Fluid-filled fractures and fissures often determine the pathways and volume of fluid movement. They are critically important in crustal seismology and in the exploration of geothermal and hydrocarbon reservoirs. We introduce a model for tube-wave scattering and generation at dipping, parallel-wall fractures intersecting a fluid-filled borehole. A new equation reveals the interaction of tube wavefield with multiple, closely-spaced fractures, showing that the fracture dip significantly affects the tube waves. Numerical modeling demonstrates the possibility of imaging these fractures using a focusing analysis. The focused traces correspond well with the known fracture density, aperture and dip angles. Testing the method on a VSP dataset obtained at a fault-damaged zone in the Median Tectonic Line, Japan presents evidences of tube waves being generated and scattered at open fractures and thin cataclasite layers. This finding leads to a new possibility for imaging, characterizing and monitoring in-situ hydraulic properties of dipping fractures using the tube wavefield. @en

Low-frequency Stoneley waves in a fluid-filled borehole (tube waves) have been extensively studied in order to estimate the hydraulic properties of fractures intersecting a borehole. We present a new model for tube-wave generation in a borehole (VSP) intersected by a dipping, permeable layer characterized by certain values of porosity and permeability. The new model accounts for the layer stiffness in the deformation process, and it considers realistic boundary conditions. We show a clear connection between the permeable-layer model and the multiple, parallel-wall open fracture model developed earlier. The effect of large dip angles is investigated in detail using numerical modeling (FEM). The approximate solution (the Mathieu functions) and the results of FEM reveal that the effect of the dip angle is significant for a thick permeable layer and large dip angles. We also develop an inversion method to estimate the tube to P-wave amplitude ratio using multiple receivers, whose performance is numerically tested. The preliminary results of field data application at Nojima fault in Japan show the estimated permeability to be similar to that in a foregoing study. The developed theory is crucial in evaluating field data at fault-damaged zones where steeply-dipping, cataclasite layers and fault gouges are dominant.@en