Structural pattern of the western Las Cañadas caldera (Tenerife, Canary Islands) revealed by audiomagnetotellurics

The local and regional structural pattern of volcanic edifices strongly controls the space distribution of electrical resistivity. Here we report on the structural context of the western part of the Las Cañadas caldera of Tenerife (LCC) thought to have initiated the formation of the caldera. Using a new dataset of 11 audiomagnetotelluric tensors we emphasize the resistivity distribution of Ucanca caldera and propose a major revision of its extension. We find that Ucanca caldera has a limited westwards extent and that El Cedro sector is a depression margin of the caldera. According to the extent of hydrothermalized rocks at the base of the LCC wall and the distribution of Pico Teide - Pico Viejo vents, we constrain the location and size of Ucanca caldera. The interpretation of these results also constrains the extension of the Icod Valley and proposes a headwall located below the Pico Teide - Pico Viejo Comple

Microscopic scale conductivity as explanation of magnetotelluric results from the Alps of Western Switzerland

Recent MT soundings carried out in the Penninic Alps of Valais have shown the presence of a very good, outcropping conductor. Extremely high conductivity was attributed to the presence of graphite. To verify this assumption, the electrical properties of borehole black shales were measured under simulated physical conditions (electrical frequency, hydrostatic confining pressure, internal fluid pressure, temperature). These measurements showed that under all physical conditions (electrical frequency, in the 0.005-200 Hz interval; hydrostatic confining pressure up to 39 MPa; internal fluid pressure up to 23 MPa; temperature up to 180 °C) one of the samples studied was very conductive (resistivity less than 2 Ωm under all physical conditions). Interestingly, despite similar macroscopic aspect, other samples from a nearby borehole were found to be only slightly conductive. Chemical, mineralogical and petrographic investigations revealed that the enhanced electrical conductivity is mostly due to textural characteristics (such as grain size and carbon film distribution at the grain boundaries) rather than to chemical difference

Fluorimètre portable et dispositif d’injection de traceur pour les suivis des variations rapides des débits de cours d’eau

Field fluorometer and injection device for gauging streams with fast variations. We present a system developed for the measurement of fast periodic stream gauging. The fluorometer consists of a field probe, a data logger with LCD display and flash cards, and a cable of 15 to 30 meters which connects the two parts. This system is adequate for measurements using the technique of tracer dilution (so-called " global method") in which a given mass of tracer is injected upstream. If necessary, a colorless tracer can be used. We associate the fluorometer to an apparatus for injection of fluorescent tracer, based on the tipping bucket rain gauge. Using these two pieces of equipment allows us to perform a series of unattended flow rate measurements at a fixed time interval over a duration of one to several days. The advantage of this system is that once installed, it periodically measures the flow rate of the water without operator intervention, as long as the solution reservoir is not depleted.Nous présentons ici le système que nous avons développé pour le jaugeage périodique de cours d’eau. Il s’agit d’un fluorimètre de terrain constitué d’une sonde, d’une valise d’acquisition de données avec affichage et support mémoire, et d’un câble de 15 à 30 mètres qui relie les deux parties. Ce matériel se prête parfaitement à la technique de jaugeage par dilution de traceur (méthode dite «globale » avec injection instantanée d’une masse de traceur). Si nécessaire, un traceur incolore peut être utilisé. Nous associons le fluorimètre à un dispositif d’injection de traceur fluorescent, basé sur une bascule à augets, inspirée du pluviomètre. L’utilisation de ces deux pièces d’équipement nous permet d’effectuer une série de jaugeages à intervalle de temps fixe, sur une durée de un à plusieurs jours. L’avantage de ce système, c’est qu’une fois installé, il mesure périodiquement le débit du cours d’eau sans intervention de l’opérateur, aussi longtemps que la réserve de solution n’est pas épuisée.Schnegg Pierre-André. Fluorimètre portable et dispositif d’injection de traceur pour les suivis des variations rapides des débits de cours d’eau. In: 35es journées de l’hydraulique de la Société Hydrotechnique de France. Hydrométrie 2013. Paris, 15-16 mai 2013. 2013