VSP applications in overburden anisotropy and fracture characterization for unconventional resources

Abstract

An extensive vertical seismic profile (VSP) program was conducted in the Appalachian basin, USA, to study the Marcellus shale. The data was acquired with conventional vertical vibrators and consisted of near-offset, offset, walkaway, and walkaround VSP. SV- and SH-waves observed in the offset and walkaway VSP data showed clear traveltime differences at long offsets. The shear waves were likely generated near the sources. Anisotropic parameters δ, ε and γ were extracted from P- and SH-wave traveltimes for three depth intervals. These results show that the VSP data can be used to estimate the SH-wave anisotropic parameter γ in addition to the P- and SV-wave anisotropic parameters δ and ε. A polarization analysis of waveform data around first arrivals in the walkaround VSP survey found non-linear particle motion in the data and a pattern of azimuthal variation in linearity. A further waveform examination found that split shear-waves may have originated from a mode-converted shear-wave near the receiver. The fast and slow shear-wave directions closely correlate with the linearity variation pattern. It supports the interpretation that the observed non-linear particle motion is caused by split shear-waves following the direct P-waves. Because minimal data processing is required before the analysis, the first arrival linearity analysis provides a quick measure to detect and characterize local azimuthal anisotropy.