Response characteristics of shear waves scattered by fractures with borehole observation system

Abstract

Fractures play a crucial role in various fields such as hydrocarbon exploration, groundwater resources management, and earthquake research. The determination of fracture location and the estimation of parameters such as fracture length and dip angle are the focus of geophysical work. In borehole observation system, the short distance between fractures and detectors leads to weak attenuation of elastic wave energy, and high-frequency source makes it easier to identify small-scale fractures. Compared to traditional monopole logging methods, dipole logging method has advantage of exciting pure shear waves sensitive to fractures, so its application is becoming increasingly widespread. However, since the reflected shear waves and scattered shear waves of fractures correspond to different fracture properties, how to distinguish and analyze these two kinds of waves is crucial for accurately characterizing the fracture parameters. To address this issue, numerical simulation of wave responses by a single fracture near a borehole in rock formation is performed, and the generation mechanism and characteristics of shear waves scattered by fractures are investigated. It is found that when the dip angle of the fracture surpasses a critical threshold, the S-wave will propagate to both endpoints of the fracture and generate scattered S-waves, resulting in two distinct scattered wave packets on the received waveform. When the polarization direction of the acoustic source is parallel to the strike of the fracture, the scattered SH-waves always have larger amplitude than the scattered SV-waves regardless of changing the fracture dip angle. Unlike SV-waves, the SH-waves scattered by the fracture do not have any mode conversion. Additionally, propagation of S-waves to a short length fracture can induce dipole mode vibration of the fracture within a wide frequency range. The phenomena of shear waves reflected and scattered by the fracture are further illustrated and verified by two field examples, thus showing the potential of scattered waves for fracture evaluation and characterization with borehole observation system.