Jean-Lambert Join

Using principal components analysis and Na/Cl ratios to trace groundwater circulation in a volcanic island: the example of Reunion

Abstract The development of the limited groundwater resources of a remote tropical island, in particularly rugged volcanic terrain, requires a thorough understanding of the hydrogeology. The cost of a classical approach by borehole investigations often precludes a ‘reasoned’ approach, resulting in a somewhat haphazard development of resources wuth varying success. This paper describes the use of multivariate statistical analysis to trace groundwater circulation in volcanic terrains. The method involves the use of principal components analysis (PCA) based on both structural and hydrochemical parameters of 243 springs of Reunion (western Indian Ocean). As a result, we propose the use of Na/Cl analysis in differentiating three types of volcanic aquifers. This analysis is consistent with a geological and hydrogeological conceptual model developed from a combined hydrochemical and geological reconnaissance of 27 springs.

Deposits related to degradation processes on Piton des Neiges Volcano (Reunion Island): overview and geological hazard

Abstract Piton des Neiges (PN) Volcano on Reunion Island offers a rare opportunity to study deposits related to degradation processes in a deeply eroded oceanic shield volcano. Both the inner parts and flanks reveal a large amount of resedimented volcaniclastic material, including extensive debris avalanche deposits. PN litho–structural units, first studied by Upton and Wadsworth [1965, Philos. Trans. R. Soc. Lond., A 271, pp. 105–130], are re-examined. This review highlights the importance of long volcanic repose periods and erosion processes during PN history. volcaniclastic deposits have been studied in the field in order to evaluate the spatial and temporal distribution of the three main types of PN degradation processes. The deposits of these processes have been classified into: (1) talus, (2) mudflow and debris flow, and (3) debris avalanche. Lithology, frequency and estimated volumes of each deposit type imply that the structural evolution of PN can be considered in terms of the competition between the volcanic productivity and the degradation and erosion processes. The occurrence of huge catastrophic avalanches produced by flank failure is convincingly linked to the basaltic activity of PN, which implies a very low risk at present. On the contrary, mudflows and debris flows pose an important risk due to the high population density focussed around the basin outlets. Moreover, if smaller debris avalanches can occur in the cirques of PN, another major risk must be evaluated.

Geoelectrical imaging of a thick regolith developed on ultramafic rocks: groundwater influence

A thick regolith with supergene nickel ore deposits, developed from ultramafic rocks in New Caledonia (southwest Pacific), was investigated using electrical resistivity tomography (ERT). This paper presents the results of an electrical survey and monitoring of the lateritic weathering mantle of Tiebaghi plateau. Resistivity pseudosections, validated by many borehole logs, contributed to defining a four-layer geoelectric model corresponding to four main weathering horizons: (i) ferricrete with soft nodular horizon; (ii) red mottled zone and yellow fine saprolite; (iii) coarse saprolite; and (iv) bedrock. All geoelectric sections reveal undulated bedrock topography, with each high representing a resistive bedrock ridge and each swale corresponding to a conductive saprolite trough. When parallel pseudosections are correlated, most ridges and troughs have a prominent northeast – southwest strike. Seasonal monitoring of a selected ERT profile shows that the geoelectric structure is well preserved through time. Local resistivity variations occur in the mottled zone – fine saprolite layer and along a major fracture zone, and are interpreted as being due to water content variations in groundwater pathways. On the basis of field and laboratory experiments, it is proposed that the resistivity variations are not due to dewatering but could result from fluctuations in groundwater chemistry. ERT appears to be a useful tool to document the geometry of the regolith and the groundwater flow pattern in weathering horizons and bedrock.

New geological and hydrogeological implications of the resistivity distribution inferred from audiomagnetotellurics over La Fournaise young shield volcano (Reunion Island)

Abstract An audiomagnetotelluric survey has been performed along an inactive flank of La Fournaise volcano massif in Reunion island, to study the subsurface resistivity structure. One-dimensional modelling of the AMT data at each site revealed an extensive low-resistivity (less than 10 ω m) zone at a few hundred meters below the surface. The significance of this unexpected conductive substratum is discussed in relation with the proposed impact of volcano-tectonic processes (caldera and landslides collapses), and a new interpretation of the geological structure is proposed. Moreover, it is likely that these conductors are poorly permeable argilaceous materials; then they coincide with a limit in permeability and determine groundwater behavior.

Hydrothermal system mapped by CSAMT on Karthala volcano, Grande Comore Island, Indian Ocean

Abstract Controlled source audio-magnetotelluric (CSAMT) has been used to investigate the resistivity structure of the summit region of Karthala volcano. The major purpose of this CSAMT survey is to locate the active hydrothermal system. The presence of the hydrothermal system had already been inferred from surface evidence of hydrothermal activity and from self-potential (SP) mapping of the Karthala summit zone. The results of the 1D CSAMT inversion indicate the following: (1) a highly resistive (500–5000 Ω m) 200- to 400-m thick surface layer, that is characteristic of dry basaltic rocks, and made up of lava flows and/or tuff breccias; (2) a 300- to 1200-m thick layer of intermediate resistivity (20–400 Ω m), thought to be representative of the groundwater body; and (3) a deep conductor with a resistivity of less than 2 Ω m, which might be related to the active hydrothermal system. This hydrothermal system appears to be bounded by caldera edges and shows the same north–south trend. Its depth ranges from more than 1 km to less than 0.7 km in the northern part of the caldera and its resistivity ranges from 2 to 0.5 Ω m in the northern part of the caldera. Less resistive zones and the shallowest depths of this conductive layer are well correlated with the largest SP positive anomalies and are assumed to be generated by hot fluid circulation. The most active hydrothermal zone is situated in the northern part of the caldera.

Caractérisation géostructurale des émergences et typologie des nappes d’altitude en milieu volcanique insulaire. (Ile de la Réunion)

ResumenSur l’Ile de la Reunion, l’alimentation en eau constitue un handicap majeur au developpement agricole et urbain du versant sous le vent du massif volcanique du Piton des Neiges. En particulier dans les zones d’altitude (superieure a 900m), les ressources en eau souterraine ne sont pas accessibles par forage. Sur les pentes externes du Piton des Neiges, quatre sites pilotes riches en emergences ont fait l’objet d’investigations hydrogeologiques dans le but d’evaluer la faisabilite de galeries de captage des eaux souterraines dans les hauts de l’ile.Le lever d’une carte geologique au 1/10 000 et la reconnaissance systematique du contexte geostructural des emergences ont permis : - de mettre en evidence la diversite des conditions hydrogeologiques du milieu volcanique,- de montrer l’existence de sources profondement encaissees dans le massif, qui traduisent la presence de reserves aquiferes significatives dont l’exploitation par galeries drainantes serait envisageable.- de proposer une typologie des n...