Estimation of P‐wave anisotropy parameters from 3D vertical seismic profile with distributed acoustic sensors and geophones for seismic imaging in the CO2CRC Otway Project

Abstract

ABSTRACTThe quality and accuracy of a seismic image can be significantly affected by anisotropy. When anisotropy is significant, neglecting it can lead to smearing of the image, dispositioning of reflections and distortion of amplitudes. Thus, it is imperative to estimate anisotropy and take it into account, especially for long‐offset or wide‐azimuthal seismic data. This study estimates P‐wave anisotropy parameters at the Otway site (Victoria, Australia). A wide range of available offsets acquired within the CO2CRC Otway Project provides a unique opportunity for anisotropy estimation from 3D vertical seismic profile data. We estimate P‐wave anisotropy from direct‐wave arrivals in vertical seismic profile data acquired with geophones and distributed acoustic sensors installed in two wells. Data analysis reveals seismic anisotropy of the subsurface with a significant presence of both polar and azimuthal anisotropy. We estimate key parameters such as normal moveout velocity, azimuthal ellipticity, polar anellipticity and fast‐velocity azimuth by nonlinear fitting of the analytical travel‐time approximation to observed direct P‐wave travel times from 3D vertical seismic profile data. Obtained anisotropy parameters show that polar anellipticity remains almost constant and equal to 0.1 for the whole depth range, while azimuthal anisotropy changes significantly with depth: it is relatively weak in the shallow section then increases significantly below a depth of 600 m. The fast‐velocity azimuth coincides with the orientation of the maximum horizontal stress in the study area. The results show that despite their low sensitivity for large offsets, the distributed acoustic sensor receivers provide sufficient information on anisotropic travel times over the entire length of the well, which is critical for reflection imaging from both vertical seismic profile and surface seismic data.