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

Although tube-wave generation and tube-wave scattering in VSP experiments are extensively used to detect and characterize subsurface hydraulic fractures (i.e., open fractures and/or permeable layers), the majority of the earlier studies consider only single generation and single scattering (reflection and transmission) responses. We formulate a new integral equation which represents total tube wavefield including multiple reflection, transmission and generation due to multiple fractures. The accurate prediction of the complex signatures of total tube wavefield is crucial for further characterizing the hydraulic properties of highly-fractured areas, such as a fault-damaged zone. Deriving the integral equation further enables us to develop a new approach to imaging and characterizing hydraulic fractures. We numerically show that the proposed integral equation can be used to predict the total tube wavefield. A spatial-focusing analysis is useful to identify the positions of the hydraulic fractures. We applied the developed theory to field VSP data obtained at a fault-damaged zone in the Median Tectonic Line, Shikoku, Japan. Our preliminary result shows evidence that tube waves are generated and scattered at open fractures and thin layers of the cataclasites at this field@en

Tube waves are low frequency guided waves that propagate along a fluid-filled borehole. The analysis of tube waves is a promising approach to image and characterize hydraulic fractures intersecting a borehole. It exploits tube waves generated by an external seismic wavefield which compresses fractures and injects fluid into the borehole. It also utilizes the attenuation of tube waves due to fluid exchange between the fracture and the borehole, which creates scattered waves (reflection and transmission). Conventional approaches consider tube waves due to a single fracture. However, when the spacing between multiple fractures is short relative to the wavelength of the tube waves, the generated and scattered tube waves interfere with each other, making it difficult to isolate the effect of a single fracture. The analysis of closely spaced fractures is important in highly fractured areas, such as a fault zone. In this study, we explore the possibility of prediction and utilization of generated and scattered tube waves due to multiple fractures. We derive a new integral equation of the full tube wavefield using 1D wavefield representation theory incorporating nonwelded interfaces. We adapt the recent developments in modeling tube wave generation/scattering at a fracture. In these models, a fracture is represented as a parallel wall or a thin poloelastic layer. This allowed us to consider the effects of a dynamic fracture aperture with fracture compliances and the permeability. The representation also leads to a new imaging method for the hydraulic fractures, using multiply-generated and scattered tube waves. This is achieved by applying an inverse operator to the observed tube waves, which focuses the tube waves to the depth where they are generated and/or scattered. The inverse operator is constructed by a tube wave Green’s function with a known propagation velocity. The Median Tectonic Line (MTL) is the most significant fault in Japan, extending NE-SW for over 1000 km across the Japanese Islands. We observed multiple tube waves in a P-wave VSP experiment in a 250 m deep, vertical borehole located on the MTL at Shikoku, Japan. The borehole televiewer and the core studies show that below 40 m depth, the Sambagawa metamorphic rocks contain highly fractured zones which consist of more than 100 open fractures and more than 30 cataclasites. We predict the full tube wavefield using the values of fracture depth and thickness known from the borehole televiewer. We model the open fractures as parallel-wall fractures and the cataclasites as thin poroelastic layers. Furthermore, we estimate the depth of the hydraulic fractures by applying the inverse operator. The results show that the tube waves could be generated and scattered at these permeable structures. Our preliminary results also indicate the possibility that the effect of the open fractures is more dominant in the generation and scattering of tube waves than that of the cataclasites in this field. The formulation and the results presented in this study and the following discussion will be useful in analysis of tube waves in highly fractured zones, in order to localize and characterize hydraulic fractures. @en