Anisotropic attenuation analysis of crosshole data generated during hydraulic fracturing

A of seismic waves is sensitive to the physical properties of the subsurface and has been observed in vertical seismic profiling (VSP) and reflection data. De et al. (1994) report measurements of the Pand S-wave quality factors from VSP surveys and sonic logs. Klimentos (1995) measures compressional and shear attenuation from sonic logs in sandstone formations with variable oil, water, and gas saturation and observes that QP and QS can be used for pore-fluid discrimination. Maultzsch et al. (2007) evaluate P-wave azimuthal attenuation anisotropy from 3D VSP data acquired over a fractured hydrocarbon reservoir and infer fracture directions from attenuation analysis. Attenuation anisotropy has also been observed in P-wave reflection data (Clark et al., 2009; Vasconcelos and Jenner, 2005). Seismic attenuation is most commonly measured using the spectral-ratio method. Zhu et al. (2007) extend the spectral-ratio method to anisotropic media and apply it to physical modeling data acquired for a transversely isotropic (TI) sample. Computing spectral ratios helps eliminate the source spectrum and can be used to obtain accurate effective and interval attenuation coefficients of Pand S-waves in layered anisotropic media (Behura and Tsvankin, 2009a; Shekar and Tsvankin, 2011). Here, we present a case study with crosshole data generated by perforation shots in a horizontal borehole and recorded in a vertical borehole. The spectral-ratio method applied to all pairs of traces yields a system of equations for the P-wave angle-dependent attenuation coefficient. Since the acquisition aperture is narrow, we approximate the directional dependence of the attenuation coefficient by a quadratic polynomial of the polar and azimuthal angles. The inversion results show that taking attenuation anisotropy into account reduces the data misfit and reveals variations of the attenuation coefficient between perforation stages. Analysis of the shear-wave attenuation coefficient is hampered by the generally poor quality of S-wave data. Although interpretation of the attenuation measurements is not straightforward, it provides indications of temporal changes related to hydraulic fracturing.