Détection et reconnaissance de l'état des cavités souterraines par sismique réflexion haute résolution (SHR)

L'évaluation du risque d'effondrement dans les zones contenant des cavités naturelles (dissolution de gypse, calcaire) ou anthropiques (carrière et mines exploitées) est complexe. La connaissance des caractéristiques et le comportement à long terme des cavités souterraines représentent une information importante pour le contrôle de ces phénomènes d'effondrements. Les recherches portant sur les techniques géophysiques visant à détecter les cavités souterraines sont toujours d'actualité. Parmi ces techniques, la sismique réflexion haute résolution a fait l'objet, ces dernières années, d'un large champ d'application pour l'imagerie du sous-sol proche (domaine des hautes fréquences). L'étude présentée ici s'inscrit dans le cadre d'un programme de recherche sur 1'analyse, la prévention et la maîtrise des mouvements de terrains liés à la présence de cavités souterraines, financé par le Ministère de l'Aménagement du Territoire et de l'Environnement. Une démarche d'évaluation précise de la technique SHR a été conduite. Elle est basée sur une expérimentation d'un site sous-miné et inaccessible, mais pour lequel des informations liées à l'exploitation minière et à la géologie étaient disponibles. Le site sous-miné sélectionné est situé en Provence près de la commune de Gréasque

High resolution seismic prospection of old gypsum mines - evaluation of detection possibilities

International audienceThe aim of this work is to evalute the location and detection possibilities of the old Underground gypsum mines, appearing in the region of Paris, and being now unexploited, which represents a real danger both for population and building activities. The experiments were conducted in the site affected recently by serious accidents, caused by ground collapses. They included the application of the high resolution seismic reflection, microgravimetry and resistivity methods. The results obtained were tested by the drilling. The positions of the researched galleries were generally unknown and their occurence was only suspected due to the presence ground-collapse (GC) areas caused by the natural collapse of a cavity. Some of the GCs were observed directiy at the surface, the others were detected by means of the comparison of the old and recent aerial photographs. shows the general geological data of this site and the locations ofthe seisnüc profiles S2,S3,S4 considered in this report . The seismic data was recorded using the 100 Hz geophones and the silenced "betsy gun" as a source (designed in laboratory). The closest offset had been individually adapted, based on the approach of the Optimum offset window

Détection des marnières de Haute Normandie par sismique haute résolution

National audienceThis paper presents examples of application of the high resolution reflection seismics for the detection and localisation of underground voids which are old explorations of marl and chalk in Upper Normandy region in France. The voids occur at depths of 35-40 m. The survey is conducted using the classical impulsive down-hole source and the new portable high résolution P wave vibrator. The results show that detection is possible, reliable and precise.Ce papier présente des exemples d'application de la sismique haute résolution à la détection et localisation des cavités souterraines qui résultent d'exploitation de marne et craie dans la région de Haute Normandie. Les vides se trouvent à profondeurs 35 - 40 m. L'investigation est réalisée avec des sources classiques (impulsionnelles) ainsi qu'avec un nouveau mini-vibrateur de haute résolution à ondes P. Les résultats démontrent que la détection est possible, fiable et précise

Detection of underground Marlpit mine using high resolution seismic

International audienceThis paper presents examples of application of high resolution reflection seismic for the detection and location of underground marlpit quarries in the region of haute-Normandie (north-west France) . Since some of these voids are unknown they represent a real danger both for population and building activities. The experiments are carried out at three sites with similar geological setting but differing in depth of voids (30-45 m), and in ground absorption (inelastic attenuation). For each site an explosive source is used. An additional survey is performed using new portable high resolution P wave vibrator, over one of the profiles. This provides an interesting comparison concerning the choice of the source type. In spite of high ground absorption and natural irregularities of the geological structure, the results show that the detection of marlpit underground quarries, often considered as unresolved, is possible

Comparison of durability indicators obtained by Non Destructive Testing methods to monitor the durability of concrete structures

International audienceThis paper deals with the use of non destructive testing methods (NDT) to assess indicators of concrete durability and mechanical properties of reinforced concrete structures. On site, NDT methods based on electromagnetic or ultrasonic wave propagation (such as radar, impact echo, ultrasonic transmission deviceÉ) are used because they are more or less sensitive to water content and mechanical properties depending on the method. It has been shown, in a former project [1, 2], that the NDT results called Òobservablesî are linked to mechanical and durability indicators (YoungÕs modulus, compressive strength, porosity and saturation degree). Meanwhile, the relationship between observables and indicators depends on the concrete mix design. A calibration protocol is then proposed to get this relationship for the right mix of the reinforced structure studied by using a minimal number of cores. The cores are non-destructively characterised in laboratory or used to determined reference indicators by standardised destructive methods. The aims of this paper are first to present the ND calibration protocol on cores and then to validate this proposed calibration protocol. To achieve this goal, some NDT results obtained on site and on the corresponding core are compared and durability indicators deduced from NDT calibration are compared with reference durability indicators

Identification of inhomogeneous concrete cover by non-contact ultrasonic method

International audienceConcrete cover degradation is induced by aggressive agents in ambiance, chemicals, moisture, or temperature variations. Due to this degradation concrete becomes inhomogeneous and usually a thin surface layer appears with porosity and elastic modulus different than the properties of deeper sound concrete. Therefore the on-site nondestructive evaluation of concrete cover is important to monitor the integrity of concrete structures and prevent their irreversible damage. In this paper the methodology applied by the classical technique used for ground structure recovery called Multichannel Analysis of Surface Waves (MASW) is used as the NDT tool to characterize the thickness and elastic modulus of concrete cover. The procedure consists in generation and reception of surface waves within the required frequency band, using the non-contact transducers which scan evaluated profiles. Then the phase velocity dispersion characteristic V(f) is extracted, and concrete cover is characterized using as the proposed velocity gradient. The dispersion characteristics are exploited by the inversion software in order to obtain the variation of shear wave velocity as a function of depth

Evaluation of concrete structures by combining non-destructive testing methods (SENSO project)

The management and maintenance of the built heritage is one of the main interests of the owners of concrete structures. The engineers wish to obtain quantitative information about concrete properties and their variability. Non-destructive testing (NDT) is very popular in this context as it quickly provides relevant information on the integrity and evolution of the material, but several kinds of indicators representative of the concrete condition need to be evaluated. A French Project, named SENSO, aims to develop methods for the non-destructive evaluation of concrete based on a multi-techniques approach. Several families of techniques are concerned (ultrasonic, electromagnetic, electrical, etc.). The main objective is to define the sensitivity of the techniques and the variability of the evaluation for each indicator concerned. To achieve this, a large experimental programme, involving a representative range of concretes and several indicators, has been carried out. A large database, linking the NDT observables and the indicators, allows the different observables to be distinguished in terms of quality (linked to the variability) and in terms of relevance for the characterisation of each indicator. The improvement of the indicator evaluation by means of technique combinatio

The Ramsey multiplicity of K_4

With the help of computer algorithms, we improve the lower bound on the Ramsey multiplicity of K4, and thus show that the exact value of it is equal to 9