SUMMARY The dam of Lampy (Black Mountain, Aude, France) is considered as one of the oldest dams in France. A geophysical survey is performed to better understand the pattern of groundwater flow downstream of this dam in the granitic substratum. Induced polarization is first used to image both electrical conductivity and normalized chargeability. Eight core samples of granite from this site are measured and analysed in the laboratory. Their electrical conductivity and normalized chargeability are expressed as a function of the porosity and cation exchange capacity (CEC). The field data and the petrophysical results are used to image the water content, the CEC and the permeability distribution of the substratum. Then, self-potential is used as a complementary passive geophysical technique, which, in absence of metallic bodies, is directly sensitive to groundwater flow through the so-called streaming potential effect. Indeed, the excess of electrical charges in the vicinity of the solid grains, in the so-called double layer, is dragged by the ground water flow generating in turn an electrical (streaming) current and therefore an electrical field. A map of the resulting self-potential signals is done over the area covered by the induced polarization profiles. This map shows a large positive anomaly with an amplitude of ∼80 mV possibly associated with upwelling groundwater in an area where the soil is water-saturated. A groundwater flow simulation is performed to model this anomaly. This is done in two steps. A preliminary groundwater flow model is built using the permeability and water content distributions obtained from the induced polarization data. Then, this groundwater flow model is updated using the information contained in the self-potential data including the electrical conductivity distribution obtained through resistivity tomography. The algorithm for the inversion of the self-potential data is validated through a 2-D numerical test. This analysis yields a groundwater flow model with the flow being focused through a high permeability zone. This study shows how three geoelectrical methods (self-potential, induced polarization and electrical resistivity) can be efficiently combined to image groundwater flow in the vicinity of a dam.
Abstract Creep processes may relax part of the tectonic stresses in active faults, either by continuous or episodic processes. The aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space. Results are presented from microstructural studies of natural samples collected from San Andreas Fault Observatory at Depth borehole drilled through the San Andreas Fault, which reveal the chronology of the deformation within three domain types. (i) A relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution. (ii) Shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition: abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others, (iii) zones with mainly dissolution trended toward the present‐day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite. This study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep. In parallel, the authors discuss how the creeping mechanism, mainly controlled by the very low friction of the saponite in the first 3–4 km depth, may evolve with depth.
SUMMARY The petrophysical properties of 41 volcanic samples from La Soufrière volcanoe (Guadeloupe Island, Eastern Caribbean, France) are investigated. We first measure the complex conductivity spectra of these rock samples at 4 salinities (NaCl) at laboratory conditions (∼20 °C). For each rock sample, we determine the (intrinsic) formation factor, the surface conductivity and the Cole–Cole normalized chargeability. We also measure the compressional wave velocity (dry and saturated), the shear wave velocity in saturated conditions, the (dry and saturated) thermal conductivity, the dry specific heat capacity and the permeability of the rock samples as well as their cation exchange capacity (CEC) and connected porosity. The formation factor versus porosity obeys Archie's law with a cementation exponent of 2.16 ± 0.10. The surface conductivity and the normalized chargeability are proportional to each other and to the CEC divided by the tortuosity of the material (product of the formation factor by the connected porosity) as predicted by the dynamic Stern layer model. Permeability can be predicted from the normalized chargeability and the formation factor inside one order of magnitude. The thermal conductivity and the seismic properties can be evaluated from the connected porosity of the core samples formation factors. A non-linear relationship is established between the shear wave velocity and the compressional wave velocity for the present data set and other data from the literature. Finally, we show on a specific example, how to convert an induced polarization survey on a stratovolcano into a seismic velocity model (P- and S-waves velocity distributions). We perform a specific application to Papandayan Volcano, a stratovolcano located in Java Island (Indonesia). This work paves the way to the joint inversion problem of seismic and induced polarization surveys for volcanic unrest monitoring.
Le fluage est une deformation ductile affectant certaines failles actives, voire toutes. Il existe en effet deux types de fluage. Le premier, le fluage permanent, concerne certaines failles. La deformation s'effectue de facon continue et constante dans le temps. Le second, plus ponctuel, est frequemment enregistre a la suite d'un seisme. On peut alors parler de deformation post-sismique. La comprehension des mecanismes a l'origine de ces deux types de fluage apparait etroitement liee a celle de la succession des differentes phases du cycle sismique. Notre etude, basee a la fois sur des observations naturelles et sur des experimentations de laboratoire, a demontre que le fluage resultait d'une combinaison de processus interagissant les uns avec les autres et favorisant la mise en place d'un mecanisme dominant. Nous avons mis en evidence que les deux parametres les plus importants a l'etablissement de cette dominance etaient la composition mineralogique de la roche et son etat d'endommagement. Dans une zone de faille, ces deux parametres varient avec le temps, la profondeur et avec l'eloignement a la zone de glissement. Par consequent, les mecanismes de fluage evoluent eux aussi en fonction de ces donnees. Notre approche microstructurale d'echantillons provenant de la zone en fluage actif de la faille de San Andreas, fournis par le forage SAFOD, nous a permis d'etablir une chronologie des deformations subies par les roches de cette zone. Un modele d'evolution des mecanismes de fluage a ainsi emerge. Cette evolution est capable d'entretenir le fluage au cours du temps de facon permanente, soit par un enrichissement de la zone en mineraux a faible coefficient de friction, comme revele par les echantillons de la Faille de San Andreas, soit par un equilibre entre processus de fracturation et de cicatrisation, maintenant la resistance de la roche a un seuil trop faible pour un fort chargement. Dans ce dernier cas, nos experimentations de laboratoire ont montre que si les processus de cicatrisation devenaient dominants, il y avait creation d'heterogeneites de resistance a l'interieur de la roche. A l'echelle d'une zone de faille, ces heterogeneites peuvent etre suffisamment importantes pour mener a l'initiation d'une rupture consequente. Ces experiences sont analogues au cas de la Faille de San Andreas, ou dans un contexte de fluage permanent, des microasperites locales generent de la sismicite.
