A portable air permeameter for easy in situ measurements in rough conditions

International audienc

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

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 ohm 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 landslipes 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 ground-water behavior. (Résumé d'auteur

Groundwater in ultramatic mined massifs of New Caledonia

Groundwater in ultramatic mined massifs of New Caledonia. Ultramafic rocks outcrop over more than one third of New Caledonia's main island. Under tropical conditions, thick lateritic mantles with nickel concentrations developed on these rocks by geochemical weathering. Groundwater in ultramafic mined massifs represents a valuable resource, but also a severe constrain for mining engineering. Previous works describe several water tables in the various layers of the weathering mantle. From a hydrologic study of the Tiebaghi massif, the hydraulic continuity across the weathering layers down to the bedrock is proposed

Combined electromagnetic prospecting and ground water modelling to study shield volcano hydrogeology: Piton de La Fournaise volcano case study (Reunion Island)

International audienc

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: (1) 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 hove 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

Assessing spatio-temporal patterns of groundwater salinity in small coral islands in the Western Indian Ocean

Peer reviewedPublisher PD

Mapping coastal aquifers by joint inversion of DC and TEM soundings : three case histories

Electrical and electromagnetic methods are well suited for coastal aquifer studies because of the large contrast in resistivity between fresh water-bearing and salt walter-bearing formations. Interpretation models for these aquifers typically contain four layers : a highly resistive unsaturated zone ; a surficial fresh water aquifer of intermediate resistivity ; an underlying conductive, salt water saturated aquifer ; and resistive substratum. Additional layers may be added to allow for variations in lithology within the fresh water and salt water layers. Two methods are evaluated : direct current resistivity and time domain electromagnetic soundings. Use of each method alone produces nonunique solutions for resistivities and/or thickness of the different layers. We show that joint inversion of vertical electric and time domain electromagnetic soundings produces a more tightly constrained interpretation model at three test sites than is produced by inversion methods applied to each data set independently. (Résumé d'auteur

Géophysique des sols et des formations superficielles : abstracts et résumés étendus

A.C.S.A.M.T measurements campaign has been carried out on Beaux Songes site with a new equipment Stratagem (Geometrics). The controlled source is an antenna that sends radio frequencies signals in the range of 1 kHz to 70 kHz, from a distance of the MT recording station far enough to valid the far field approximation. Quick 2D interpretations are directly available on the field. 44 stations have been set up to cover the 10 km2 area. An inversion method of the results provides the topography of conductive substratum. (Résumé d'auteur