Seismic Traveltime Anisotropy of the Marcellus Shale: Feasibility Study for Fracture Characterization

In this study we investigate the feasibility of measuring azimuthal anisotropy in the Marcellus shale from a 3D-3C, wide-azimuth data set to invert traveltime information for fracture characterization. These data are acquired in Pennsylvania, USA, in a complex near-surface and topographic environment. This presents a challenge to obtain the necessary azimuthal velocity and S-wave splitting information for characterizing multiple fracture sets in an orthotropic medium. Below the near surface, the overburden is relatively flat and homogeneous, but the Marcellus formation is conformable with the local tectonics of the Syracuse salt, exhibiting folding and faulting. The ultimate goal is to use PS-wave seismic data, not pure mode S-waves, to invert for the two background P- and S-wave velocities, fracture density of two fracture sets, and a fluid factor. Data quality of the 3D-3C Bradford survey appears to be very good for accurate elliptical NMO velocity analyses of the P-wave and the two split S-waves. The high S/N and resolution of the PS-wave data, and a well defined event at the top and bottom of the Marcellus suggest an accurate inversion of traveltime information is feasible for fracture characterization. Observed P- and S-wave azimuthal anisotropy is around 10% and 5%, respectively.