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Abstract:
The presence of fluids in closely aligned fractures is important for a range of processes within the Earth. In the near-surface, understanding systems of fluid-filled fractures is important in various applications such as geothermal energy production, monitoring CO2 storage sites and exploring for metalliferous sub-volcanic brines (e.g., Blundy et al., 2021). In the mantle, melting is an important geodynamic process, exerting control over mantle composition and dynamic processes. Upper mantle melting weakens the lithosphere, facilitating rifting (Kendall et al., 2005) and other surface expressions of tectonic processes. Models of aligned fluid-filled fractures, or inclusions with small aspect ratios, predict both velocity and attenuation anisotropy for shear-waves (e.g., Hudson, 1982; Chapman 2003). Forward modelling shows that attenuation anisotropy is highly sensitive to important fracture properties, such as fracture length and orientation, and can be frequency dependent. Attenuation anisotropy can be observed by measuring shear-wave splitting as the two separated shear-waves experience a different seismic quality factor as they propagate through the anisotropic medium.Measurements of attenuation anisotropy therefore offer a new approach to seismically detect fluids in the subsurface. Here we present a method for measuring attenuation anisotropy using an adaptation of the instantaneous frequency method of Mathenay and Nowack (1995). We explore the potential of this technique using synthetics and make measurements of attenuation anisotropy in SKS data collected at the Yellowknife array, Canada, and at stations across the Main Ethiopia Rift. These results highlight the potential for attenuation anisotropy as a tool to detect geofluids in the subsurface.
