies in the Sutton Island member of the Murray formation. In association with ripple cross-stratification and possible eolian cross-bedding, these facies indicate a transition from longer-lived perennial lakes recorded by older strata to younger lakes characterized by intermittent exposure. The transition from perennial to episodically exposed lacustrine environments provides evidence for local to regional climate change that can help constrain Mars climate models