Acoustic imaging methods were used to investigate compressional wave anisotropy -caused by multiple parallel fractures. The theoretically predicted transmission coefficients from the displacement discontinuity theory are compared with the measured compressional wave transmission. The displacement discontinuity theory for wave propagation across fractures was determined to be robust except for glancing angles (75°-89°) and parallel (90° of incidence. to the fracture. The anisotropic transmission coefficient is observed to be frequency dependent and results in an apparent frequency-dependent stiffness.
Des methodes acoustiques de formation image ont ete employees pour etudier l'anisotropie de p-wave formee par des ruptures parallèles multiples. Les coefficients theoriquement prevus de transmission de la theorie de "displacement-discontinuity" sont compares à transmission mesuree. La theorie de displacement-discontinuity pour la propagation de vague à travers des ruptures a ete determinee pour être robuste excepte jeter sou-parallèle, (75°-89°) et parallele (90°) de l'incidence à la rupture. On observe le coefficient de transmission a dependent dans le frequence et resultats d'une rigidite liee à la frequence apparente.
Akustische Methoden wurden verwendet, um die Druckwelleanisotrophie nachzuforschen, die durch mehrfache parallele Brueche verursacht wurde. Die theoretisch vorausgesagten Übertragung Koeffizienten von der displacement- discontinuity theorie werden mit der gemessenen Druckwelleuebertragung verglichen. Die Displacement-discontinuity theorie fuer Welle Ausbreitung ueber Bruechen wurde festgestellt, robust zu sein, ausserhalb um (75°-89°) und (90°) der Ausdehnung zum Bruch. Der anisotrope uebertragende Koeffizient wird beobachtet, um Frequenzabhangiges und resultate in einer offensichtlichen Frequenzabhangiges Steifheit zu sein.
Designing and constructing structures in rock requires an accurate map of the mechanical and hydraulic properties of a site. The mechanical properties of rocks are commonly observed to be anisotropic. The source of mechanical anisotropy may be attributed to the rock fabric (e.g. mineral composition) and/or structural features (e.g. micro-cracks). Fractures are a structural feature in rock and are a principle source of hydraulic and mechanical 'anisotropy as well as heterogeneity in a rock mass. These discontinuities are planes of mechanical instability and often have effective permeabilities that far exceed the permeability of the rock matrix. In this paper, we focus on the effect of fractures on seismic anisotropy.
Traditionally, effective medium theories have been used to analyze wave propagation through fractured rock. Effective medium theories (Nur, 1971; Hudson, 1981; Crampin, 1980) use a static approximation to develop analytic expressions for the elastic moduli of a cracked or fractured rock by incorporating the additional. compliance caused by a dilute population Of micro-cracks. This approaches assumes that a fracture will. reduce the modulus of the rock and thus reduce the seismic velocity. This approximation results in the effect of the fracture being distributed throughout the bulk material causing the discreteness of the fracture to be lost. Hence a reduction in velocity is observed, but the cause of the reduction cannot be located. In addition, for a purely elastic system, effective moduli theories predict frequency independent seismic velocities. In this paper, we show how advances in locating and characterizing fractures using seismic methods can be achieved through understanding the role of fractures.
