High-resolution velocity field imaging around a borehole: Excavation-damaged zone characterization

Abstract

The excavation of a deep underground structure induces a stress field redistribution that creates an Excavation Damaged Zone (EDZ). Study of EDZ physical properties is of prime importance in the framework of nuclear waste geological storage, where EDZ constitutes a preferential pathway for storage materials towards the biosphere. Use of ultrasonic wave propagation inside the rock media is of great help as a near field non-intrusive technique for this. The significant research activity in the laboratory offers a large number of boreholes. These boreholes form linear excavations with a perfectly round section. It is for this reason that a specific probe and inversion method have been developed in order to reconstruct the image of the velocity field all around the environment of the boreholes. In order to recover all data needed with high accuracy and minimum effort, the probe is fully monitored for automatic data acquisition.This numerical inversion method has been preliminarily applied to a multi layer test bed in order to validate and optimise the experimental and numerical procedures. Based on those results, an in situ experiment has been performed in the Meuse/Haute-Marne Underground Research Laboratory (URL) in France for the safety assessment of nuclear waste storage in the Callovo-Oxfordian argillaceous layer.

Static and dynamic pressure sensitivity anisotropy of a calcareous shale

“…; Hsu et al . ) and field (Balland and Renaud ) to induce considerable azimuthal variations in the compressional and shear wave speeds. These azimuthal variations are also referred to as an “anisotropy”, but while it is related to the formation anisotropy due to mineral, crack, and fracture alignment, layering, or in situ stress, one must be aware that stress concentrations contribute to these additional changes in the material properties around the borehole (Plona et al .…”

Caractérisation non destructive de la zone endommagée des galeries souterraines excavées dans les roches argileuses

“…Les deux études majeures utilisant la diagraphie sismique sont présentées de manière synthétique dans le Tableau 2. Pour la problématique qui nous intéresse, les travaux de Balland et Renaud (2009) sont probablement les plus aboutis. Ces auteurs ont développé une sonde microsismique permettant de faire varier la distance source-récepteur (et donc le volume d'investigation autour du forage) et la position angulaire des récepteurs autour de l'axe de la sonde.…”

“…Les degrés de liberté des liaisons récepteurs-sonde et l'utilisation d'un code d'inversion adaptée à cette technique offrent la possibilité d'obtenir une image 3D de champs de vitesse autour du forage. Cette technique a été employée dans le laboratoire souterrain de Meuse/Haute-Marne et met en évidence une zone endommagée anisotrope s'étendant à 0,35 rayon de forage (Balland et Renaud, 2009).…”

Numerical simulation and development of data inversion in borehole ultrasonic imaging

“…every 10°(with an azimuthally directional source, Balland and Renaud, 2009). This acquisition geometry will make it possible to check the damaged volume around the borehole, especially its symmetry (with respect to the principal axes of stresses).…”

“…The excavated damaged zone around a nuclear waste storage cavity is to be examined using borehole ultrasonic imaging. It is necessary to establish that useful data are likely to be recorded with this tool (Balland and Renaud, 2009) and that the data can be satisfactorily inverted. The first aspect of this program focuses around a vertical single borehole at the roof of URL gallery.…”