Role of the sediments in scavenging inorganic contaminants in the Syr Daria River and the Small Aral Sea (Kazakhstan)

9-11 july 2008International audienceThis study is focused on the measurement and the behaviour of the inorganic pollutants in the Suspended Particulate Material (SPM) and the Bottom Sediments (BS) of the Syr Daria River in its Kazakh course, including its outlet in the Small Aral Sea. Two field campaigns were carried out during the low and the high water period. The results display that the current Syr Daria River sediments quality is influenced by several phenomena seasonally and locally controlled: carbonate precipitation, absorption by phytoplankton, scavenging by clay mineral, upstream ground leaching during the snow melting, various discharges (drainage canals, urban and industrial waste waters), connection with delta lakes during the flooding period and impact of the geochemical background. The study of the solid – liquid partition coefficients emphasizes the contrasted behaviour of trace elements with respect to different scavenging phases. As a result it appears that in spite of the mitigating action of the dam reservoirs, of the irrigated areas, of the overflow lakes and flood plains, the Aral Sea sediments are not completely preserved from the contamination

GEOTHERMAL EXPLORATION IN MARTINIQUE BASED ON MAGNETO-TELLURIC AND CONTROLLED-SOURCE EM SURVEY

International audienceResistivity imaging is a key input in many geothermal exploration programs, and particularly in volcanic environment. It is assumed that resistivity variations allow imaging the caprock, but also hydrothermal weathering and preferential geothermal fluid flow. The latter is generally a brine which drastically decreases the electrical resistivity and the temperature increase also lowers resistivity. Several authors have described resistivity signatures for some geothermal reservoirs context. Magnetotelluric (MT) is generally used to image resistivity variations down to a relevant investigation depth of several kilometers. Within the framework of a global French program oriented towards the development of geothermal energy, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-method approach, with the aim to define precisely the potential geothermal structures previously highlighted. Amongst the common investigation methods deployed, we carried out three magnetotelluric (MT) surveys located above three out of the most promising geothermal areas of Martinique, namely the Anses d'Arlet (focus of this presentation), the Montagne Pelée and the Pitons du Carbet prospects. Based on natural source signal, the quality of broad band MT data depends on the source activity (lightning strikes and sun) during the survey but also on the local noise conditions. In many cases, because industrial activities have been developed close to the geothermal resource, the noise conditions make it difficult to obtain reliable MT tensors and therefore a good resistivity distribution. A common way to improve the data quality is to record longer time series and to apply robust processing with remote reference including noise filtering

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

Will the river Irtysh survive the year 2030? Impact of long-term unsuitable land use and water management of the upper stretch of the river catchment (North Kazakhstan)

International audienceThe Irtysh river basin all the way from river spring in China across Kazakhstan as far as the Russian part of Siberia is among the most ecologically endangered and affected regions on our planet. The study provides a summary of the historical reasons for anthropological interventions in this area, which began with the construction of plants of the military—industrial complexes in the forties of the last century during World War II. These plants have a major share in extreme high concentrations of heavy metals in surface as well in groundwaters locally. The Semipalatinsk nuclear polygon plays a specific role as a source of contamination of local waters. The release of top secret data enabled us to gain knowledge about serious problems related to high radioactivity of groundwaters, which should spread uncontrollably through a system of secondary fissures activated by nuclear blasts. Another serious problem in this region is the quantitative aspect of contamination. Model simulations of water balance indicate that large industrial development in the spring area in China and continuously increasing water consumption in Kazakhstan may lead to desiccation of the lower stretch of this large river in Siberia during the summer months of 203

Geochemical Study of a Crater Lake: Lake Pavin, France:A View through Li-O-H Isotopes

AbstractHere we report δ18O values and for the first time δD and lithium isotopic data for the Pavin crater lake (French Massif Central). In its deeper unmixed part, this meromictic lake presents low H and O isotope compositions as a result of CO2 intrusion and the presence of mineral water ingress. This influence of sub-lacustrine water contribution is confirmed by lithium isotopes with low δ7Li values

Géothermie haute température : le défi énergétique

Alors qu'elle représente un potentiel important pour la production d'électricité dans la plupart des îles volcaniques de l'Outre-mer, qui voient grimper leur facture énergétique et leurs rejets en CO2, la géothermie haute température a du mal à s'y développer. Les objectifs ambitieux du Grenelle de l'environnement qui visent à produire, à l'horizon 2020, 50 % de l'électricité de l'outre-mer grâce aux énergies renouvelables, sont une opportunité, sans précédent, pour mettre en place une stratégie cohérente de développement de la géothermie dans ces îles

Weathered profiles in tropical volcanic islands by combined geochemical and geophysical approaches

International audienceTropical volcanic islands commonly have pronounced relief and high runoff rates, and consist of easily weathered volcanic material. Intense mechanical and chemical weathering in volcanic terrains has been recognized as being an important component in the transport of the global dissolved load to the oceans [1]. High chemical weathering rates are mainly due to the impact of hydrothermalism inputs with subsurface water circulations [2]. A helicopter-borne TDEM (Time Domain ElectroMagnestism) and magnetic survey was conducted by BRGM (French Geological Survey) over 3 islands of Guadeloupe, Martinique and Reunion in 2012 and 2013 for a total of 20,000 km of flightlines. TDEM method uses the diffusion of a transient EM field to determine the electrical resistivity versus depth. Erosion timescales were calculated from U-series analyses of river sediments. Our results show a broad range: 0 to150 ka in Martinique, 0 to 60 ka in Guadeloupe and 55 to 90 ky for Piton de la Fournaise (Réunion). At watershed scale, the estimated weathered profile (WP) depth obtain by using U-series method are consistent with TDEM helicopter-borne geophysical imagery method, ranging from 0 to 70 m. WP are locally impacted by hydrothermal circulations with associated secondary minerals (halloysite, tridymite…). Among the combined impact of all parameters (climate, runoff, slopes, hydrothermalism inputs, vegetation etc.), basin age seems to be the key control parameter: the younger the basin, the higher the weathering rate is

Resistivity Imaging for Geothermal Exploration, using Controlled-Source EM where Magneto-Telluric is Not Applicable: Model and Field Study

International audienceResistivity imaging is a key parameter in most geothermal exploration programs, and particularly in volcanic environment. It is assumed that resistivity variations allow imaging the caprock, but also hydrothermal weathering or preferential water flow. The geothermal fluid is generally brine water which drastically decreases the electrical resistivity and the temperature increase also decreases resistivity. The resistivity signatures for several conceptual geothermal reservoirs can be found in the literature. Magneto-telluric (MT) is generally used to image resistivity variations down-to a relevant investigation depth of several kilometers. Based on natural source signal, MT prospection efficiency depends on the Sun activity during the survey but moreover depends on the noise conditions. In many cases, because industrial activities have been developed close to the geothermal resource, the noise conditions make it difficult to obtain reliable MT tensors and hence a good resistivity image. Generally, the only way to counter is recording longer time series, to apply robust noise filtering and combining with a remote filtering. The noise issue in MT prospection is even more drastic in Island context. We investigated the substitution of controlled-source EM instead of MT measurements close to urbanized area. The distance constraints to respect the far field conditions are so challenging that they are often logistically not possible. Indeed, when it is possible to measure the receiver stations far enough from the source, the distortion due to the controlled-source disappear. But when the conditions for applying standard processing of CSAMT are not fulfilled, it is necessary to consider the source effect to interpret properly the EM response. We have studied and performed EM imaging in the near field, using a pair of pre-existing boreholes to inject high current into the ground through metallic casings. Based on numerical modeling, we propose a methodology to obtain maps and section of apparent resistivity. The results obtained were confronted to previous geophysical campaign results performed in the 1980's and recent shallow prospection. This measuring protocol was applied to prospect the Lamentin area (Martinique, Lesser Antilles, France). This survey was performed in 2013 within the framework of a large exploration program founded by the FEDER, ADEME, Regional Council and SMEM designed to explore the geothermal potential of Martinique

Occurrence of K-feldspar-bearing hydrothermal breccias in the Bouillante geothermal field (Basse Terre - Guadeloupe)

International audienceSurficial indicators of recent to present geothermal activity are present around Bouillante (Guadeloupe, French West Indies). Until lately, most mineralogical parageneses identified in this area were typical of low temperature alteration ( 200°C) have been identified in the Bouillante bay (Anse Marsolle). Their petrographic study revealed several hydrothermal parageneses typical of a multistage alteration process. Stage 1) An early potassic alteration facies typical of a high-temperature geothermal system characterized by K-feldspar/adularia + quartz + pyrite. K-feldspars have been shown to be present both in the clasts (replacement) and cement of these breccias. They are associated with quartz whose textural properties have revealed fracturing associated with boiling. Stage 2) An acid-sulphate advanced argillic alteration facies with the occurrence of jarosite, gypsum ± silica (≥ 150°C) as usually observed in the upper part of epithermal systems worldwide as marker of deeper boiling. Stage 3) An argillic alteration facies (illite/smectite mixed layers ± smectite ± calcite). With temperatures typically ranging from 100 to 200°C, this alteration facies is associated with near neutral fluids of mainly meteoric origin as known in the present geothermal reservoir (pH = 5.4). This alteration is the later one as evidenced by petrographic observations. These results highlight mineral assemblages and mineral textures characteristic of high temperature hydrothermal alteration in epithermal settings. The occurrence of these breccias involved the existence of eruptive events (magmatic/hydrothermal explosion) which ejected this material. Isotopic (oxygen, argon) and geochemical (trace elements) studies are now necessary to clarify the timing of these breccias and the nature and the connection of the original fluids with current geothermal fluid