3-D Magnetotelluric Investigations for geothermal exploration in Martinique (Lesser Antilles). Characteristic Deep Resistivity Structures, and Shallow Resistivity Distribution Matching Heliborne TEM Results

Within the framework of a global French program oriented towards the development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-methods approach, with the aim to define precisely the potential geothermal ressources, previously highlighted (Sanjuan et al., 2003). Amongst the common investigation methods deployed, we carried out three magnetotelluric (MT) surveys located above three of the most promising geothermal fields of Martinique, namely the Anses d'Arlet, the Montagne Pel{\'e}e and the Pitons du Carbet prospects. A total of about 100 MT stations were acquired showing single or multi-dimensional behaviors and static shift effects. After processing data with remote reference, 3-D MT inversions of the four complex elements of MT impedance tensor without pre-static-shift correction, have been performed for each sector, providing three 3-D resistivity models down to about 12 to 30 km depth. The sea coast effect has been taken into account in the 3-D inversion through generation of a 3-D resistivity model including the bathymetry around Martinique from the coast up to a distance of 200 km. The forward response of the model is used to calculate coast effect coefficients that are applied to the calculated MT response during the 3-D inversion process for comparison with the observed data. 3-D resistivity models of each sector, which are inherited from different geological history, show 3-D resistivity distribution and specificities related to its volcanological history. In particular, the geothermal field related to the Montagne Pel{\'e}e strato-volcano, is characterized by a quasi ubiquitous conductive layer and quite monotonic typical resistivity distribution making interpretation difficult in terms of geothermal targets. At the opposite, the resistivity distribution of Anse d'Arlet area is radically different and geothermal target is thought to be connected to a not so deep resistive intrusion elongated along a main structural axis. Beside these interesting deep structures, we demonstrate, after analyzing the results of the recent heliborne TEM survey covering the whole Martinique, that surface resistivity distribution obtained from 3-D inversion reproduce faithfully the resistivity distribution observed by TEM. In spite of a very different sampling scale, this comparison illustrates the ability of 3-D MT inversion to take into account and reproduce static shift effects in the sub-surface resistivity distribution.Comment: Wordl Geothermal Congress 2015, Apr 2015, Melbourne, Australi

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Multi-scale study of river surface temperature using thermal infrared remote sensing : examples in the Rhône basin (South East France)

Dans un contexte de changement climatique, la compréhension du régime thermique des cours d’eau est un enjeu important. En mesurant le rayonnement dans le spectre électromagnétique de l’infrarouge thermique (IRT : 0,4-14µm), la télédétection IRT offre la possibilité d’obtenir une cartographie de la température de surface à différentes échelles spatiales. L’approche multi-échelle est ainsi le fil directeur de ce travail.Dans le premier temps, nous utilisons des images satellites Landsat ETM+ pour caractériser les structures thermiques longitudinales et temporelles d’un grand continuum fluvial : le Rhône français (500 km). Une méthode automatique supprimant les pixels contaminés par les entités exondées, est développée, améliorant ainsi la précision des données. Les images nous permettent de comprendre les effets thermiques des affluents et des centrales nucléaires. L’Isère est la principale source d’eau froide, alors que les centrales nucléaires du Bugey, de Saint-Alban et de Tricastin réchauffent le fleuve. Nous mettons en évidence des anomalies thermiques au niveau des aménagements hydroélectriques. Par rapport aux canaux, les Rhône court-circuités (RCC) sont plus sensibles aux conditions extérieures du fait de leur géométrie et de leurs conditions hydrauliques.Dans un second temps, les travaux se focalisent sur un tronçon plus court (50 km) : l’Ain dans sa basse vallée où quatre campagnes IRT aéroportées sont réalisées. Nous développons une méthode statistique permettant de calculer l’incertitude de mesure associée à la construction des profils longitudinaux de température de l’eau. Les artefacts des vraies tendances longitudinales sont ainsi différenciés. Pour comprendre ces tendances, un modèle 1D (thermo-hydraulique) est mis en place sur 21 kilomètres. Il considère les flux de chaleur à l’interface eau-air et les propriétés géométriques ainsi qu’hydrauliques de la rivière. Les arrivées phréatiques associées aux bras morts et aux suintements latéraux sont identifiées sur les images thermiques et intégrées au modèle. Ces arrivées phréatiques peuvent refroidir l’Ain de 0,6°C en été lorsqu’elles représentent 15,7% du débit total.Une échelle plus fine est explorée enfin. Le travail porte cette fois sur neuf tronçons en tresses (1 km) pour lesquels des images IRT à très haute résolution spatiale sont acquises. En caractérisant les distributions spatiales de la température, nous identifions deux types de tronçons. Le premier montre une très faible variabilité thermique spatiale tout au long de la journée. Les cours d’eau de ce type ont bien souvent un régime hydrologique proglaciaire avec des débits estivaux élevés, ce qui tend à homogénéiser la température. Le second type présente une hétérogénéité thermique élevée. La température des chenaux courants varie avec la température de l'air. En revanche, la température des chenaux alimentés par des eaux souterraines est relativement constante au cours de la journée. Nous proposons une méthode ne nécessitant pas d’images IRT pour identifier les tronçons montrant une variabilité thermique élevée.À travers ce travail, nous montrons qu’il est nécessaire de coupler les approches spatiales et temporelles pour comprendre la température des cours d’eau. Longtemps, les mesures ont été effectuées avec des thermomètres. L’aspect spatial a ainsi souvent été ignoré. La télédétection IRT a permit de mieux appréhender les structures spatiales de température. Toutefois, pour comprendre ces dernières il est indispensable de considérer les changements temporels de température. Il est également nécessaire d’intégrer une approche plus physique permettant de simuler différentes situations pour évaluer l’importance des différents facteurs affectant la température.In a context of global warming, understanding the thermal regime of rivers is a key issue. By measuring the radiation in the electromagnetic spectrum of thermal infrared (TIR: 0.4-14µm), TIR remote sensing offers the possibility of obtaining surface temperature maps at multiple scales. The multi-scale approach is thus the guiding principle of this work.First we use satellite thermal infrared images from Landsat ETM+ to investigate longitudinal and temporal variations in the thermal patterns of a large river continuum, the French Rhône (500 km). An automated water extraction technique is developed to remove pixels contaminated by terrestrial surfaces. This method improves the accuracy of our data. The images allow us to understand the thermal effects of tributaries and nuclear power plants: the Isère is the main source of cold water while the Bugey, Saint-Alban and Tricastin nuclear power plants warm the river. We show temperature differences within the largest hydroelectric bypass facilities between the bypass section and the canal. The factors responsible for these differences are the length and minimum flow of the bypass section as well as tributaries coming into this reach.Second, we focus on a shorter river (50 km): the lower Ain in France where four airborne TIR surveys are performed. Based on a statistical analysis of temperature differences between overlapping images we calculate the measurement uncertainty associated with TIR derived profiles. This uncertainty allows for the discrimination between artifacts and real longitudinal thermal trends. To understand these trends, we use a 1D determinist model which predicts water temperature at an hourly time step along a 21 km reach. The model considers heat fluxes at the water-air interface as well as the geometrical and hydraulic characteristics of the river. Based on TIR images, groundwater inputs associated with backwaters and lateral seepages are identified. They are inserted into the temperature model. These groundwater inputs can mitigate high water temperatures during the summer by cooling the river up to -0.6°C when they represent 15.7% of the total discharge.A finer scale is finally explored. The work focuses on nine braided reaches located in the French Alps (1 km) where very high spatial resolution TIR images are acquired. By characterizing the spatial distributions of water temperature, we identify two types of reaches. The first type shows a very low thermal spatial variability throughout the day. Rivers of this type often have a proglacial hydrological regime with high summer flows, which tends to homogenize the temperature. The second type exhibits a higher thermal variability with changes during the day. The temperature of flowing channels changes during the daytime according to the air temperature. In contrast, the temperature of groundwater-fed channels exhibits smaller changes which creates thermal variability over space and time. We propose a method which does not require TIR images in order to identify reaches showing high thermal variability.Through this work, we show that it is essential to combine both spatial and temporal approaches to understand river temperature. Thermometers have been used for many years. Thus, the spatial aspect has often been ignored. TIR remote sensing has allowed a better characterization of spatial thermal patterns. However, to understand these patters it is necessary to consider temporal changes of water temperature. It is also necessary to integrate a more physical approach in order to simulate different scenarios and to assess the importance of the different factors affecting water temperature

Effects of human infrastructures on thermal pattern of two bypass sections of the Rhône River using airbone thermal remote sensing

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Effects of human infrastructures on thermal pattern of two bypass sections of the Rhône River using airbone thermal remote sensing

International audienc

Effet des barrages sur la température du Rhône : que nous apprennent les images Landsat ETM+

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Thermal patterns of the French Rhône River using Landsat ETM+ TIR images

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Caractérisation des méso-habitats fluviaux par imagerie drone

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Thermal patterns of the French Rhône River using Landsat ETM+ TIR images

International audienc

Longitudinal and temporal Thermal Patterns of the French Rhône River using Landsat ETM+ Thermal Infrared (TIR) Images

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