Yves Albouy

Improvement in TDEM sounding interpretation in presence of induced polarization. A case study in resistive rocks of the Fogo volcano, Cape Verde Islands

Abstract A Time Domain Electromagnetic (TDEM) survey was carried out in and around the caldera of the Fogo volcano, Cape Verde Islands, to detect the low resistive structures that could be related to groundwater. A sign reversal in the sounding curves was encountered in central-loop measurements for the soundings located in the centre of the caldera along three main radial profiles. The negative transients are recorded in the early channels between 6.8 and 37 μs. Negative values in an early time transient is an unusual field observation, and consequently the first step was to check the data to ascertain their accuracy and quality. In the second step, three-dimensional (3D) effects are evaluated and ruled out in this zone, while an Induced Polarization (IP) phenomenon is observed using Direct Current (DC) sounding measurements. In the third step, the IP effect is called upon to explain the TDEM distortions; a Cole–Cole dispersive conductivity is found to be adequate to fit the field data. However, the more relevant one-dimensional (1D) model is recovered when both central-loop and offset-loop data are jointly taken into account, thus indicating that an effect of dispersive conductivity is necessary to explain the field data. The 1D electrical structure exhibits four layers, with decreasing resistivity with depth. Only the first layer is polarizable and its Cole–Cole parameters are m =0.85, c =0.8 and τ =0.02 ms for chargeability, frequency dependence and time constant, respectively. However, the Cole–Cole parameters deduced from TDEM forward modelling remain different from those deduced from DC/IP sounding. In this volcanic setting, this IP effect may be caused by the presence of small grains of magnetite and/or by the granularity of effusive products (lapillis). As a conclusion, it is shown that a modelling using different TDEM data sets is essential to recover the electrical structure of this area.

New insights into the hydrogeology of a basaltic shield volcano from a comparison between self-potential and electromagnetic data: Piton de la Fournaise, Indian ocean

Abstract In order to investigate aquifers, several geophysical surveys have been carried out in the Baril area of the southern flank of Piton de la Fournaise volcano on Reunion in the Indian Ocean using audiomagnetotelluric (AMT), very-low-frequency (VLF) and self-potential (SP) methods. We present the results with emphasis on a comparison between SP data and the findings of geoelectric surveys. AMT soundings have indicated, from the surface downward, three layers: (i) resistive volcanic rocks, (ii) an intermediate resistivity layer, and (iii) a conductive basement attributed to a seawater-bearing aquifer. VLF measurements allow the mapping of the first layer apparent resistivity, and therefore its bottom, when the true resistivity is supposed to be isotropic and homogenous. When this assumption does not hold, only the SP method permits the mapping of this bottom. Because of the good agreement between the SP and electromagnetic results, we propose the SP method as the first tool that should be used in studying shallow hydrogeological structures in volcanic areas.

Origin and function of a closed depression in equatorial humid zones: The Lake Télé in North Congo

Lake Tele has a nearly perfect ellipsoidal shape and is surrounded by swamps and barely penetrable flooded forests in the heart of the Congo—Zaire watershed basin, and has intrigued the international scientific community for decades. In June 1992, a first Franco—Congolese multidisciplinary scientific expedition was finally able to reach the lake which is vast (23 km2) and shallow (3 m). Its volume is estimated at 71 × 106m3 and is 40% filled with an organic silt layer a metre thick. Its hydrological exchanges are almost exclusively vertical with very little lateral contribution from the surrounding swamp. This leads to the observation that the waters are very slightly mineralized (< 3 mg 1−1), but are very rich in organic carbon (44% of suspended matter and of total dissolved matter) and are very acidic (pH < 4). In addition, a magnetic anomaly of some hundred nanoTesla from a magnetic body at shallow depth has been detected in the lakes northern half, although its origin remains unknown. Botanical and palynological observations suggest the persistence of a strongly hydromorphic forest environment for at least the last 6600 years. Identified pollen taxa, whether ancient or recent, come mainly from pioneer and colonizing taxa such as Macaranga, which tends to indicate that the forest is continuing to gradually fill in the lake.

Apport de l'association des méthodes TDEM (Time-Domain Electromagnetism) et électrique pour la connaissance de la structure du glissement-coulée de Super Sauze (bassin de Barcelonnette, Alpes-de-Haute-Provence, France)

The aim of this study is to determine an investigation protocol that permits the Super Sauze flowslide to be described along continuous lines, through the association of electrical and TDEM (Time-Domain Electromagnetism) methods. The transition from one resistivity range to another often corresponds to the three layer limits defined by geotechnical soundings. The results presented show that the combined methods are well suited to the question investigated and that they are validated by the geotechnical data.